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Discussion on the petrogenesis of granites
Abstract
As a major component of continental crust, granites have been served as the most important subject in geology. Based on advancements obtained during past decades, this paper provides a comprehensive overview about the issues related to granitic formation. As for genetic types, the classifications between I-, S-and A-type granites are sometimes difficult, especially for those of highly fractionated rocks. It is stated that the concentrations of zirconium in whole-rock and titanium in zircon can be used to provided information on the temperature of partial melting and magma crystallization, but the pressure under which the source partially melted is hard to estimate. The granites are mostly occurred in the subduction zones and post-orogenic extensional settings, where the inputs of volatile and heat resulted in crustal partial melting of orogenic roots, and then the formation of granites. The traditionally used geochemical diagrams for the tectonic discrimination could not get right answer in most cases. This paper also presents a concise summary about the recent achievements of granitic study in China. Finally, potential breakthroughs for the Mesozoic granites in eastern China are explored.
... According to the nature of the magma source, granite is mainly divided into I-, S-and A-type granites (Loiselle & Wones, 1979). However, when the granite undergoes high differentiation crystallization, its mineral composition and chemical composition are similar to the low eutectic granite, which makes the identification of the above three types of granite difficult (Wu et al., 2007(Wu et al., , 2017. ...
... However, in many cases, highly differentiated granites also have a high Ga/Al ratio similar to A-type granite (Wu et al., 2017). The critical feature of A-type granite is high Zr contents (>250 ppm) and high formation temperature (>800 C) (Bonin, 2007;King et al., 1997;Wu et al., 2007). The Laoyu granite shows low Zr contents of 89.6-150 ppm. ...
... The zirconium saturation temperature of the whole rock is 733-781 C (Watson & Harrison, 1983), which is significantly lower than the formation temperature of typical A-type granite (Huang, Chen, & Sun, 2023;King et al., 1997;Wang et al., 2022;Whalen et al., 1987). Meanwhile, alkaline mafic minerals, diagnostic minerals of A-type granite, are not found in the Laogyu granite (Wu et al., 2007). Therefore, the Laogyu granite should belong to highly fractionated granite rather than A-type granite. ...
The North Qinling Orogenic Belt (NQB) records the pivotal geological information for understanding the Palaeozoic evolution of the Qinling Orogenic Belt (QOB). Previous studies mainly focused on the subduction-collision process along the Shangdan suture zone before the Early Devonian. However, as the significant interim period between the Early Palaeozoic and Mesozoic orogeny, the Late Palaeozoic tectonic evolution of the NQB remains poorly understood. Fortunately, the Carboniferous magmatic rocks discovered this time provide substantial geological evidence for revealing the Late Palaeozoic tectonic history of the Qinling Orogenic Belt. This paper will provide a detailed analysis of zircon U-Pb ages, geochemical characteristics and Sr-Nd-Hf isotopic compositions of Carboniferous rocks. The Carboniferous magmatic rocks are categorized as highly differentiated S-type granite and monzo-diorite, formed at 350 ± 2.4 Ma and 353 ± 5.2 Ma, respectively. The granite shows weak peraluminous (A/CNK = 1.02-1.11) and shoshonite nature. Based on the isotopic composition and trace element characteristics, we propose that the Qinling Group paragneiss is the primary source of the Carboniferous granite. Monzodiorite is characterized by enriched LREEs and LILEs (Ba, K and Pb), depleted HFSEs (Th, Nb, Ta and Ti) and enriched Sr-Nd isotopic composition. Monzodiorite magma source region consists of continental crustal material and lithospheric metasomatized mantle. Carboniferous magmatic rocks are the product of an extensional tectonic setting, which indicates the NQB tectonic regimes transition from compression to extension during the Carboniferous.
... Similarly, the zircon saturation temperatures of the nearby Shibaogou and Zhongyuku granitic intrusions in the Yuku ore field are constrained at ~776 and 762 • C, respectively (Xue et al., 2018), which also are cold granites. Generally, the formation temperature of granite is correlated with tectonic settings, the granite that formed in subduction zone or collision orogenic belt usually has formation temperatures below 800 • C (cold granite), whereas the hot granite (above 800 • C) is mostly generated in tectonic settings related to crustal extension and thinning (Miller et al., 2003;Wu et al., 2007). Combining with the tectonic evolution of the Qinling Orogen (Yang et al., 2019), the zircon saturation temperatures of these Late Mesozoic granitic intrusions in the Yuku ore field (central Luanchuan region) indicate a subduction-related tectonic setting (Wu et al., 2007). ...
... Generally, the formation temperature of granite is correlated with tectonic settings, the granite that formed in subduction zone or collision orogenic belt usually has formation temperatures below 800 • C (cold granite), whereas the hot granite (above 800 • C) is mostly generated in tectonic settings related to crustal extension and thinning (Miller et al., 2003;Wu et al., 2007). Combining with the tectonic evolution of the Qinling Orogen (Yang et al., 2019), the zircon saturation temperatures of these Late Mesozoic granitic intrusions in the Yuku ore field (central Luanchuan region) indicate a subduction-related tectonic setting (Wu et al., 2007). Previous studies argued that the southward intracontinental subduction of the NCC beneath the Yangtze Craton occurred at ca. 160-125 Ma (Dong et al., 2011), the water-bearing fluids conducive to crustal melting might be partially provided by the dehydration of the subducted continental crust of the NCC. ...
... The muscovite monzogranite in this study yielded a zircon U-Pb mean age of 126.8 Ma, correlating with the transitional period from compression to extension tectonics in the Qinling Orogen and the lithospheric thinning of the southern margin of the NCC after ~130 Ma (Li et al., 2012b;Li et al., 2018;Wang et al., 2020;Yang et al., 2019). The formation temperature of muscovite monzogranite has been defined in the range of 670-742 • C, which also belongs to cold granite (Wu et al., 2007), but it formed at a much lower temperature than biotite monzogranite in the Huangbeiling pluton. Considering that the protolith of muscovite monzogranite is clay-rich metamorphic mudstone, it suggests that the dehydration of water-bearing minerals (e.g., muscovite) in protolith further reduces magma temperature during melting. ...
The East Qinling Orogen as an important segment of central China that records a complex tectonic evolution history. The Huangbeiling pluton is a rare S-type granite in this orogen and has vital significance in understanding the tectonic evolution of the Qinling Orogen during the Late Mesozoic. The biotite monzogranite of this pluton is the major ore-hosting rock of the Yumugou Mo-W deposit. In this study, we newly report a muscovite monzogranite from this pluton. We conducted detailed petrological, geochemical (whole-rock, biotite, muscovite major and trace elements), and zircon U-Pb dating investigations on both the biotite and muscovite monzogranites from Huangbeiling pluton, with an view to reveal their physico-chemical conditions of formation, petrogenesis and tectonic evolution. Zircon U-Pb dating yielded a weighted mean 206Pb/238U age of 126.8 ± 0.6 Ma for muscovite monzogranite, integrated with published zircon U-Pb age (156–132 Ma) of biotite monzogranite, indicating the Huangbeiling pluton was produced by two episodes of magmatism during the Late Mesozoic. The pluton shows formation temperatures in the range of 670–772°C, and is classified as cold granites formed in a subduction-related setting. The emplacement depth of Huangbeiling pluton is constrained at ca. 0.3–4.6 km, with an average depth of 2.0 km. The biotite monzogranite has higher oxygen and fluorine fugacities than the muscovite monzogranite, suggesting high potential for Mo mineralization. Geochemically, the muscovite monzogranite is identified as a S-type granite sourced from crust-derived clay-rich mudstone, and underwent muscovite dehydration melting and subsequent fractional crystallization of K-feldspar and plagioclase during magma ascend. In combination with the syn- to post-collision geochemical affinities of the Huangbeiling pluton and the Late Mesozoic tectonic evolution of the Qinling Orogen, we propose two episodes of magmatism in the Huangbeiling pluton during the tectonic transition (150–125 Ma) from compression to extension in the Qinling Orogen. The first episode (156–132 Ma) of magmatism is correlated with the delamination of lithospheric mantle and intra-continental subduction of the North China Craton under compressional tectonics, whereas the second episode (~126 Ma) of magmatism occurred through muscovite dehydration melting during pressure decrease.
... Peak magmatism occurred during the late Hercynian-early Yanshanian period in the Western Kunlun region. However, there is no chronological evidence of magmatic activity from 230 to 250 Ma [56], which may be because the strong compressive stress was not conducive to magmatism [58]. When the tectonic stress changes to post-collisional extension, decompression melting and magmatic upwelling are more likely to occur [58], leading to intense magmatism. ...
... However, there is no chronological evidence of magmatic activity from 230 to 250 Ma [56], which may be because the strong compressive stress was not conducive to magmatism [58]. When the tectonic stress changes to post-collisional extension, decompression melting and magmatic upwelling are more likely to occur [58], leading to intense magmatism. With the development of extension and collapse of the orogenic belt, a substantial amount of heat was generated, thereby causing the partial melting of crustal material that ascended and was emplaced. ...
... With continuous extension and lithospheric thinning, large-scale magma upwelling formed the Late Triassic intrusive rocks in the Western Kunlun region. cause the strong compressive stress was not conducive to magmatism [58]. When the tectonic stress changes to post-collisional extension, decompression melting and magmatic upwelling are more likely to occur [58], leading to intense magmatism. ...
Triassic granitoids are abundant on the northwestern margin of the Tibetan Plateau. The Dahongliutan pluton, located in the eastern Western Kunlun orogen, formed in the Late Triassic.Previous field studies have identified potential mixing of crustal and mantle magmas. In this study, we used zircon U–Pb ages and major and trace elemental analyses to investigate the tectonic evolution of the pluton, and to determine whether any exchange of mantle-derived material occurred between the pluton and the source area. We found that the pluton has relatively high SiO2 contents, and the aluminum saturation index is consistent with peraluminous high-K calc-alkaline granite. The pluton is enriched in light rare earth elements; both light and heavy rare earth elements are highly fractionated. The magma that formed the pluton was predominantly derived from the crust; however, a small amount of upper mantle material was involved in the early stages of magma formation. The pluton underwent composite emplacement as a result of tectonic extension and magmatic emplacement, which may have occurred in the late Triassic post-collisional orogenic stage. Late Triassic magmatism provided heat and ore-forming material for Pb–Zn, Cu, Fe, and rare metal mineralization, which is of considerable importance for geological prospecting.
... As the key constituent of the continental crust (Collins, Beams, White, & Chappell, 1982), granitic rocks are commonly divided into M-, I-, S-, and A-types (Whalen, Currie, & Chappell, 1987;Wu, Li, Yang, & Zheng, 2007). The Early Cretaceous granite porphyry from Taipingtun exhibits high contents of SiO 2 (75.76-78.66 ...
... As research has developed, the definition of A-type granites has been continuously abundant, which are not only alkaline, anhydrous, and non-orogenic (Clemens, Holloway, & White, 1986;Loiselle & Wones, 1979), but also include some aluminous and peraluminous granitic rocks (Bonin, 2007;King et al., 1997;Wu et al., 2007). A-type granites are typically characterized by high Na 2 O + K 2 O, FeO T /MgO, Ga/Al; high-field-strength element (HFSE) values; and low Ba, Sr, P, ...
... The material sources of A-type granites are diverse, and there remains no consensus on their origins (Bonin, 2007;Wu et al., 2007). Several models have been proposed to explain the genesis of A-type granites: ...
New LA‐ICP‐MS zircon U–Pb geochronology, whole‐rock geochemistry and Hf isotopic data have been studied on the Taipingtun granite porphyry to determine its petrogenesis and constrain the tectonic setting of the central Great Xing'an Range, northeastern China, during the Early Cretaceous. The results of zircon U–Pb dating suggest that the Taipingtun granite porphyry was emplaced during the Early Cretaceous (122–123 Ma). Geochemically, the granite porphyries have high contents of SiO2 (75.76–78.66 wt%) and (K2O + Na2O) (7.14–8.29 wt%), and low Al2O3 (11.09–12.81 wt%) and CaO (0.15–0.29 wt%), indicating peraluminous and high‐K calc‐alkaline features. Additionally, these rocks are characterized by strong Th, U, K, and Rb enrichment; Ba, Sr, P and Ti depletion; high Ga/Al ratios (10,000*Ga/Al = 2.60–4.92) and negative Eu anomalies (δEu = 0.02–0.13). According to the zircon saturation thermometer results, zircon saturation temperatures range from 756 to 959°C with an average of 823°C, displaying high magmatic temperature for the parental magma. Based on these geochemical characteristics, it can be reasonably inferred that the initial composition of the Taipingtun granite porphyry melts exhibited an A‐type magmatic affinity. Furthermore, zircon grains within the Taipingtun granite porphyry have εHf(t) values of 7.12 to 10.99 and TDM2 model ages of 476–726 Ma. The Ba/La (average 3.89), Nd/Th (average 2.25), Zr/Hf (average 22.55), Ti/Zr (average 2.96) and Rb/Sr (average 4.21) values display the characteristics of crust‐derived magmas. Moreover, these rocks have obviously negative Eu anomalies (average 0.13) and relatively high Y (average 37.83 ppm) and Yb (average 4.15 ppm) contents, suggesting the magmas are generated in a low‐pressure environment. Thus, we consider the primary magmas of the Taipingtun granite porphyry may have been predominantly derived from the partial melting of juvenile crustal rocks at low pressure. Combined with the regional geological data, we suggest that the Taipingtun granite porphyry was formed in an extensional tectonic environment, and possibly related to the subducted Palaeo‐Pacific Ocean Plate in the Early Cretaceous. The Taipingtun granite porphyry in the central Great Xing'an Range was emplaced during the Early Cretaceous (ca. 122–123 Ma). Their geochemical signatures indicated that they exhibited A‐type affinity, and were probably derived from the partial melting of juvenile crustal rocks, suggesting that they were formed in an extensional tectonic setting related to the retreat of the subducted Palaeo‐Pacific Ocean (PPO) Plate.
... On FeO t /MgO versus (Zr + Nb + Ce + Y) and Zr versus 10,000 × Ga/Al diagrams, the samples plot in the fractionated granite, I-type granite, A-type granite fields, and on the boundary between the I-and A-type granite fields (Fig. 10c, d). However, Wu et al. (2007b) proposed that many highly fractionated Iand S-type granites can have 10,000 × Ga/Al ratios of >2.6 and plot in the A-type field. Additionally, the zircon saturation temperatures of most of the early Paleozoic igneous rocks in the LXZGCR range from 486 to 867 • C (Watson et al., 2006), significantly lower than those of A-type granites (> 900 • C; Eby, 1992). ...
... Additionally, the zircon saturation temperatures of most of the early Paleozoic igneous rocks in the LXZGCR range from 486 to 867 • C (Watson et al., 2006), significantly lower than those of A-type granites (> 900 • C; Eby, 1992). Furthermore, the lack of strong negative Eu anomalies in most samples indicate that they are not A-type granites (Wu et al., 2007b). We, therefore, infer that the early Paleozoic granitoids in the LXZGCR are mostly I-type granites. ...
... The granodiorites, collected from the Tongjiang-Fuyuan area, have relatively high SiO 2 , Al 2 O 3 and Na 2 OþK 2 O contents, and low MgO, Fe 2 O 3 and CaO, indicating that the magma originated from partial melting of crustal materials (Barbarin, 1999). In addition, these samples are enriched in LREEs and LILEs, and deficient in HREEs and HFSEs, indicating that the magma was formed in the lower crust (Taylor & McLennan, 1985;Hofmann, 1988;Wu et al., 2007;. ...
... A large number of magmatic belts were formed along the Northeast Asian continental margin along with the subduction of the Palaeo-Pacific plate (e.g. Wu et al., 2007Wu et al., , 2011Sun et al., 2013, Xu et al., 20132015a;Wilde, 2015). Notably, a Mesozoic subduction-related magmatic belt with NNE-trending distribution is widely developed in the eastern continental margin of the Northeast Asia, extending from the Russian Far East, via NE China to the SW Japan (e.g. ...
Typical ophiolitic rock assemblages such as siliciclastic rocks, basalts and gabbros, together with the subduction-related intermediate-acidic intrusive rocks, are newly discovered in the Tongjiang-Fuyuan area of the Heilongjiang Provence, NE China. To determine the formation age and genesis of the mafic rocks (basalts and gabbros) and intermediate-acidic intrusive rocks (granodiorites) in the area, as well as their geodynamic settings, the whole-rock geochemical analysis and zircon LA-ICP-MS U-Pb dating were carried out. Zircon U-Pb results suggest that the granodiorites are 93–95 Ma and gabbro is 95 Ma, respectively. Geochemical results show that the gabbros and basalts exhibit characteristics of ocean island basalt (OIB) affinity and are typically related to having originated from mantle plumes. While the granodiorites show the nature of the island-arc magmatic rocks and may originate from the lower crust. Based on the coeval igneous rock associations and regional tectonic evolution, we conclude that the late Cretaceous magmatic rocks in the Tongjiang-Fuyuan area are the product of continuous subduction of the Palaeo-Pacific plate and reflect the subduction rollback process of the Palaeo-Pacific plate.
... As calcalkaline rocks in orogenic belts are mostly I-type granites, this further indicates that the granites in the study area have mineral chemical characteristics of the I-type granite series, which is consistent with the results obtained by Huang et al. (2020) [10] using whole-rock geochemical analysis. Previous studies have also shown that I-type granitic rocks are mainly derived from the remelting of intermediate-basic igneous rocks from the middle and lower crust that have not undergone weathering, and their formation process may involve contamination with mantle-derived components or other crustal materials [44,45]. The chemical composition of biotite is related to the tectonic environment, and its characteristic composition can reflect the genetic type and formation environment of the host rock. ...
... As calc-alkaline rocks in orogenic belts are mostly I-type granites, this further indicates that the granites in the study area have mineral chemical characteristics of the I-type granite series, which is consistent with the results obtained by Huang et al. (2020) [10] using whole-rock geochemical analysis. Previous studies have also shown that I-type granitic rocks are mainly derived from the remelting of intermediate-basic igneous rocks from the middle and lower crust that have not undergone weathering, and their formation process may involve contamination with mantle-derived components or other crustal materials [44,45]. The MgO contents of the biotites in the host rock and MMEs ranged from 6.79% to 14.68% and from 12.39% to 15.62%, respectively, showing certain mantle-derived magma characteristics. ...
The mafic microgranular enclaves (MMEs) from Mesozoic intermediate-acid magmatic rocks, widely developed along the Fujian coast, are considered to be the results of large-scale crust–mantle interaction by magma mixing. This paper is based on zircon U-Pb chronology, along with zircon Hf isotope and mineral analyses for the host granite and MMEs from Langqi Island, in order to investigate the magma mixing mechanism of the Langqi pluton in Fuzhou, Southeast China. The results indicate that the MMEs were emplaced during the late Early Cretaceous (98.9 ± 2.2 Ma), identical to the age of the granite (100.1 ± 1.1 Ma) within the error range. The zircon εHf(t) values for the granite and MMEs are in the ranges of −2.1~0.0 and −1.7~+1.1. The zircon Hf isotope data indicate that both the granite and MMEs were predominantly derived from the ancient crustal basement of Cathaysia, with a partial mantle-derived contribution. The An values of plagioclase phenocrysts with oscillatory zoning patterns in the MMEs show oscillatory changes from the core to the rim, indicating multiple mixing events between the two magmas with different compositions. Amphiboles in the MMEs show characteristics of crust–mantle contamination, and the Ti migrated from the mafic magma with high concentration to the felsic magma with low concentration during the magma mixing process. Biotites in the host rock and MMEs belong to primary biotite, and they have relatively high MgO contents (ave. 12.78 wt.%) and relatively low FeOT/(MgO + FeOT) ratios (ave. 0.56), showing characteristics of crust–mantle contamination. The crust–mantle magma interaction in a crystal, mushy state played a significant role in controlling the formation and evolution of the Langqi pluton. The magmatism was predominantly sourced from mixing between the mantle-derived mafic magma and the crust-derived felsic magma during the subduction of the Paleo-Pacific Plate, resulting in the formation of the Langqi doleritic veins, granites, and MMEs.
... and poor CaO (average 0.75%~0.82%) [85], enriched in HFSEs (Zr and Hf) and Ce, poor in Ba, Sr, Eu, P, and Ti, and show a typical right-dipping seagull-type REE distribution pattern, which conforms to the characteristics of A-type granite. Overall, in the case of high differentiation (aluminous A-type granite), I-type, S-type, and A-type granites are difficult to distinguish [86]. ...
... Overall, in the case of high differentiation (aluminous A-type granite), I-type, S-type, and A-type granites are difficult to distinguish [86]. They often have the same mineralogical and geochemical characteristics [85]. Therefore, they still need to be distinguished. ...
The Sayashk tin (Sn) deposit is located within the southern part of the Eastern Junggar orogenic belt in Xinjiang Province and forms part of the Kalamaili alkaline granite belt. There are many Sn polymetallic deposits in the area. To constrain the age, genesis, and tectonic setting of the Sayashk tin deposit in the East Junggar region, we conducted a bulk-rock geochemical analysis of the granite porphyry (SR1) and medium- to fine-grained granite (SR2) hosts of the deposit, LA–ICP–MS zircon U–Pb dating and Lu–Hf isotopic analysis, as well as molybdenite Re-OS dating and combined our results with the metallogenic conditions and other geological characteristics of the deposit. The results show that the Sayashk Sn deposit is indeed spatially, temporally, and genetically closely related to the granite porphyry and medium-fine-grained granite. Both zircon U–Pb ages are 308.2 ± 1.5 Ma and 310.9 ± 1.5 Ma, respectively. The isochron age of molybdenite is 301.4 ± 6.7 Ma, which represents the crystallization age of the granite porphyry and medium-fine-grained granite. Therefore, all of them formed in the late Carboniferous epoch. The medium-fine-grained granites and granite porphyry are characteristically rich in Si and alkali, poor in Ca and Mg, rich in high field-strength elements (HFSE, e.g., Zr, Hf) and Ce, and deficient in Ba, Sr, Eu, P, and Ti. They are typical A-type granites, showing the characteristics of a mixed crustal mantle source. The εHf(t) values of the zircon from the granite porphyry (SR1) range from 10.27 to 16.17 (average 13.71), εHf(t) values of the zircon from the medium-fine-grained granites (SR2) are between 5.72 and 9.21 (average 7.08), and the single model ages (TDM1) and two-stage model ages (TDM2) of the granite porphyry (SR1) fall within the ranges of 319~535 Ma and 339~644 Ma. The single model ages (TDM1) and two-stage model ages (TDM2) of the medium-fine-grained granites (SR2) fall within the ranges of 346~479 Ma and 309~557 Ma. There is little difference between their two-stage model ages and zircon U–Pb ages, indicating that the Sayashk granite may be the product of partial melting of juvenile crustal. Combined with previous research results, the Sayashk Sn deposit formed in a post-collision extensional tectonic setting after the late Carboniferous in the Kalamaili area.
... The Mokeri pegmatite and syenogranite exhibit relatively lower Al 2 O 3 and weakly peraluminous affinity (Fig. 5c), distinguishing them from S-type granite (Chappell and White, 1974). Although garnet, an Almineral typically associated with S-type granite (Chappell, 1999;Wu et al., 2007), is present in the Mokeri pegmatite, it should be noted that some highly fractionated I-type granites like Jialuhe biotite monzogranite and Siguniangshan garnet leucogranite (Zhao et al., 2020a) can also contain garnet. Notably, both rocks exhibit low P 2 O 5 content (0.01 -0.02 for the pegmatite and 0.02 -0.06 for the syenogranite) and display an anticorrelation with SiO 2 between syenogranite to pegmatite (Fig. 7e), indicative of I-type granite tendencies due to apatite's low solubility in metaluminous and weakly peraluminous melt (Wolf and London, 1994). ...
The newly discovered pegmatite, along with associated syenogranite and gabbroic diorite in the Mokeri area of the East Kunlun Orogen (EKO), were comprehensively investigated utilizing geochronological, whole rock and mineral chemical analysis. Zircon U-Pb dating results indicate that the pegmatite and syenogranite were emplaced almost simultaneously at 236 ± 4 Ma and 236 ± 2 Ma, respectively, while the gabbroic diorite was emplaced at 240 ± 3 Ma. The Mokeri pegmatite and syenogranite are chemically classified as high-K calc-alkaline, weakly peraluminous I-type granites. They exhibit similar geochemical compositions with high SiO 2 (73.70-78.57 wt%), low MgO (0.04-0.11 wt%), Fe 2 O 3 T (0.23-2.00 wt%) and TiO 2 contents (0.02-0.20 wt%), as well as low Nb/Ta (3.81-12.05) and Zr/Hf ratios (12.64-31.04), indicating their common crustal magma source. Geochemical and mineralogical evidences suggest an evolutionary relationship between the Mokeri syenogranite and pegmatite, characterized by higher SiO 2 and Na 2 O contents, as well as elevated Rb/Sr and Rb/ Ba ratios, alongside lower Fe 2 O 3 T , MgO and TiO 2 concentrations in the pegmatite. The pegmatite exhibits a high differentiation index (DI) of 94 (mean value), which signifies a continuous sequence of magma evolution in conjunction with the moderately differentiated syenogranite (DI = 90). Plagioclase components in the pegmatite (An 7 to An 18) are more evolved than those in syenogranite (An 19 to An 31). Garnet, derived from two pegmatite dykes, belongs to almandine-spessartine series and exhibits a progressive enrichment in manganese with increasing distance from the parental body-syenogranite. The Mokeri gabbroic diorite exhibits compositional depletion of Nb, Ta, and Ti contents and enrichment of Sr, Ba, and Pb contents, resembling the observed patterns in the island arc basalts (IAB). The elevated ratios of Th/Yb, Ba/Zr and Rb/Y suggest it derivates from an enriched lithospheric mantle modified by subducted slab-fluid. The Mokeri pegmatite-syenogranite-gabbroic diorite association was formed during a transitional phase between syn-collision and post-collision extension (240-236 Ma), indicating the termination of collision between the East Kunlun Terrane and Bayan Har Terrane at 236 Ma. These magmatic events likely represent a tectonic-magmatic response to slab break off during the post-collision.
... The XL granites samples display ε Nd (t) values of − 2.9 − +3.0 and ( 87 Sr/ 86 Sr) i ratios of 0.695777-0.702456, however, two tested points (17GW084 and 17GW085) have extremely high 87 Rb/ 86 Sr ratios nearly close to 10 (7.84 and 9.43), which are not proper to discuss the petrogenesis (Wu et al., 2007). XL gabbros show relatively low ( 87 Sr/ 86 Sr) i ratios (0.702796-0.702839) ...
... The Dongcao muscovite granite has a high Ga/Al ratio (ranging from 4.01 to 4.29, with an average of 4.14) and is enriched with Rb, U, Ta, Pb, P, and Hf and depleted of Ba, Sr, Zr, Ti, REEs, and other elements, showing characteristics of A-type granites [32][33][34][35][36]. However, many scholars have found that the I-type and S-type granites with excessive differentiation also have a relatively high Ga/Al ratio (e.g., higher than 2.6) and are enriched with high field strength elements (HFSEs), which are similar to A-type granites [37][38][39]. The Dongcao muscovite granite has a high differentiation index (the sum of normative percentages of quartz, orthoclase, albite, nepeline, leucite, and kalsilite) [40] values (varying from 89.97 to 92.34, with an average of 90.95), Rb values (ranging from 1490 mg/kg to 1695 mg/kg, with an average of 1602 mg/kg), and Cs values (ranging from 210 mg/kg to 247 mg/kg, with an average of 229 mg/kg); low K/Rb ratios (ranging from 17.95 to 19.46, with an average of 18.59); low Nb/Ta ratios (ranging from 0.97 to 1.55, with an average of 1.27); and low Zr/Hf ratios (ranging from 10.71 to 12.86, with an average of 11.62), indicating that it has experienced high crystallization differentiation and is a highly differentiated granite [39,[41][42][43]. ...
Dongcao muscovite granite, as the product of the second stage of the magmatic intrusion of the Ganfang composite pluton, is closely related to the mineralization of Li–Nb–Ta rare metals in the Yifeng area. This paper aims to discuss the diagenetic age, evolutionary process, and relationship with the rare metal mineralization of Dongcao muscovite granite by using petrographic, cassiterite U–Pb dating and geochemical analyses. Petrographic analysis shows that the lithology of the Dongcao muscovite granite is medium– to fine–grained muscovite monzogranite. The cassiterite U–Pb dating results show that the diagenetic age of the Dongcao muscovite granite is 139.7 ± 6.7 Ma, which is Early Cretaceous. The geochemical analysis indicates that the rock is characterized by high Si, abundant aluminum alkalis, low Ca and Fe, and low Mg, which indicates that this granite is a strongly peraluminous rock. Moreover, the Dongcao muscovite granite is enriched with Rb, U, Ta, Pb, P, and Hf and depleted of Ba, Sr, Ti, and rare earth elements (REEs), with a tetrad effect of REEs. Based on this analysis, the Dongcao muscovite granite is a highly differentiated granite that formed in the tectonic transition from continental collisional to post–collisional settings related to the subduction of the Paleo–Pacific plate. A high degree of crystallization differentiation occurred at the early stage of magmatic evolution, resulting in the initial enrichment of Li–Nb–Ta–Sn. The melt–fluid interaction in the late stage is significant to the high enrichment of Li–Nb–Ta–Sn until the final mineralization.
... wt%; K 2 O = 5.06-6.09 wt%), showing affinity to A-type granites ( Fig. 5h; Collins et al., 1982;Wu et al., 2007). However, several models have been proposed for the petrogenesis of A-type felsic rocks, including (1) anatexis of the felsic continental crust (Clemens et al., 1986); (2) the direct fractionation of mantle-derived tholeiitic and alkaline magmas (Turner et al., 1992); and (3) a mixing effect of crust-derived felsic magma with mantle-derived mafic magma (Eyuboglu et al., 2011). ...
... These geochemical characteristics are similar to the A-type rhyolites. Especially, A-type rhyolite experienced extreme fractional crystallization is depleted in Zr (< 100 ppm) and Zr + Nb + Ce + Y contents (King et al., 2001;Wu, Li, et al., 2007), while the samples exhibit high Zr (312.95 ppm in average >250 ppm) and Zr + Nb + Ce + Y (451.39 ppm in average >350 ppm) contents, indicate that the samples are not highly fractionated A-type rhyolites. ...
As the largest petroliferous basin in northeast (NE) China, Songliao Basin preserves continuous Cretaceous sedimentary-volcanic records, providing an excellent opportunity to recover the palaeo-environment. The Yingcheng Formation in the Songliao Basin contains ample gas reservoirs that attract widespread attention. Whereas the timing, petrogenesis and geodynamic mechanism of the volcanic rocks in the Yingcheng Formation are still controversial, which largely constrain our understanding of the formation and evolution of the Songliao Basin. Here, we present an integrated investigation of zircon U–Pb ages and Hf isotope, as well as whole-rock elements and Sr-Nd isotopes data for a suite of rhyolites of Yingcheng Formation from the SK2 Borehole of NE China. Zircon U–Pb age dating results of nine samples have yielded a concentrated age of ~110 Ma. These Yingcheng rhyolites are characterized by high SiO2 (66–78 wt%) and alkali (Na2O +K2O =7.80–11.70 wt%) content, high 10000*Ga/Al (1.26–3.82, mostly >2.6) and FeOT/(FeOT +MgO) ratios (0.81–0.95), which show geochemical affinities with A-type rhyolites. They have relatively low Y/Nb (0.69–1.78, average 1.19) and Rb/Nb (1.58–6.52, average 4.34) ratios, suggesting that the Yingcheng volcanic samples belong to A1-type rhyolites which formed in an intraplate environment. These Yingcheng rhyolites show depleted Nd-Hf isotopic compositions (εNd(t) =2.43–4.87 and zircon εHf(t) =4.22–9.88) comparable with the Early Cretaceous A-type and I-type rhyolites in the Songliao Basin, suggesting that they were originated from a juvenile continental crust. They were most likely derived from the partial melting of anhydrous lower crust instead of differentiation of mantle-derived or mixing with alkaline basaltic magma, due to their low Mg# and absence of the coeval mafic rocks. Combined with the previous studies, A-type rhyolites in the Yingcheng Formation erupted lasting at least 10 Myr and were widely distributed in the rifts of the Songliao Basin. We propose that the Yingcheng rhyolites were most likely generated from the rollback of the Palaeo-Pacific Plate in the Early Cretaceous, thus highlighting the significance of the subduction of the Palaeo-Pacific Plate during the secular evolution of the Songliao Basin.
... The granites can be classified into M, I, A and S types according to their petrogenetic background and magma sources (e.g., Chappell and White, 1974;Wu et al., 2007Wu et al., , 2017. Generally, A-type granites are characterized by low H 2 O, alkali compositions and produced in the intraplate setting (e.g., Loiselle and Wones., 1979;Liu et al., 2022). ...
The Gangdese magmatic belt of southern Tibet is an ideal place to study Neo-Tethyan subduction, continental crustal growth and reworking. However, there are still controversies with regard to the evolution of the Neo-Tethys Ocean, the magma source and the detailed diagenetic processes of igneous rocks in the Gangdese belt. The Early Cretaceous magmatic exposures are sporadic in the Gangdese magmatic belt. Thus the finding of the new exposure is key to understanding the scenarios of the Neo-Tethys Ocean and geological background of southern Tibet during the Early Cretaceous. In this contribution, we undertook systematic geochronology, whole-rock geochemistry and zircon Lu-Hf isotopic studies on a newly identified granitic pluton in the middle Gangdese belt (Quesang area), southern Tibet. The results show that zircon U-Pb dating of three representative samples yielded a weighted age of 120 ± 1.4 Ma, 117.3 ± 2.5 Ma and 114.0 ± 1.3 Ma, respectively, which indicate the emplacement and crystallization age belonging to the Aptian stage of the Early Cretaceous in response to the northward subduction of the Neo-Tethyan oceanic lithosphere beneath the Lhasa terrane. In situ zircon Lu-Hf isotopic analyses indicate that ƐHf(t) values of the studied granitic pluton are predominantly positive ranging from 7.2 to 11.4, and one zircon shows negative ƐHf(t) value (−6.26), implying that a small amount of ancient continental crustal materials might have participated in the magma evolution. Geochemically, the granite samples are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), but depleted in heavy rare earth elements (HREE), indicating arc-type geochemical characteristics or subduction-related tectonic setting. In addition, combined with mineral assemblages, zircon Lu-Hf isotopic features and low molar Al2O3/(CaO+Na2O+K2O) (A/CNK) ratios of 0.91–1.10, the studied samples show a close affinity with I-type granites. Moreover, zircon oxygen fugacity results show that Ce4+/Ce3+ values range from 185 to 12, with a mean value of 78, indicating a low oxygen fugacity setting similar to the Chile ore-barren granitic plutons. In combination with published data, we argue that the Gangdese magmatic belt may have developed continuous magmatism (145–105 Ma), and the notion of magmatic lull might deserve more consideration during the Early Cretaceous. In this study, the Early Cretaceous granitic pluton might be the result of the northward subduction of Neo-Tethys oceanic lithosphere beneath the Lhasa terrane at a normal angle.
... The formation of I-type granites usually has three mechanisms: (1) the mixing of mantle-derived melts and crust-derived felsic melts (Wu et al. 2007;Gray and Kemp 2009;Han et al. 2021); (2) the partial melting of lower crust metaigneous rocks (Chappell and White 1992;Altherr et al. 2000;Zhou et al. 2020); (3) crystallization differentiation of mantle-derived mafic magma (Sisson et al. 2005;Nandedkar and Ulmer 2014;Zhang et al. 2015). Note that mixing of two geochemically contrasting end melts would result in obvious geochemical variation in the resultant melt. ...
The Qimantag in the East Kunlun Orogenic Belt has widespread Triassic magmatic rocks that have received scant attention, with an unresolved issue relating to its petrogenesis and geodynamics. In this paper, we used zircon U–Pb–Hf isotopes and whole-rock geochemistry to trace the petrogenesis and tectonic settings of the moyite and monzogranite from the Qimantag Alananshan, East Kunlun. The moyite and monzogranite are silicic (SiO2 ~ 69.9–76.41%), highly alkali (Na2O + K2O ~ 7.29 to 8.96 wt.%), with Mg# about 10.4–30.34, indicative of a high-K calc-alkaline rock series. The rare earth element patterns diagram is right-leaning, with a negative Eu anomaly (δEu = 0.31–0.68). They are enriched in Rb, K, and light rare earth elements but depleted in Nb, Ta, and Ti, with abundant amphibole, typical of I-type granites. U–Pb on zircon constrained the emplacement of the moyite at 223.9 ± 2.6 Ma and monzogranite at 226.9 ± 2.9 Ma. The εHf(t) values range from − 2.8 to + 0.1 except for one outlier value of − 7.0, corresponding two-stage model age of 1249–1437 Ma. Our combined geochemical and isotopic results indicate that the moyite and monzogranite were derived from partial melting of the lower thicken crust with the contribution from the older basement materials. These rocks formed in a post-collision setting that is transitional between compressive collision and extension orogeny.
... The Tianshui granitic rocks showed strong negative Eu anomalies in chondritenormalized REE patterns, indicating that plagioclase was an important residual phase. Numerous experimental studies have shown that plagioclase dominates residues at pressures of <1.0 GPa (depth < 30 km) [130][131][132]. Stevens et al. (1997) [133] conducted experiments of biotite dehydration melting of metapelites and metagreywackes at 0.5 GPa in a temperature range of 750 to 830 • C and produced peraluminous leucogranites in equilibrium with granulite-facies residual mineral assemblages. ...
The source and petrogenesis of peraluminous granitic rocks in orogenic belts can provide insights into the evolution, architecture, and composition of continental crust. Neoproterozoic peraluminous granitic rocks are sporadically exposed in the Tianshui area of the western margin of the North Qinling Terrane (NQT), China. However, the source, petrogenesis, and tectonic setting of these rocks still remain unclear, which limits our understanding of the Precambrian tectonic and crustal evolution of the Qinling Orogenic Belt (QOB). Here, we determined the whole-rock geochemical compositions and in situ zircon U–Pb ages, trace-element contents, and Hf–O isotopic compositions of a series of peraluminous granitic mylonites and granitic gneisses in the Tianshui area at the west end of North Qinling. Zircon U–Pb dating revealed that the protoliths of the studied granitic mylonites and granitic gneisses crystallized at 936–921 Ma. The granitic rocks displayed high A/CNK values (1.12–1.34) and were enriched in large-ion lithophile elements (e.g., Rb, Ba, Th, U, and K) and light rare earth elements, and they were depleted of high-field-strength elements (e.g., Nb, Ta, and Ti). These rocks showed variable zircon εHf(t) (−12.2 / 9.7) and δ18O (3.56‰ / 11.07‰) values, suggesting that they were derived from heterogeneous crustal sources comprising predominantly supracrustal sedimentary rocks and subordinate igneous rocks. In addition, the U–Pb–Hf isotopic compositions from the core domains of inherited zircons were similar to those of detrital zircons from the Qinling Group, suggesting that the Qinling Group was an important crustal source for the granitic rocks. The lithological and geochemical features of these granitic rocks indicate that they were generated by biotite dehydration melting of heterogeneous sources at lower crustal depths. Combining our results with those of previous studies, we suggest that the NQT underwent a tectonic transition from syn-collision to post-collision at 936–874 Ma in response to the assembly and breakup of the Rodinia supercontinent.
... Granites are usually divided into I, S, A, and M types according to their mineral composition, chemical composition, source rock characteristics, and tectonic environment [29][30][31][32][33][34][35][36][37]. ...
The East Kunlun Orogenic Belt is located in the western part of the Central Orogenic Belt of China, with a large number of Triassic igneous rocks parallel to the Paleo-Tethys ophiolite belt, which provides a large amount of geological information for the tectonic evolution of the Paleo-Tethys Ocean. The granitoids studied in this paper are located in the Ela Mountain area in the eastern part of the East Kunlun Orogenic Belt. Zircon U-Pb dating results show that these different types of granitoids were crystallized in the Triassic. The 247.5 Ma porphyritic granites from Zairiri (ZRR) displayed calc-alkaline I-type granite affinities, with the zircon εHf(t) values being mainly positive (−0.5 to + 3.8, TDM2 of 1309–1031 Ma), indicating that they are derived from the partial melting of the juvenile crust and mixed with ancient crustal components. The 236.8 Ma Henqionggou (HQG) granodiorites and 237.5 Ma Daheba (DHB) granodiorites are high-K calc-alkaline I-type granite, and both have mafic microgranular enclaves (MMEs), showing higher and more varied Mg# (39.73–62.73), combined with their negative Hf isotopes (εHf(t) = −2.6 to −1.6, TDM2 = 1430–1369 Ma), suggesting that their primary magmas were the products of partial melting of the Mesoproterozoic lower crust that mixed with mantle-derived rocks. The 236.4 Ma DHB porphyritic diorites showed characteristics of high-K calc-alkaline I-type granitoids, with moderate SiO2 contents, medium Mg# values (40.41–40.65), with the Hf isotopes (εHf(t) = −2.9 to −0.5; TDM2 = 1451–1298 Ma) indistinguishably relative to contemporaneous host granodiorites and MMEs. The petrographic and geochemical characteristics indicate that the porphyritic diorites are the product of well-mixed magma derived from the Mesoproterozoic lower crust and lithospheric mantle. Based on the results of this paper and previous data, the chronology framework of Late Permian–Triassic magmatic rocks in the eastern part of the East Kunlun Orogenic Belt was constructed, and the magmatic activities in this area were divided into three peak periods, with each peak representing an extensional event in a particular tectonic setting, for example, P1 (slab roll-back in subduction period; 254–246 Ma), P2 (slab break-off in transition period of subduction and collision; 244–232 Ma), P3 (delamination after collision; 230–218 Ma).
... The samples do not contain Al-rich minerals (e.g., garnet and corundum) that are common in S-type granites but contain sphene and apatite that are common in I-type granites. The aluminum content index (A/CNK < 1.1) indicates that they are I-type granites (Wu et al., 2007b). Combined with the data of other stocks near the study area, the Zr-10000Ga/Al diagram shows the sample plot in the I-A-type transition area ( Figure 6A). ...
In the western Mangling orefield, the molybdenum (Mo) polymetallic deposits are closely related to the ore-bearing porphyry stocks (individual outcrop size: <1 km ² ). In this study, we have discovered several granitic stocks at Yaozhuang. Systematic petrologic, zircon U-Pb-Hf isotope and whole-rock geochemical studies show that both the granitic stocks of porphyritic granite (157 ± 2 Ma) and the intruding monzogranite dike (153 ± 1 Ma) were emplaced in the Late Jurassic. These granitic stocks are characterized by high SiO 2 (66.83–75.63 wt%), high K 2 O (4.15–5.05 wt%), high Al 2 O 3 (12.90–16.93 wt%), and low MgO (0.06–0.73 wt%) and are metaluminous to weakly peraluminous, being highly fractionated I-A-type transition granites. The content of the total rare Earth element (ΣREE) of the porphyritic granite (139.6–161.7 ppm) is lower than that of the monzogranite (151.4–253.6 ppm). The porphyritic granite has weakly negative Eu anomalies (Eu/Eu* = 0.77–0.93), whereas the monzogranite has weakly positive Eu anomalies (Eu/Eu* = 0.97–1.21) and are more enriched in light rare Earth elements. Both of them are enriched in large ion lithophile elements (LILEs, e.g., K, Rb, and Ba) but depleted in high-field-strength elements (HFSEs, e.g., Nb, Ta, Ti, Zr, and Hf). The zircon ε Hf (t) values of all the samples range from −16.1 to −6.9, and the two-stage model ages (t DM2 ) are 1.78–2.16 Ga. The magma may have originated from partial melting of the lower crust (more than 40 km in depth) caused by mantle-derived magma underwelling. The plutons and stocks were emplaced into the intersection of the early EW-trending faults and the late (Yanshanian) NE-trending faults. The fertile magma with high water content (H 2 O > 4%) and high oxygen fugacity (Delta FMQ > 1.5) indicates that the Yaozhuang area has significant potential for porphyry Mo polymetallic ore discovery.
The geodynamic mechanisms that controlled magmatic activity in South China during the late Mesozoic have been a cutting-edge focus of recent research. Southeastern Guangxi, which is located at the juncture between the Pacific and Neo-Tethyan tectonic domains, is a transitional zone characterized by the occurrence of widespread Jurassic–Cretaceous magmatic rocks. Investigation of this region can shed light on the late Mesozoic tectonic setting of South China. We conducted U–Pb geochronological and geochemical analyses of the Liuwang granodiorite and quartz porphyry, which are exposed in southeastern Guangxi. Zircon U–Pb dating yielded an age of 161.8 ± 1.2 Ma for the granodiorite and 97.89 ± 0.68 Ma for the quartz porphyry, indicating that they formed during the Late Jurassic and Late Cretaceous, respectively. The Liuwang granodiorites are weakly peraluminous high-K calc-alkaline rocks characterized by enrichment in large ion lithophile elements (including Rb) and high field strength elements (including Th, U, Pb, and Ta) and depletion in Ba, Nb, and Sr. The granodiorites also exhibit weak rare earth element (REE) fractionation and slightly negative Eu anomalies. Conversely, the Liuwang quartz porphyry is weakly peraluminous and belongs to the potassic syenite series, transitioning into the high-K calc-alkaline series. It is characterized by enrichment in Rb and high field strength elements (including Th, U, Pb, and Ta), with depletion in Ba, Nb, Sr, and Zr. It does not exhibit REE fractionation but does yield prominent negative Eu anomalies. The granodiorite and quartz porphyry yield εHf(t) values of −23.26 to −2.48 and −4.4 to +0.8, respectively, with tDM2 ages of 2642–1270 and 1411–1081 Ma, respectively. These data suggest that the Liuwang granodiorite formed under a background of Late Jurassic lithospheric extension–thinning and was derived from partial melting of Palaeoproterozoic–Mesoproterozoic metasedimentary sandstones with a minor contribution from mantle-derived melts. In contrast, the Liuwang quartz porphyry was derived from partial melting of Mesoproterozoic pelitic rocks and formed in a Late Cretaceous tectonic setting linked to the northward subduction of the Neo-Tethys Ocean beneath South China.
The research on highly fractionated granite has significant implications for both the evolution and compositional maturation of the continental crust and metallogenic exploration. As a means of further understanding crustal evolution and promoting ore exploration in the Bangong–Nujiang metallogenic belt (BNMB), we present the petrography, zircon LA–ICP–MS U–Pb age, and Hf isotopic data, along with the whole-rock geochemical and Sr–Nd isotopic composition on Kese highly fractionated granite in the Baingoin area within the BNMB, central Tibet. The results show that Kese granite possesses a zircon U–Pb age of 127.8 ± 1.7 Ma and a relative enrichment in zircon Hf isotopic composition (−12.8~+0.3) with a two-stage Hf model age of 1.2~2.0 Ga. This granite belongs to the high-K calc-alkaline series, characterized by a strongly peraluminous feature, and is enriched in large-ion lithophile elements (LILEs) and Nd isotopes (−7.86~−7.74). The granite was likely to have been derived from the mixed melts derived from 40%~45% juvenile basaltic lower crust, 15%~20% ancient lower, and 40% middle–upper, following intense fractional crystallization processes involving amphibole, biotite, plagioclase, and some accessory minerals during the magma’s evolution. We infer that Kese highly fractionated granite can be formed from the continental collision of the Lhasa–Qiangtang terranes initiated before 128 Ma. The reworking of pre-existing juvenile and ancient crustal materials drove the composition of the northern Lhasa terrane to that of a mature continental crust. Moreover, the distinctive geochemical features have shown that the high degree of differentiation led to intense magmatic–hydrothermal interaction during the formation of Kese granite. A comparison of the geochemical characteristics of mineralized and barren granites suggests that the highly fractionated granites in Baingoin from the BNMB have a high economic potential and are suitable for preliminary exploration of Sn–W-(U) deposits.
To ascertain the Early-to-Middle Jurassic tectonic setting in the central Great Xing’an Range, this study investigated the Early and Middle Jurassic granitoids exposed in the Chaihe area in the central Great Xing’an Range based on isotopic chronology and petrogeochemistry. The results of this study show that the Early and Middle Jurassic granitoids have emplacement ages of 179–172 Ma. Moreover, the Early and Middle Jurassic granitoids are high-K calc-alkaline unfractionated I-type granitoids and high-K calc-alkaline fractionated I-type granitoids, respectively. The magma sources of the Early and Middle Jurassic granitoids both originated from the partial melting of newly accreted lower crustal basaltic rocks. Meanwhile, the Middle Jurassic magma sources were mixed with mantle-derived materials or ocean-floor sediments formed by the dehydration and metasomatism of subducted slabs. The Early and Middle Jurassic granitoids in the study area were formed in the subduction environment of the oceanic crust, in which the Mongol-Okhotsk oceanic plate was subducted southward beneath the Eerguna and Xing’an blocks. Moreover, the Siberian plate began to collide and converge with northeast China during the Middle Jurassic.
The Woduhe area of Heilongjiang Province is located in the eastern side of the Greater Xing'an Range. This area has experienced the evolution of the Palaeo‐Asian Ocean, the Palaeo‐Pacific, and the Mongolia–Okhotsk Ocean, and it can be used to discuss the transformation process of these palaeo‐oceanic plates to continental plates. However, controversies exist over its time period and the process of transformation. The 1:250000 Woduhe Geological Map shows that Early Cretaceous granitic batholith developed near the Dajinshan area. However, the LA‐ICP‐MS zircon U–Pb dating presented in this paper shows that the trondhjemites from the southern part of the Dajinshan area were formed at 156.9 ± 2.2 Ma, syenogranites were formed at 131.7 ± 1.6 Ma, and diorite porphyrite dikes were formed at 126.0 ± 3.1 Ma. Therefore, Early Cretaceous batholiths should be reclassified as different, separate units. The Late Jurassic intrusive rocks belong to TTGG assemblages (tonalite–trondhjemite–granodiorite‐granite). Trace element spider diagrams show that the rocks are enriched in K, Pb, Zr, and Hf; depleted in Nb, Ta, and Eu; with higher Sr; and lower Y content. The rare earth element pattern is right‐sloping (La/Yb > 12). The Early Cretaceous intrusive rocks belong to G assemblages (granodiorite). Combined with the tectonic settings, we determined that the Late Jurassic and the Early Cretaceous granites were formed in a magmatic arc setting related to subduction. Late Jurassic intrusive magma mainly originated from the partial melting of the thickened lower crust under the Xing'an arc, and was likely contaminated by mantle material. Early Cretaceous intrusive magma originated from the partial melting of the Xing'an sub‐arc crust. Based on the distribution of the Late Jurassic TTG (N and NE) and G (S and SW) assemblages, intrusive records of the southward and southwestward subduction of the Mongolia–Okhotsk Ocean can be observed. In addition, the grade of maturity of the Xing'an arc is increasing during Late Jurassic to Early Cretaceous.
The North Qinling Orogenic Belt (NQOB) is a composite orogenic belt in central China. It started evolving during the Meso‐Neoproterozoic period and underwent multiple stages of plate subduction and collision before entering intra‐continental orogeny in the Late Triassic. The Meso‐Cenozoic intra‐continental orogeny and tectonic evolution had different responses in various terranes of the belt, with the tectonic evolution of the middle part of the belt being particularly controversial. The granites distributed in the Dayu and Kuyu areas in the middle part of the NQOB can provide an important window for revealing the geodynamic mechanisms of the NQOB. The main lithology of Dayu and Kuyu granites is biotite monzogranite, and the zircon U–Pb dating yielded diagenetic ages of 151.3 ± 3.4 Ma and 147.7 ± 1.5 Ma, respectively. The dates suggest that the biotite monzogranite were formed at the end of the Late Jurassic. The whole‐rock geochemistry analysis shows that the granites in the study areas are characterised by slightly high SiO 2 (64.50–68.88 wt%) and high Al 2 O 3 (15.12–16.24 wt%) and Na 2 O (3.55–3.80 wt%) contents. They are also enriched in light rare earth elements, large ion lithophile elements (e.g. Ba, K, La, Pb, Sr), and depleted in high field strength elements (HFSEs) (e.g. Ta, Nb, P, Ti). Additionally, the granites demonstrated weakly negative‐slightly positive Eu anomalies (δEu = 0.91–1.19). Zircon Lu–Hf isotopic analysis showed ɛ Hf (t) = −6.1–−3.8, and the two‐stage model age is T 2DM(crust) = 1.5–1.6 Ga. The granites in the study areas are analyzed as weak peraluminous high‐K calc‐alkaline I‐type granites. They formed by partial melting of the thickened ancient lower crust, accompanied by the addition of minor mantle‐derived materials. During magma ascent, they experienced fractional crystallisation, with residual garnet and amphibole for a certain proportion in the magma source region. Comprehensive the geotectonic data suggest that the end of the Late Jurassic granite magmatism in the Dayu and Kuyu areas represents a compression‐extension transition regime. It may have been a response to multiple tectonic mechanisms, such as the late Mesozoic intra‐continental southward subduction of the North China Craton and the remote effect of the Paleo‐Pacific Plate subduction.
The development of trench-arc-backarc (TABA) systems is uniquely associated with modern-style plate tectonics on Earth. The Qilian orogenic belt in NW China records the evolution history of the Proto-Tethys Ocean at the transition time from the Proterozoic to Phanerozoic. This paper presents systematic studies of petrography, U-Pb chronology and geochemistry on various rocks from a MOR-type ophiolite belt, active continental margin and back-arc basin in the Qilian orogenic belt to address the development of a modern-style TABA system. Arc magmas include felsic intrusions with ages of 530–477 Ma and felsic-mafic arc volcanic rocks with ages of 506–439 Ma, showing distinctive features of typical magmatic rocks formed at an Andean-type continental margin. The back-arc basin is recorded by a 490–448 Ma SSZ-type ophiolite with boninite, and Silurian turbidite flysch formation. We establish a three-stage tectonic history from the initiation of subduction to the formation of a mature Japan-Sea-type back-arc basin at the active continental margin in the Early Paleozoic era. (1) Northward subduction of Proto-Tethys Ocean initiated and the Andean-type continental arc developed at ~530–500 Ma with continual crustal thickening; (2) a tectonic transition occurred from an Andean-type active continental margin to a West Pacific-type active continental margin at ~500–490 Ma with rapid thinning of crust to ~35 km, and (3) mature ocean basins and back-arc-basin (BAB) ophiolites were formed in the back-arc extensional environment at ~ 490–450 Ma.
The Fanchang volcanic basin (FVB) is located in the Middle and Lower Yangtze Metallogenic Belt (MLYMB) between the ore districts of Ningwu and Tongling. The existing ore deposits in the FVB are relatively small in scale and related to late Mesozoic A-type granites. In this paper, the crystallization age, major and trace element composition, and Sr-Nd and Hf isotope compositions of the A-type granites are summarized from the literature; in addition, the magnetite composition, H and O isotopes of fluid inclusions, and sulfur isotope composition of metal sulfides in some typical ore deposits in the FVB are also summarized to give insights into the petrogenesis and mineralization of the A-type granites intruding into the FVB. The results show that: (1) Orthopyroxene, plagioclase, K-feldspar, and biotite are the main fractionating minerals controlling the evolution of the magmas of A-type granites in the FVB and other areas in the MLYMB. (2) The whole-rock Sr-Nd and zircon Hf isotopic characteristics show that the source of A-type granite magma is complex and includes the enriched mantle, lower crust, and upper crust, probably with stronger participation of Archaean–Paleoproterozoic crustal materials in the FVB granites than in other regions of the MLYMB. (3) The ores in the FVB are dominated by skarn and hydrothermal deposits. H and O isotopes of fluid inclusions indicate that ore-forming fluids have been derived from mixtures of magmatic hydrothermal fluid, meteoric waters, and deep brine related to gypsum layers. S isotopes of metal sulfides indicate that the sulfur may be a mixture of magmatically derived sulfur and sulfur originating from the Triassic gypsum-bearing layers. The deposit and ore characteristics of the main deposits in the FVB are also illustrated, and the evaluation of metal resources indicates that the skarn and hydrothermal iron–zinc ores in the FVB also have potential as sources of Cd, Ga, and Se. In addition, in terms of the oxygen fugacity, rock type, and geochemical characteristics of magmatic rocks, the metallogenic characteristics and potential of the A-type granites in the FVB are evaluated. It is considered that in addition to the dominant constituents of iron and zinc and the minor constituents listed above, the FVB could have the potential for providing copper, gold, molybdenum, uranium, and other metals as well.
The North Qaidam (NQ) has numerous Paleozoic–Mesozoic magmatism and metamorphism that contain significant traces of the tectonic evolution of this orogen; however, the precise timing and mechanism of subduction, and the collisional and post-collision history of the NQ and neighboring orogens during the Paleozoic–Mesozoic are not well understood. We conducted an integrated investigation involving detailed field observations, petrology, zircon U–Pb geochronology, Lu–Hf isotopes, and whole-rock geochemical data from volcanic rocks from the Maoniushan Formation and intermediate-acid intrusive rocks in the Dadakenwulashan, Habuqige, and Kongquegou areas of NW China to determine their ages and petrogenesis. We also employed a compilation of recently published high-quality data in our assessment, and provided a better understanding of the regional tectonic evolution. Devonian volcanic rock and Triassic intrusive rock samples yielded zircon U–Pb ages of 407 Ma, 378 Ma, 377 Ma, 241 Ma, 232 Ma, and 230 Ma. Using the existing regional data and spatiotemporal distribution of magmatism, possible models to explain the tectonic evolution of the NQ from the Paleozoic to the Mesozoic, which records two Wilson cycles within a branch of the Tethyan orogenic system, were proposed. (1) The transition from compressional to extensional tectonic regimes may have occurred at ∼410 Ma in the NQ. (2) Twice rapid crust uplift and erosion formation of the Maoniushan Formation molasse with a volcanic interlayer have been recognized, which respond to the tectonic regime of slab break-off (410 Ma-390 Ma) and lithosphere delamination (380 Ma-360 Ma), respectively. (3) The tectonic evolution of the NQ was a remote response to subduction of the Paleo-Tethys Ocean during the Mesozoic.
The East Kunlun Orogen (EKO) is located in the western part of China and has witnessed the Late Permian–Triassic tectonic evolution involving the A'nyemaqen Ocean (a part of the Palaeo‐Tethys Ocean). In this study, geochronology, geochemistry, and Hf isotope analysis of Late Permian–Triassic granitoids were carried out. Zircon U–Pb dating of Dachagou granodiorite and monzogranite in the western part of the EKO (W–EKO) is 233.4 ± 1.1 and 225.8 ± 1.2 Ma, respectively. The Dachagou granodiorite and monzogranite are characterized by low values of 10, 000 Ga/Al ratios (1.75–2.06, 1.42–1.60) similar to those of I‐type granite. The εHf (t) (−2.4 to +0.1, −3.0 to +0.2) indicate that the magma forming these granites was derived from a mafic Mesoproterozoic lower crust. Zircon U–Pb age of Lalingzaohuo quartz diorite is 244.5 ± 2.6 Ma, and its εHf(t) values (−0.7 to +2.5) indicate the origin of crust–mantle mixing. Zircon U–Pb ages for Nagengkangqieer granodiorite and monzogranite in the eastern part of the EKO (E–EKO) are 250.1 ± 2.8 and 238.9 ± 2.2 Ma, respectively. These rocks are also characterized by low 10, 000 Ga/Al ratios (2.37–2.66 and 2.49–2.63) again similar to I‐type granite. The εHf(t) (−3.9 to 0.4, −3.3 to 1.1) indicate that the magma was derived from the crustal materials. As contemporaneous rocks, the Dachagou and Nagengkangqieer granodiorites and the Lalingzaohuo quartz diorites are formed in a subduction environment. However, the Dachagou and Nagengkangqieer monzogranites are formed in a collision environment. Based on this result, the closure time of the W–EKO and E–EKO of the A'nyemaqen Ocean had an age gap of ~15 Ma, hence the closure mode of the A'nyemaqen Ocean should be a “scissor‐type” closure. The W–EKO and E–EKO show different closure timings by 15 Ma. Geochronological and geochemical constraints reveal scissors‐type Ocean closure.
Late Triassic granites and related magmatic-hydrothermal Mo-W mineralization are common in the South Qinling orogen of central China, for which the Sihaiping granite pluton is a typical example. However, the petrogenesis of the Sihaiping granite remains enigmatic. Unraveling its petrogenesis can aid in our understanding of the Mo-W metallogeny in the South Qinling orogen. In this study, we present in situ zircon U-Pb ages, whole-rock geochemistry, and Sr-Nd-Hf isotopes of the Sihaiping granite. Zircon U-Pb dating by laser ablation inductively coupled plasma mass spectroscopy yielded Late Triassic ages of 206.6 ± 0.94, 206.5 ± 0.96, and 205.9 ± 0.75 Ma. The granite displayed high SiO2, K2O, and Rb contents, but low MgO, P2O5, TiO2, Sr, Ba, and Eu contents, indicating highly fractionated I-type granite affinity. The magma was interpreted to have been derived from Proterozoic crystalline basement in the South Qinling given its relatively low (⁸⁷Sr/⁸⁶Sr)i (0.704006 to 0.705039), negative εNd(t) (–4.9 to –3.5), and zircon εHf(t) (–6.2 to +1.6) values, with Nd model ages ranging from 1.39 to 1.27 Ga and two-stage Hf model ages ranging from 1.36 to 1.04 Ga. The strong fractional crystallization and suitable oxygen fugacity (ΔFMQ values mainly concentrated between +2 to +8) in the magmatic system played a major role in the Mo-W mineralization, indicating that the pluton has significant Mo(-W) metallogenic potential. Overall, the findings suggest that the Triassic Sihaiping granite and its related Mo-W mineralization likely formed in the transitional stage from syn- to post-collision between the South China Block and South Qinling along the Mianlue suture.
Triassic igneous rocks are widely exposed in the central Qiangtang terrane, Tibet. However, Triassic tectonic–thermal evolution in the central Qiangtang terrane and the evolution of the Paleo-Tethyan Ocean are still unclear. In this study, we report zircon U–Pb ages, and whole-rock major- and trace- element compositions, and Sr–Nd isotope data for the newly discovered Late Triassic Tumen volcanic rocks. The volcanic rocks consist of interlayered dacite and rhyolite (SiO2 = 63.8–69.7 wt%) with a thickness of approximately 150 m. New zircon U–Pb dating reveals the eruption of this volcanic suite at 211 Ma. Geochemical data show that the volcanic suite has a wide range of K2O (1.36–5.46) values and high-molar Al2O3/(CaO + K2O + Na2O) values (0.97–1.36). All samples clearly show the enrichment of light rare earth element, with (La/Yb)N = 6.79–11.03 and (La/Sm)N = 3.23–4.44, and distinct negative Eu anomalies (Eu/Eu* = 0.44–0.70). All samples exhibit the depletion of high field strength elements (e.g., Ti, Nb, and Ta), and enrichment in large-ion lithophile elements (e.g., Rb and K). The dacite–rhyolite suite has a narrow range of (⁸⁷Sr/⁸⁶Sr)i ratios (0.7102–0.7132) and εNd(t) values (−10.2 to −10.0), with Nd isotope model ages of 1.76–1.82 Ga. Zircon saturation temperatures (860℃–921℃) and geochemical characteristics show that the samples are A-type granitoids and were likely generated by partial melting of the Qiangtang continental crust. The rise of the asthenospheric mantle caused by the slab break-off provides a large amount of heat and materials for the igneous rocks in the Shuanghu–Longmucuo suture zone during the Late Triassic.
The Laurani Au–Ag–Cu deposit is a typical high-sulfidation (HS) epithermal deposit genetically related to volcanic rocks in the Bolivian Altiplano polymetallic belt (BAPB), yet few detailed studies have been carried out on these volcanic rocks. Detailed petrographic observations show that the volcanic rocks comprise monzogranite porphyries, dacites, dacitic porphyries and tuffs. Zircon U–Pb dating show that they formed at 7.44 ± 0.15 Ma ∼7.65 ± 0.09 Ma in the late Miocene during the rapid uplift of the Altiplano. Geochemical features indicate the Laurani volcanic rocks belong to high-K calc-alkaline to shoshonite series, enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) and depleted in high field strength elements (HFSE) and heavy rare earth elements (HREE), with no Eu anomalies (Eu/Eu* = 0.80–1.11, mean = 0.95). We suggest that the magma sources of Laurani volcanic rocks are primarily generated from the partial melting of the thickened lower crust, for: (1) No Eu anomalies indicate that plagioclase is absent in the residue, and the extensively depleted HREE and positively correlated high (Gd/Yb)N, (Dy/Yb)N, and (La/Yb)N ratios indicate that garnet is a dominant residual mineral in their source, which are consistent with the thickened lower crust; (2) ¹⁷⁶Hf/¹⁷⁷Hf ratios (0.282409–0.282544) and negative εHf(t) values (−13.15 to −8.37) confirm that the magmas are mainly evolved from the partial melting of the old lower crust; (3) Inherited zircons ages of 1.87–1.62 Ga are consistent with the basement ages of the Arequipa Massif between 1.9 Ga and 1.8 Ga, indicating that the magma sources are mainly generated from the partial melting of the Paleoproterozoic crust. These observations coincide with the thickened (60–80 km) felsic to intermediate lower crust beneath the Altiplano. Combined with the geodynamic process of rapid uplift of the Altiplano, we propose that the removal of the lower crust and/or mantle lithosphere and/or its interaction with the asthenosphere provide heat to contribute to the partial melting of the thickened lower crust to generate parental magma, and parental magma emplace along the strike-slip Eucaliptus fault to form the Laurani volcanic rocks. Furthermore, numerous inherited zircon ages with a weighted mean of 1189 ± 30 Ma reflect the intense Grenvillian metamorphism events in the Mesoproterozoic and the two ages 259.9 ± 5.11 and 231.7 ± 2.2 Ma indicate the weak Permian magmatic events in northern Bolivia. From this study, we infer that partial melting of the thickened lower crust during the rapid uplift of the orogenic belt may be an important magma formation mechanism.
The Alubaogeshan intrusion, consisting of porphyritic monzogranite in the central part and syenogranite porphyry in the edge, is genetically related to the Maodeng–Xiaogushan Sn–polymetallic deposit. This paper aims to investigate the genetic link between these two granite phases and the relationship between magma evolution and Sn mineralization. LA-ICP-MS zircon U–Pb dating of the porphyritic monzogranite and syenogranite porphyry yielded ages of 134 ± 1.0 Ma and 133 ± 0.5 Ma, respectively. The Alubaogeshan intrusion has high SiO2 and total alkali (Na2O + K2O) contents, with low Al2O3, Fe2O3, FeO, and MgO contents, belonging to high-K calc-alkaline metaluminous–weakly peraluminous granites. Chondrite-normalized rare earth element (REE) patterns are characterized by light REE enrichment and heavy REE depletion, with extremely negative Eu anomalies. In addition, the intrusion was also enriched in Rb, Th, U, K, Zr, Hf, and light REE and strongly depleted in Ba, Sr, P, and Ti. Mineralogical and geochemical characteristics, together with high zircon saturation temperature, indicate that the Alubaogeshan intrusion belongs to transitional I–A2-type granites. Compared with the porphyritic monzogranite, the syenogranite porphyry reveals higher SiO2 content and lower TiO2, Al2O3, Fe2O3, FeO, CaO, MgO, and P2O5 contents, with more obvious negative anomalies of Eu, Ba, Sr, P, and Ti. The temporal and spatial relationship and geochemical characteristics of the two rocks indicate that the syenogranite porphyry evolved from the porphyritic monzogranite. The modeling results show that the fractional crystallization of amphibole, plagioclase, K-feldspar, biotite, apatite, monazite, and allanite occurred during the magma evolution. The granitic magma differentiation plays a key role in the formation of the Maodeng–Xiaogushan Sn–polymetallic deposit. The εHf(t) values of zircon range from 2.58 to 7.08, and the two-stage Hf model ages range from 619 Ma to 850 Ma, implying that the Alubaogeshan intrusion was predominantly derived from partial melting of a Neoproterozoic juvenile crust. The TE1,3 values of the ore-related granites from W–(Sn) deposits are generally higher than 1.1, suggesting that fluid–melt interaction is crucial for W mineralization. The degree of magmatic evolution is one of the controlling factors for the size of Sn–polymetallic deposits in the southern Great Xing′an Range.
Granites from the eastern Tibetan Plateau record information about the Tethys evolution. This paper reports a systematic study on the N‐S‐trending Late Cretaceous granite belt in the Yidun Terrane. The studied rocks are A‐type granites and I‐type granites with adakitic affinity. From north to south, the studied rocks have intermediate ages and whole‐rock geochemistry that shows a transitional composition. The granites from the northern Yidun Terrane were emplaced at 106–90 Ma. They are characterized by high silica (SiO 2 > 70 wt.%) contents, low Mg # (10–38), Eu/Eu*(0.09–0.70), Sr/Y (1–12), and La/Yb (5–21) values and positive to negative ε Hf( t ) (−7.6 to +2.5). In contrasts, granites from the southern Yidun Terrane were emplaced at 87–75 Ma and are intermediate to felsic with SiO 2 < 70 wt.%, and higher Mg # (34–69), Eu/Eu* (0.64–1.01), Sr/Y (16–71), La/Yb (22–41) values, and negative ε Hf( t ) values (−9.1 to −1.3). These features together with the Sr–Nd isotopic compositions show that granites from the northern Yidun Terrane were derived from partial melting of ancient crustal from the Yidun Terrane (Xiuwacu granite), with contributions of mantle‐derived components (Queershan and Genie batholiths). Granites from the southern Yidun Terrane likely originated from ancient thickened‐crustal source. The increase of Sr/Y and (La/Yb) N ratios from north to south suggests a crustal thickening, which influenced the magmatic process, resulting in the transition of geochemical composition of the studied granites. An eastward migrating arc post‐collisional model is proposed for the Lhasa‐Qiangtang collision when compared to the eastward younging Late Cretaceous granites in the Qiangtang Terrane. The southward migrating granitic magmatism in the Yidun Terrane was triggered by the eastward migrating rollback of the flat‐subducted Bangong–Nujiang oceanic slab beneath the Qiangtang Terrane.
This study provides relevant data on the granodiorite in southern Zhuguangshan, South China Block (SCB), including mineralogical information, zircon U–Pb ages, zircon Hf isotope data, whole‐rock geochemical data, and Sr–Nd–Pb isotopic data. These data indicate that the granodiorite in southern Zhuguangshan crystallized at approximately 448.7 Ma. The hornblende and micas of the granodiorites are magnesio‐hornblende and Mg‐rich biotite, respectively. The granodiorite samples are characterized by relatively high SiO2 contents of 61.72–66.19 wt.%, K2O contents of 3.77–5.18 wt.%, Na2O contents of 2.42–3.16 wt.%, low FeOT contents of 3.93–5.79 wt.%, and relatively high MgO contents of 1.78–2.55 wt.%, with Mg# values (molar Mg/[Mg + Fe] × 100) of 43–50. The contents of rare earth elements (REEs) in the granodiorite specimens are 166–283 ppm, and the REEs show both a chondrite‐normalized REE pattern that inclines rightward and slightly negative Eu anomalies (0.62–0.90). The Fuxi granodiorite is rich in large‐ion lithophile elements (LILEs; Rb, Th, U, and K) but is depleted in high‐field‐strength elements (HFSEs; Nb, Ta, and Ti). All the samples have similar Sr–Nd–Pb and zircon Hf isotopic compositions and have a (87Sr/86Sr)i ratio of 0.707319–0.710888, a (143Nd/144Nd)i ratio of 0.511705–0.511760, a (206Pb/204Pb)t ratio of 17.918–18.459, a (207Pb/204Pb)t ratio of 15.711–15.813, and a (208Pb/204Pb)t ratio of 37.953–38.563. The εNd(t) and εHf(t) values range from −7.12 to −5.88 and from −9.81 to −4.45, respectively, with corresponding two‐stage Nd model ages of 1.66–1.76 Ga and two‐stage Hf model ages of 1.55–1.78 Ga. These findings indicate that the granodiorite is magnesian andesite and that the magmas are derived from a mixture of crustal and mantle materials. Based on these findings in combination with previous research results, it can be concluded that the mantle beneath the Cathaysia Block was moderately depleted during the Early Palaeozoic and that Early Palaeozoic magmatism may be related to Early Palaeozoic oceanic crust subduction. Sketch map showing the known Early Palaeozoic granites, mafic rocks, and metamorphic rocks of the South China Block.
The Lanping‐Simao Basin is located on the southeastern Tibetan Plateau, China, and contains massive evaporites. The origin of evaporites in the basin have been debated due to the strong transformation by tectonic movement. In this study, 40 halite samples fromboreholeMK‐3 in Mengyejing area of the basin were collected, and were analyzed by XRD, Cl‐Sr isotopes and chemical compositions to trace the origin of evaporites in the basin. The Br∗103/Cl ratios of halite samples are between 0 and 0.55, most of which are synchronized with the law of seawater evaporation and in the stage of halite precipitation from seawater, indicating the evaporites are mainly marine origin. The 87Sr/86Sr ratios range from 0.707489 to 0.711279. After correction, the 87Sr/86Sr145Ma ratios range from 0.704721 to 0.707611, equivalent with the 87Sr/86Sr ratios of seawater in the 145 Ma, indicating marine origin. The decay of 87Rb in the evaporite during deposition, change of the depositional environment and the unsealed environment in the later period resulted in the present87Sr/86Sr ratios of some samples are high. The δ37Cl value compositions range from −0.38‰ to 0.83‰, which is consistent with the δ37Cl value composition of the world marine halite (−0.6‰ to 0.4‰), further confirming that seawater is the main origin. In addition, the high δ37Cl value of some samples at the boundary of the upper and lower evaporite layer may be related to the influence of δ37Cl‐rich brine and the incomplete dissolution of the halite.
Systematic geochronology and geochemistry were performed on the volcanic rocks in the Ganhe area, northern Great Xing’an Range, northeastern China. Zircon U–Pb data indicate that there are three periods of volcanism in the Ganhe area, namely the Manketouebo Formation (166.3 ± 2.5 Ma), Manitu Formation (155.2 ± 1.4 Ma), and Baiyingaolao Formation (123.3 ± 1.0 Ma). The rhyolites of Manketouebo Formation and Baiyingaolao Formation exhibit chemical affinities to A-type granites, implying an extensional environment. Their geochemical features indicate that they are the result of the partial melting of the crust. The andesites of Manitu Formation exhibit chemical affinities to the magmatic rocks associated with a subduction zone. Their geochemical features indicate that they were generated by partial melting of a mantle wedge that had been metasomatized by subduction fluids or subducted oceanic slab. According to the geochemical features of coeval igneous rocks from the northern Great Xing’an Range and its adjacent areas, we propose that the Manketouebo Formation was generated in a continental back-arc extensional setting caused by the subduction of the Mongolia-Okhotsk plate, the Manitu Formation was related to the subduction zone metasomatism caused by the southward subduction of Mongolia-Okhotsk plate, and the Baiyingaolao Formation was generated in a continental back-arc extensional setting caused by the westward subduction of Paleo-Pacific slab or a post-collisional extensional setting after the closure of Mongolia-Okhotsk Ocean.
Eastern Central Qilian Block (E-CQB) is located in the Qilian orogenic belt, characterised by large outcrops of granite. The subduction age and geochemical processes of E-CQB are not precise. In this study, we conducted a comprehensive study of petrology, geochronology and whole-rock geochemistry of the Middle Ordovician granites (Sanlian rock mass) in the E-CQB. The results provide three key findings. First, the magmatic emplacement age of monzonite granites is about 466 Ma. Analysis derived from geochemical and geochronological revealed that the sample contains high-K, calc-alkaline, strongly peraluminous characteristics, and enriched in large-ion lithophile elements (LILEs; e.g., K, Th, and Rb). Depleting intensely in high field-strength element (HFSEs; e.g., Ti and P) then weakly in Nb, Ba, Sr and weak negative Eu. Monzonitic granites belong to S-type granites, can be divided into cordierite-bearing peraluminous granites, and the provenance is mainly partial melting of metapelite and metagreywacke. Second, combining the previous research and the new data obtained in this paper, the subduction age of E-CQB can be further refined to 444~466 Ma.
Permian magmatic activity is widespread in the Austroalpine Unit stretching from Eastern Alps to the Western Carpathians and Pannonian region, the link between magmatism and the subduction of Paleo-Tethys Ocean is not well constrained up to now. Here, we report, for the first time, a Middle Triassic granite in the Austroalpine basement, which is at odds with a common opinion of the passive margin setting of this unit. The Tweng and Schladming Complexes representing a Neoproterozoic to Early Paleozoic magmatic arc were intruded by Permian and Triassic granites, related to back-arc extension triggered by the subduction of Paleo-Tethys Ocean. To reveal the tectonic affinity of these granitic rocks, we systematically analyzed their zircon geochronology and whole-rock geochemistry. The zircon U-Pb data show that the Tweng granitic gneisses and leucogranites were formed at 261 Ma and 244 Ma, respectively. The Schladming granitic gneisses were formed at 244 – 241 Ma, their εHf(t) values (-0.9 to +5.6) indicate their derivation from the lower crust and involved continent material. Geochemically, the granitic rocks from the Tweng Complexes have common features with subduction-related continental arc andesitic affinity. The granitic gneisses of Tweng and Schladming Complexes show A-type granite-affinity with high zircon crystallization temperatures of 784 – 835 ℃ and 781 – 818 ℃, respectively, whereas the Tweng leucogranites exhibit I-type granite characteristics and low zircon crystallization temperatures of 720 – 724 ℃. The geochemical features suggest that the Tweng granitic gneisses and leucogranites originated from the lower crust, while the Tweng leucogranites are explained by remelting of Tweng granitic gneisses, and interacted with subduction-related fluids. Therefore, we propose that the Permian-Triassic granitic rocks of Tweng and Schladming Complexes were formed in a back-arc setting during the subduction of the Paleo-Tethys Ocean, which finally resulted in the opening of the Meliata oceanic back-arc basin during Middle Triassic times.
The Bozhushan granites, located in the southeastern Yunnan Province W–Sn polymetallic metallogenic belt in SW China, have considerable W-Sn endowment. However, their petrogenesis remains uncertain, as both crustal and mantle origins have been proposed. Numerous microgranular enclaves (MEs) are found in the Bozhushan granites, and the origin of these MEs can give insights into the petrogenesis of these ore-forming granites. Here we present an integrated study of the major and trace elements, whole-rock Sr–Nd isotopes, and zircon Hf–O isotopes of the MEs. The MEs from the Bozhushan granites are commonly composed of monzodiorite and granodiorite, and they have lower SiO2 (51.7–66.4wt.%) and higher TiO2 (0.83–1.67wt.%), MgO (1.68–4.49wt.%), and CaO (1.74–4.26wt.%) contents than those of their hosts (SiO2=64.3–76.9wt.%; TiO2=0.4–0.67wt.%; MgO=0.86–1.33wt.%; CaO=2.08–3.25wt.%). Furthermore, the chemical compositions of the MEs and host granites are discontinuous and do not show linear trends in element variation diagrams, as might be expected if the MEs represented restites, thus implying disparate genesis. The observed near-linear element variation most probably reflects mixing of mafic and felsic magma end-members in various proportions. In addition, the MEs have Sr–Nd–Hf–O isotopic characteristics (⁸⁷Sr/⁸⁶Sri=0.7117–0.7138, εNd=−12.59 to −9.26, εHf(t)=−13.1 to −6.1, and δ¹⁸O=8.39‰–9.19‰) and zircon U–Pb ages (87.4±1.0Ma to 86.5±0.4Ma) that are similar to those of their host granites. We conclude that the similarity of trace-element contents and Sr–Nd isotopic compositions between the MEs and host granites was caused by diffusion and partial re-equilibration. However, if the magma temperature of the MEs was 850°C, 950°C, or 1050°C, the required Zr contents for zircon saturation would be 238–854ppm, 497–1785ppm, or 928–3334ppm, respectively, which are substantially higher than those measured for the bulk rocks (44–436ppm). Furthermore, zircons from the MEs have Ti-in-zircon temperatures (Tzr; MEs, 669–884°C; host, 790–842°C) and Th/U ratios (MEs, 3.43–8.85; host, 2.68–11.86) that are indistinguishable from those of their host granites. Thus, on the basis of our new data, we propose that the MEs of the Bozhushan granites might have been derived from mantle-derived mafic magma. Zircons from the MEs did not crystallize directly within the MEs but rather were xenocrysts that formed during the early stage of magmatic evolution at the bottom of the granitic magma chamber and were subsequently incorporated into the MEs when mafic magma was injected into the granitic magma chamber. The Hf–O isotopes of zircons in this case cannot be used to constrain the primary composition of the magma that formed the MEs. During the Late Cretaceous, owing to lithospheric extension and asthenospheric underplating, the lower crust was partially melted to form high-temperature (790–842°C) granitic magma in the Bozhushan area. During the formation of the W–Sn mineralization associated with the Bozhushan granites, 10%–20% underplated mafic magma mixed with granitic magma and contributed both heat and mantle-derived material. Therefore, we consider that the Bozhushan granites are clearly characteristic of A-type granites, but the W–Sn mineralization was formed as a result of high-temperature partial melting of Sn–W-rich metasedimentary sources triggered by additional heat provided by mantle-sourced mafic magma.
The Pingmiao W (Cu) deposit is located in the central part of the newly-discovered giant Dahutang ore-concentrated district, South China. Field relationships and mineral assemblages confirm that the W (Cu) mineralization in the Pingmiao deposit is genetically related to multiphase late Mesozoic granites, which include the porphyritic two-mica granite, biotite granite porphyry, fine-grained muscovite granite, and Li-mica albite granite. Zircon U–Pb dating shows that these granites emplaced at ca. 146–143 Ma, which are coeval with the W–Cu–Mo-bearing granitoids from other ore deposits in the Dahutang district. The strongly peraluminous (A/CNK >1.1) feature, negative correlations between Rb and Th and between Rb and P2O5, low Zr+Nb+Ce+Y values (mostly lower than 200), and relatively low Zr saturation temperatures (lower than 800 ℃) suggest that these granites are typical S-type granite. These granites show similar Nd isotopes (–8.91 to –4.61), CaO/Na2O (0.2–0.57), and Al2O3/TiO2 (mostly ranging of 50–90) ratios with those W–Cu–Mo-bearing granitoids from other ore deposits in the Dahutang district, suggesting that they derived from partial melting of metapelites in the Shuangqiaoshan Group (or the Proterozoic metamorphic basement geochemically similar to the Shuangqiaoshan Group), with involving of minor mafic–ultramafic volcanic rocks. Geochemical fractionation trends recorded by whole-rocks and micas permit to distinguish the evolutionary trend from the porphyritic two-mica granite and biotite granite porphyry to the fine-grained muscovite granite and Li-mica albite granite. The apparent rare earth element tetrad effect, high Li and F contents, low K/Rb, Zr/Hf, and Nb/Ta ratios, as well as the zoned features of micas suggest that the melt–fluid interaction occurring during the formation of Li-mica albite granite. The Pingmiao granites have reduced redox state, revealed by their low whole-rock Fe2O3/FeO ratios (mostly <0.5), biotite Fe3+/Fe2+ ratios (ranging of 0.02–0.07), zircon Ce4+/Ce3+ ratios (median value of 18.70), and oxygen fugacity (near or below the FMQ buffer). The data in this study indicate that the late Mesozoic granitoids in the Dahutang district are favorable for W, Cu (< 1 Mt), and Mo (< 0.3 Mt) mineralization. Notably, the more evolved Li-mica albite granite has potential for W, Sn, Nb and Ta mineralization.
Zircon Hf-isotopic mapping can be regarded as a useful tool for evaluating the coupling relationship between lithospheric structure and metallic mineralization. Hence, this method shows important significance for mineral prediction. To explore this potential, the published granite zircon Hf isotope data from the Sanjiang Tethyan Orogen were systematically compiled. This study uses the Kriging weighted interpolation in the Mapgis software system to contour Hf isotopes, revealing a relation between the crustal structure and metallogenesis. The mapping results suggest that the Changning–Menglian suture zone is the boundary between ancient and juvenile crust, viz., the western terranes have ancient crust attributes, whereas the eastern terranes exhibit the properties of new juvenile crust. In addition, this study also found that the mineralization and element types in the Sanjiang Tethyan Orogen have a coupling relationship with the crustal structure. The distribution of porphyry Cu-Mo-Au deposits is mainly controlled by the new juvenile crust, whereas the magmatic-hydrothermal Sn-W and porphyry Mo-W(-Cu) deposits are closely related to the reworked ancient crust. The results of zircon Hf isotope mapping prove that the formation and spatial distribution of deposits are related to the composition and properties of the crust. Hf isotope mapping can reveal the regional metallogenic rules and explore metallogenic prediction and metallogenic potential evaluation.
The end of an orogenic Wilson cycle corresponds to amalgamation of terranes into a Pangaea and is marked by widespread magmatism dominated by granitoids. The post-collision event starts with magmatic processes still influenced by subducted crustal materials. The dominantly calc-alkaline suites show a shift from normal to high-K to very high-K associations. Source regions are composed of depleted and later enriched orogenic subcontinental lithospheric mantle, affected by dehydration melting and generating more and more K- and LILE-rich magmas. In the vicinity of intra-crustal magma chambers, anatexis by incongruent melting of hydrous minerals may generate peraluminous granitoids bearing mafic enclaves. The post-collision event ends with emplacement of bimodal post-orogenic (PO) suites along transcurrent fault zones. Two suites are defined, (i) the alkali-calcic monzonite–monzogranite–syenogranite–alkali feldspar granite association characterised by [biotite+plagioclase] fractionation and moderate [LILE+HFSE] enrichments and (ii) the alkaline monzonite–syenite–alkali feldspar granite association characterised by [amphibole+alkali feldspar] fractionation and displaying two evolutionary trends, one peralkaline with sodic mafic mineralogy and higher enrichments in HFSE than in LILE, and the other aluminous biotite-bearing marked by HFSE depletion relative to LILE due to accessory mineral precipitation. Alkali-calcic and alkaline suites differ essentially in the amounts of water present within intra-crustal magma chambers, promoting crystallisation of various mineral assemblages. The ultimate enriched and not depleted mantle source is identical for the two PO suites. The more primitive LILE and HFSE-rich source rapidly replaces the older orogenic mantle source during lithosphere delamination and becomes progressively the thermal boundary layer of the new lithosphere. Present rock compositions are a mixture of major mantle contribution and various crustal components carried by F-rich aqueous fluids circulating within convective cells created around magma chambers. In favourable areas, PO suites pre-date a new orogenic Wilson cycle.
The A-type granitoids can be divided into two chemical groups. The first group (A1) is characterized by element ratios similar to those observed for oceanic-island basalts. The second group (A2) is characterized by ratios that vary from those observed for continental crust to those observed for island-arc basalts. It is proposed that these two types have very different sources and tectonic settings. The A1 group represents differentiates of magmas derived from sources like those of oceanic-island basalts but emplaced in continental rifts or during intraplate magmatism. The A2 group represents magmas derived from continental crust or underplated crust that has been through a cycle of continent-continent collision or island-arc magmatism.
Zircon saturation temperatures (TZr) calculated from bulk-rock compositions provide minimum estimates of temperature if the magma was undersaturated, but maxima if it was saturated. For plutons with abundant inherited zircon, TZr provides a useful estimate of initial magma temperature at the source, an important parameter that is otherwise inaccessible. Among 54 investigated plutons, there is a clear distinction between TZr for inheritance-rich (mean 766 °C) and inheritance-poor (mean 837 °C) granitoids. The latter were probably undersaturated in zircon at the source, and hence the calculated TZr is likely to be an underestimate of their initial temperature. These data suggest fundamentally different mechanisms of magma generation, transport, and emplacement. ``Hot'' felsic magmas with minimal inheritance probably require advective heat input into the crust, are crystal poor, and readily erupt, whereas ``cold,'' inheritance-rich magmas require fluid influx, are richer in crystals, and are unlikely to erupt.
A delamination model applied to the Lachlan fold belt in eastern
Australia shows that lithospheric delamination began in the Early
Silurian (˜430 Ma), after crustal thickening in the Wagga-Omeo
zone. The delamination induced isostatic uplift farther east to produce
localized, transient extensional basins and coeval silicic magmas, which
formed by partial melting of the lower crust during asthenospheric
upwelling. However, the voluminous granitoid intrusions were emplaced
during and after compression, reflecting reestablishment of horizontal
compression associated with plate convergence. Continued sinking of the
slab produced an east. ward-widening orogenic belt where the granite
batholiths and associated thermally softened zones of deformation
tracked the direction of delamination. Once the slab was completely
detached at ˜400 Ma, the compressive stresses were transmitted
across the belt, and delamination recommenced about 1000 km westward
against the cratonic margin; the process was repeated at 400-370 Ma.
Within 60 m.y., the two sinking slabs had converted a 1700-2000-km-wide
continental margin into a 700-km-wide orogenic belt. Important effects
of delamination on the upper and middle crust are (1) production of
regional-scale, low-pressure metamorphic belts of limited crustal
thickness; (2) widespread voluminous silicic magmatism; (3) lack of
deep-crustal exposures even with high degrees of crustal shortening; and
(4) partial replacement of the lower crust by a mafic underplate that
cooled completely after deformation ceased.
Recent inference that Mesozoic Cordilleran plutons grew incrementally during >106 yr intervals, without the presence of voluminous eruptible magma at any stage, minimizes close associations with large ignimbrite calderas. Alternatively, Tertiary ignimbrites in the Rocky Mountains and elsewhere, with volumes of 1-5 × 103 km3, record multistage histories of magma accumulation, fractionation, and solidifi cation in upper parts of large subvolcanic plutons that were suffi -ciently liquid to erupt. Individual calderas, up to 75 km across with 2-5 km subsidence, are direct evidence for shallow magma bodies comparable to the largest granitic plutons. As exemplifi ed by the composite Southern Rocky Mountain volcanic fi eld (here summarized comprehensively for the fi rst time), which is comparable in areal extent, magma composition, eruptive volume, and duration to continental-margin volcanism of the central Andes, nested calderas that erupted compositionally diverse tuffs document deep composite subsidence and rapid evolution in subvolcanic magma bodies. Spacing of Tertiary calderas at distances of tens to hundreds of kilometers is comparable to Mesozoic Cordilleran pluton spacing. Downwind ash in eastern Cordilleran sediments records large-scale explosive volcanism concurrent with Mesozoic batholith growth. Mineral fabrics and gradients indicate unifi ed fl owage of many pluton interiors before complete solidifi cation, and some plutons contain ring dikes or other textural evidence for roof subsidence. Geophysical data show that low-density upper-crustal rocks, inferred to be plutons, are 10 km or more thick beneath many calderas. Most ignimbrites are more evolved than associated plutons; evidence that the subcaldera chambers retained voluminous residua from fractionation. Initial incremental pluton growth in the upper crust was likely recorded by modest eruptions from central volcanoes; preparation for calderascale ignimbrite eruption involved recurrent magma input and homogenization high in the chamber. Some eroded calderas expose shallow granites of similar age and composition to tuffs, recording sustained postcaldera magmatism. Plutons thus provide an integrated record of prolonged magmatic evolution, while volcanism offers snapshots of conditions at early stages. Growth of subvolcanic batholiths involved sustained multistage opensystem processes. These commonly involved ignimbrite eruptions at times of peak power input, but assembly and consolidation processes continued at diminishing rates long after peak volcanism. Some evidence cited for early incremental pluton assembly more likely records late events during or after volcanism. Contrasts between relatively primitive arc systems dominated by andesitic compositions and small upper-crustal plutons versus more silicic volcanic fi elds and associated batholiths probably refl ect intertwined contrasts in crustal thickness and magmatic power input. Lower power input would lead to a Cascade- or Aleutian-type arc system, where intermediate-composition magma erupts directly from middle- and lowercrustal storage without development of large shallow plutons. Andean and southern Rocky Mountain-type systems begin similarly with intermediate-composition volcanism, but increasing magma production, perhaps triggered by abrupt changes in plate boundaries, leads to development of larger upper-crustal reservoirs, more silicic compositions, large ignimbrites, and batholiths. Lack of geophysical evidence for voluminous eruptible magma beneath young calderas suggests that near-solidus plutons can be rejuvenated rapidly by high-temperature mafi c recharge, potentially causing large explosive eruptions with only brief precursors.
Triassic I- and A-type granites of the Chaelundi Complex, New England Fold Belt, eastern Australia, were generated in a subduction-related tectonic setting. Although isotopic ages of the suites are indistinguishable, field relations indicate that the A-type is younger. The most mafic granitoids from each suite have similar silica contents (66–68% SiO2), slightly LREE enriched patterns without Eu anomalies, low Rb/Sr and K/Ba ratios, and high K/Rb ratios, suggesting that both represent parental magmas. The A-type is distinguished mineralogically by abundant orthoclase and sodic plagioclase (total >60%), ferro-hornblende, annite and allanite. In contrast, the I-type has more hornblende and biotite, which are more magnesian in composition, and less feldspar. The parental magmas of both suites have many similar geochemical characteristics, although the A-type has slightly higher alkalis, Zr, Hf, Zn and LREE, and lower CaO, MgO, Sr, V, Cr, Ni and Fe³⁺/ΣFe. The geochemical properties characteristic of leucocratic A-type granites, such as high Ga/Al, Nb, γ, HREE and F contents, are only manifest in the more felsic members of the A-type suite. These features were produced by ∼70% fractional crystallization of feldspar, hornblende, quartz and biotite.
Both granite suites were generated by water-undersaturated partial melting of a similar source, but the A-type parent magma resulted from lower aH2O conditions during partial melting. Generation and rapid ascent of the earlier 1-type magma during disequilibrium partial melting produced a relatively anhydrous, but not refractory, charnockitic lower crust. Continued thermal input from mantle-derived magmas, during continuing subduction, partially melted the ‘charnockitized’ lower crust at temperatures in excess of 900°C, to produce A-type magmas. Charnockitic magmas (C-type) form in a similar way to A-type magmas, although their different composition reflects variations in the anhydrous lower-crustal mineral assemblages that remain after the previous (1-type) granite-forming event.
The New England Fold Belt was a subduction—accretion complex until the late Carboniferous, when the deeper parts underwent partial melting to produce S-type granites. As the I-and A-type granites intruded penecontemporaneously, a tonalitic source model for genesis of the Chaelundi A-type is untenable.
The mafic granulites from Huilanshan are outcropped on the center of the Luotian dome in the northern Dabie Mountains. The
Sm-Nd isochron defined by granulite-facies metamorphic minerals (garnet+clinopyroxene+hypersthene) yields an age of 136 ±
18 Ma indicating the early Cretaceous granulite-facies metamorphism. The cathodoluminescence (CL) images of zircons from the
granulite show clearly core-mantle-rim structures. The zircon cores are characterized by typical oscillatory zoning and highly
HREE enriched patterns, which suggests their magma origin. Some zircon cores among them with little Pb loss give SHRIMP U-Pb
ages ranging from 753 to 780 Ma, which suggests that the protolith of Huilanshan granulite is Neoproterozoic mafic rocks.
The zircon mantles usually cut across the oscillatory zone of the zircon cores have 3-10 times lower REE, Th, U, Y, Nb and
Ta contents than the igneous zircon cores but have high common Pb contents. These characteristics suggest that they were formed
by hydrothermal alteration of the igneous zircons. The part of zircon mantles with little Pb loss give a similar SHRIMP U-Pb
age (716-780 Ma) to the igneous zircon cores, which implies that the hydrothermal events occurred closely to the magmatic
emplacement. In view of the strong early Cretaceous magmatism in the Luotian dome, consequently, the Huilanshan mafic granulite
was formed by heating of the Neoproterozoic mafic rocks in mid-low crust, which caused the granulite-facies metamorphism underneath
the Dabie Mountains. The similarity between the granulite metamorphic age (136±18 Ma) defined by Sm-Nd isochron and K-Ar age
of 123-127 Ma given by amphible from the gneiss in Luotian dome suggests a rapid uplifting of the Luotian dome, which may
result in further exhumation of the ultrahigh pressure metamorphic rocks in the Dabie Mountains.
Adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones. Here we report the first example of such magmas from southern Tibet in an active continental collision environment. The Tibetan adakites were emplaced from ca. 26 to 10 Ma, and their overall geochemical characteristics suggest an origin by melting of eclogites and/or garnet amphibolites in the lower part (>=50 km) of thickened Tibetan crust. This lower-crustal melting required a significantly elevated geotherm, which we attribute to removal of the tectonically thickened lithospheric mantle in late Oligocene time. The identification of collision-type adakites from southern Tibet lends new constraints to not only the Himalayan-Tibetan orogenesis---how and when the Indian lithosphere started underthrusting Asia can be depicted---but also the growth of the early continental crust on Earth that consists dominantly of the tonalite-trondhjemite-granodiorite suites marked by adakitic geochemical affinities.
Central to understanding the exhumation history of the Himalaya is
knowing the timing of slip and magnitude of displacement on the primary
fault systems that bound the range. The widely accepted view that early
Miocene deformation in the Himalaya is characterized by simultaneous
shortening along the Main Central thrust and extension at shallower
crustal levels in part developed on the basis of knowledge of the age of
the Rongbuk granite and its apparent crosscutting relationship with the
Qomolangma detachment. This key contact, however, has not previously
been directly observed. Field mapping of the Qomolangma detachment and
its footwall reveals that no leucogranite bodies crosscut the
detachment. These observations together with Th-Pb monazite dating of
leucogranites exposed in the footwall suggest that slip was occurring
across the Qomolangma detachment shear zone ca. 17 Ma. Although there is
no evidence that requires simultaneous shortening and extension in the
High Himalaya, our observations are consistent with alternating periods
of shortening and extension in the Himalaya since the early Miocene.
In order to better constrain the paleogeographic evolution of south China we measured Sm-Nd and Rb-Sr isotopic compositions for 23 Mesozoic granites that crop out throughout the area. Tightly grouped neodymium depleted mantle model ages (1.4+/-0.3 Ga) suggest the region is underlain by relatively homogeneous Proterozoic crust and fail to define crustal provinces. Neither the isotopic nor geologic data suggest that a Mesozoic suture exists. However, granites possessing anomalously high Sm (>8 ppm) and Nd (>45 ppm) concentrations, relatively high initial epsilon neodymium (-4 to -8), and high but variable initial 87Sr/86Sr (0.759 to 0.713) form a northeast trending zone that coincides with two prominent Mesozoic basins. Southeast of the zone lie the majority of Mesozoic intrusives and Upper Triassic to Lower Cretaceous extensional basins found in south China. Mesozoic paleomagnetic poles are well clustered northwest of the zone. Pre-Cretaceous poles southeast of it are discordant with respect to those from the northwest. The only recognized tectonostratigraphic terrane in south China lies southeast of the zone. The terrane is bordered by a northeast trending sinistral fault that was active in the Mesozoic. Other faults in south China have similar attitudes, ages, and sense of shear. Together, the observations suggest that the Mesozoic tectonic regime in south China consisted of strike-slip activity plus concomitant rifting as terranes or fragments of similar crust were transported north along sinistral faults. The zone, defined by the granites enriched in Nd and Sm, demarcates displaced terranes to the southeast from relatively stable land to the northwest.
The protoliths of mafic-ultramafic plutons in the northern Dabie Mts. (NDM) (Hubei) include pyroxenite and gabbro. The zircon
U-Pb dating for a gabbro suggests that emplacement of mafic magma took place in the post-collisional setting at the age of
122.9±0.6 Ma. It is difficult to obtain a reliable Sm-Nd isochron age, due to disequilibrium of the Sm-Nd isotopic system.
Two hornblende40Ar/39Ar ages of 116.1±1.1 Ma and 106.6±0.8 Ma may record cooling of metamorphism in the mafic-ultramafic plutons in Hubei below
500°C. The hornblende40Ar/39Ar ages for the mafic-ultramafic rocks in Hubei are evidently 15–25 Ma younger than those for the same rocks in Anhui, indicating
that there is a diversity of the cooling rates for the mafic-ultramafic rocks in Hubei and Anhui. The difference in their
cooling rates may be controlled by the north-dipping normal faults in the NDM. The intense metamorphism occurring in the mafic-ultramafic
rocks in Hubei may result from the Yanshanian magmatic reheating and thermal fluid action induced by the Cretaceous migmatization.
The geochemical similarity of these mafic-ultramafic rocks wherever in Hubei and Anhui may be attributed to the same tectonic
setting via an identical genetic mechanism.
This geochemical classification of granitic rocks is based upon three variables. These are FeO/(FeO + MgO) = Fe-number [or FeOtot/(FeOtot + MgO) = Fe-*], the modified alkali-lime index (MALI) (Na2O + K2O - CaO) and the aluminum saturation index (ASI) [Al/(Ca - 1.67P + Na + K)]. The Fe-number (or Fe-*) distinguishes ferroan granitoids, which manifest strong iron enrichment, from magnesian granitoids, which do not. The ferroan and magnesian granitoids can further be classified into alkalic, alkali-calcic, calc-alkalic, and calcic on the basis of the MALI and subdivided on the basis of the ASI into peraluminous, metaluminous or peralkaline. Because alkalic rocks are not likely to be peraluminous and calcic and calc-alkalic rocks are not likely to be peralkaline, this classification leads to 16 possible groups of granitic rocks. In this classification most Cordilleran granitoids are magnesian and calc-alkalic or calcic; both metaluminous and peraluminous types are present. A-type granitoids are ferroan alkali-calcic, although some are ferroan alkalic. Most are metaluminous although some are peraluminous. Caledonian post-orogenic granites are predominantly magnesian alkali-calcic. Those with <70 wt % SiO2 are dominantly metaluminous, whereas more silica-rich varieties are commonly peraluminous. Peraluminous leucogranites may be either magnesian or ferroan and have a MALI that ranges from calcic to alkalic.
Thousands of cubic kilometers of high-K calc-alkalic magmas
intruded the Borborema Province (northeastern Brazil) during the
Neoproterozoic Brasiliano orogeny. They make up large batholiths in
which mantle-derived mafic to intermediate rocks coexist with a larger
amount of granitoids. The relatively low silica contents (61–70 wt%
SiO2) and moderate to high compatible element concentrations
(0.3–3.5 wt% MgO, 1.5–3.8 wt% CaO, as much as 150 ppm of Cr) of
the granitoids indicate that they contain an appreciable mantle component.
The similar trace element geochemical (high contents of incompatible
trace elements) and isotopic (strongly negative eNd values)
signatures of mafic and felsic rocks combined with geochemical modeling
suggest that (1) the mafic and felsic rocks are genetically linked,
(2) the granitic magmas were produced by 20%–30% partial melting
from a source having geochemical characteristics similar to the mafic
rocks, and (3) mingling and mixing of felsic magmas with subsequent
batches of mafic magmas yielded the silica-poor granitoids. Isotopic
data preclude involvement of the asthenosphere in the genesis of the
mafic melts and instead indicate their derivation from an old, enriched
lithospheric mantle. Therefore, addition of mantle material to the crust
occurred through internal lithospheric differentiation, in contrast with
conventional crustal-growth models.
The Variscan lower crust of Europe acquired its main features (seismic layering, intrusion of mantle-derived magmas, granulite grade metamorphism) when the upper crust underwent post-thickening extension through gravitational collapse. We suggest that post-thickening collapse is a geodynamic process that contributes to the growth and differentiation of continental lithosphere in collapsed mountain belts.
For the fi rst time, four distinct magmatic fabrics are documented in a composite plu-tonic body, the Tuolumne batholith, central Sierra Nevada, California, USA. One type of fabric was formed by strain caused by highly localized magma fl ow (type 1), whereas the other three chamber-wide fabrics recorded strain increments during boundary processes along batholith margins (type 2) superimposed by increments of heterogeneous regional tectonic strain (types 3 and 4). Our present work indicates that, in contrast to studies that consider magmatic fabrics to be simple structures formed by a single process, magmatic lineations and foliations in plutons may refl ect accumulated fi nite strain and form as composite structures recording multiple strain increments in relatively static, actively deforming and rheologically complex crystal mushes at lower melt percentages. We demonstrate that multiple magmatic fabrics in a single batholith may record remarkably different processes and thus preserve a temporal record of strain in the batholith from strain during internal chamber processes to postem-placement regional tectonic strain. However, it may be commonly very problematic to infer the exact nature of fl ow or even fabric-forming process from the preserved rock record. Our study also exemplifi es how examination of magmatic fabric patterns in plutons, complemented with geochronology, may provide crucial constraints on the interplay among successive magma emplacement, fabric preservation , temporal evolution of strain fi elds in a crystallizing magma chamber, and the development of crystal-mush zones.
In the hinterland of South China, there are some Cretaceous A-type granitiods or alkaline intrusive rocks, e. g., Huashan aegiriteaugite riebeckite granite in Anhui, Dayuancun aegiriteaugite arfvedsonite granite in Fujian, Ejinao nepheline sodalite syenite in Guangdong and Daheshan pyroxene quartz syenite porphyry and Sucun geode-like Kf-granite in Zhejiang. The geochronology of zircon SHRIMP and some minerals (amphibole and K-feldspar) 40Ar - 39Ar dating indicates that they were mainly formed in 137 - 86Ma. Combining with previous published isotopic ages of Late Mesozoic A-type granitiods or alkaline intrusive rocks in South China, our study also suggests that these A-type granitoids alkaline intrusive rocks may be classified as three periods. (1) Jurassic (184 - 152Ma) rocks: they distribute along the southern segment of the 'Shi-Hang rift zone' and in the south of Jiangxi, and their petrogenesis was possibly related to the strike-slip and extension due to low-speed and oblique subducting or transcurrent moving of paleo-Pacific plate or lithospheric extension which was independent of the movement of paleo-Pacific plate. (2) Early Cretaceous (139 - 123Ma) rocks: they distribute to the west of Zhenghe-Dapu fault zone and the lithospheric thinning or back-arc extension in relation to fast-speed and oblique subducting of paleo-Pacific plate is a likely responsible mechanism for their formation. (3) Late Cretaceous (101 - 86Ma) rocks: they mainly occur along the coastal of Fujian-Zhejiang area but distribute sporadically in the hinterland of South China, and they possibly resulted from the lithospheric extension owing to the collapse of the continental marginal arc or the roll-back of subducting oceanic crust.
The Yinmawanshan pluton is located in the southern part of the Liaodong Peninsula and emplaced in the Liaonan metamorphic core complex. Petrographically, three types of rocks can be distinguished from the outside to the center, i. e. , the gneissic, porphyritic and fine/medium-grained granites. The gneissic rocks, mainly composed of quartz monzodiorite and granodiorite, have evident gneissic structures and experienced ductile deformation in some locations. The direction of foliations and lineations are in accord with those of the mylonites at the lower plate of Liaonan metamorphic core complex. While the porphyritic granodiorite has weakly gneissic structure, and the fine/medium-grained monzogranite has evident massive structure. Above-mentioned characteristics show that this pluton is syn-deformed and emplaced in an extensional setting. Zircon U-Pb isotopic dating for different petrographic rocks gives similar emplacement ages ( 130 ∼ 120 Ma) , i. e. , the Early Cretaceous, not the Triassic as previously thought. The gneissic and the porphyritic rocks have high Sr contents ( > 600ppm) , low Y and Yb contents, and intense fractionation of LREE (light rare earth elements) and HREE (heavy rare earth elements), while the fine/medium-grained monzogranites have relatively low Sr concentrations, high Rb contents, and low 87 Sr/86 Sr ratio. The Sr-Nd-isotopic data indicate that the Yinmawanshan pluton was mainly derived from ancient lower crust. However, the relatively high Mg# values and large scale of Sr and Nd isotopic compositional variations suggest that materials derived from other sources (for example, lithosphere mantle, juvenile crust) had contributed to the genesis. Therefore, it is suggested that the giant Early Cretaceous magmatism in the eastern China developed in an extensional setting, which was possibly induced by lithospheric thinning in the eastern China.
Raohe Complex, located in the eastern Heilongjiang Province and composed of deep oceanic sediments and mafic igneous rocks,was considered as slice of oceanic crust and intruded by Hamahe and Taipingeun granites. Zircon U-Pb dating indicated that both of plutons were emplaced in Early Cretaceous, with the Hamahe pluton formed at three individual stages (131Ma, 124Ma and ≈ 115Ma, respectively) and the Taipingcun pluton at 111 - 114Ma. However, the gabbro in Raohe Complex was formed at 166 ± 1Ma, and the latest radiolarian in the deep oceanic sediments is about 160 - 150Ma. Therefore, it is concluded that the emplacement of the Raohe Complex took place at 150 - 131Ma of Late Jurassic-Early Cretaceous, indicating that there existed the Pacific plate subduction and striking-slip in this area before and during Late Jurassic-Early Cretaceous. Petrological studies indicate that both plutons contained magmatic cordierites and show peraluminous characteristics, suggesting their S-type affinity derived from partial melting of the sedimentary rocks. Hf isotopic analyses show that the zircons from plutons have high 176Hf/177Hf ratios and positive εHf(t) values, and the Hf isotope model age with the felsic crust rocks is 500 - 780Ma, which indicates that their protolith should be the juvenile crust formed in Neo Proterozoic-Phanerozoic. Therefore, it is suggested that there existed significant crust uplift and weathering before Late Jurassic-Early Cretaceous in the area.
A survey is given of the dimensions and composition of the present continental crust. The abundances of immobile elements in sedimentary rocks are used to establish upper crustal composition. The present upper crustal composition is attributed largely to intracrustal differentiation resulting in the production of granites senso lato. Underplating of the crust by ponded basaltic magmas is probably a major source of heat for intracrustal differentiation. The contrast between the present upper crustal composition and that of the Archean upper crust is emphasized. The nature of the lower crust is examined in the light of evidence from granulites and xenoliths of lower crustal origin. It appears that the protoliths of most granulite facies exposures are more representative of upper or middle crust and that the lower crust has a much more basic composition than the exposed upper crust. There is growing consensus that the crust grows episodically, and it is concluded that at least 60% of the crust was emplaced by the late Archean (ca. 2.7 eons, or 2.7 Ga). There appears to be a relationship between episodes of continental growth and differentiation and supercontinental cycles, probably dating back at least to the late Archean. However, such cycles do not explain the contrast in crustal compositions between Archean and post-Archean. Mechanisms for deriving the crust from the mantle are considered, including the role of present-day plate tectonics and subduction zones. It is concluded that a somewhat different tectonic regime operated in the Archean and was responsible for the growth of much of the continental crust. Archean tonalites and trond-hjemites may have resulted from slab melting and/or from melting of the Archean mantle wedge but at low pressures and high temperatures analogous to modern boninites. In contrast, most andesites and subduction-related rocks, now the main contributors to crustal growth, are derived ultimately from the mantle wedge above subduction zones. The cause of the contrast between the processes responsible for Archean and post-Archean crustal growth is attributed to faster subduction of younger, hotter oceanic crust in the Archean (ultimately due to higher heat flow) compared with subduction of older, cooler oceanic crust in more recent times. A brief survey of the causes of continental breakup reveals that neither plume nor lithospheric stretching is a totally satisfactory explanation. Speculations are presented about crustal development before 4000 m.y. ago. The terrestrial continental crust appears to be unique compared with crusts on other planets and satellites in the solar system, ultimately a consequence of the abundant free water on the Earth.
The restite (one source-component) model suggests that granitoids are derived from contrasting source rocks and that the typical linear chemical variation of Lachlan Fold Belt granitoids is produced by restite separation. However, it cannot explain the general chemical and isotopic similarity of S- and I-type granitoids in the eastern Lachlan Fold Belt, the similarity of zircon inheritance patterns between the two granite types, nor their apparently simple epsilon Nd-Sr isotopic array. A two source-component mixing model, based on the epsilon Nd-Sr isotopic array, suggests that linear chemical Variation of the granitoids reflects variable incorporation of deeply buried Ordovician sedimentary rocks and basaltic magmas. However, isotopically defined mixes do not match the predicted chemical mixes. Also systematic and sympathetic isotopic and chemical variations are observed for both the felsic and mafic granites within suites across the Bega Batholith. For a simple two-component model to apply, both the crustal and mantle end-members would have to change composition in the same way, which is unrealistic. A three source-component mixing model incorporates aspects of the other two models. It suggests that I-type magmas, formed in the lower crust by mixing of basaltic magmas with partial melts derived from the greenstone succession, are variably contaminated in the deep crust by migmatised Ordovician metasedimentary rocks. If sufficiently contaminated, the I-type granites become peraluminous, S-type granites and a restitic component may be retained. The three-component mixing model also places several important constraints on eastern Lachlan Fold Belt tectonic evolution: (i) no Proterozoic continental basement, nor Proterozoic basement terranes, need exist beneath the Lachlan Fold Belt; (ii) the Neoproterozoic-Cambrian greenstone succession and Ordovician sedimentary crust was considerably thickened before generation of the oldest S-type granites at ca 430 Ma: (iii) the greenstone succession dominated the lower crust after the crustal thickening, extending to depths between 20 to 30 km,whereas Ordovician metasediment dominated the higher crustal levels (0-20 km): (iv) the mid-crust may have been subjected to high heat-flux before generation of the I-type magmas in the lower crust, possibly associated with thermal recovery following end-Ordovician crustal thickening; (v) the remarkably homogeneous, high mu Pb crustal reservoir of the Lachlan Fold Belt is probably Ordovician sediment: (vi) the I-S line is not a major tectonic boundary, but reflects the eastern limit of Ordovician sedimentary rocks that were substantially melted: (vii) Bega Batholith granitoids show a systematic eastward decrease in degree of contamination by Ordovician sedimentary rocks, reflecting an eastward tapering Late Ordovician accretionary prism into which they intruded: (viii) the inferred west-dipping decollement surface between Ordovician metasedimentary rocks and Cambrian greenstone succession could reflect a Late Ordovician fossil subduction zone: and (ix) Siluro-Devonian Beget Batholith granitoids are probably all subduction-related, which might apply to silicic magmatism throughout the entire eastern Lachlan Fold Belt.
Lithospheric delamination is the foundering of dense lithosphere into less dense asthenosphere. The causes for this density inversion are thermal, compositional, and due to phase changes. For delamination to occur in the specific, and probably common, case where lithospheric mantle is intrinsically less dense than underlying asthenosphere due to composition differences, a critical amount of shortening is required for the densifying effect of cooler temperature to counterbalance the effect of composition. Crustal thickening that results from shortening may result in a crustal root that, due to phase changes, becomes denser than the underlying mantle lithosphere and should delaminate with it: most of the negative buoyancy resides at the top of the mantle and the bottom of the crust. In most cases composition is not known well enough to calculate the driving energy of delamination from densities of equilibrium mineral assemblages in a lithospheric column Poorly known kinetics of phase changes contribute additional uncertainties. In all cases however, the effects of delamination under a region are readily recognizable: rapid uplift and stress change, and profound changes in crustal and mantle-derived magmatism (a reflection of changes in thermal and compositional structure). Characteristics of delamination magmatism are exhibited in the Southern Puna Plateau, central Andes. The consequences of delamination for theories of crustal and mantle evolution remain speculative, but could be important. Recognition of delamination-related magmas in older (including Archean) orogens may be the best way to recognize past delamination events, because the magmas are among the most indelible and least ambiguous of delamination indicators.
New U-Pb geochronologic data indicate that the Tuolumne Intrusive Suite, California, was assembled over a period of at least 10 m.y. between 95 and 85 Ma, and that the Half Dome Granodiorite intruded over a period approaching 4 m.y. Simple thermal considerations preclude the possibility that a magma chamber the size of the Half Dome pluton could have existed as a liquid at shallow crustal depths for that long. Rather, field evidence for sheeting along the margins of the suite, the range of ages, and the regular decrease of ages toward the center of the suite and within individual plutons suggest incremental assembly. Geochronologic evidence for incremental assembly is consistent with the failure of geophysical methods to detect large magma chambers with more than ˜20% melt, even in active volcanic areas. Because it is unlikely that the individual plutons composing the Tuolumne ever coexisted as liquid-rich magmas, the chemical evolution of the suite cannot be the result of simple fractionation and/or mixing between exposed units, but instead must reflect processes occurring during magma generation.
The solubility and dissolution kinetics of apatite in felsic melts at 850°–1500°C have been examined experimentally by allowing apatite crystals to partially dissolve into apatite-undersaturated melts containing 0–10 wt% water. Analysis of P and Ca gradients in the crystal/melt interfacial region enables determination of both the diffusivities and the saturation levels of these components in the melt. Phosphorus diffusion was identified as the rate-limiting factor in apatite dissolution. Results of four experiments at 8 kbar run in the virtual absence of water yield an activation energy (E) for P diffusion of 143.6 ± 2.8 kcal-mol−1 and frequency factor (D0) of 2.23+2.88−1.26 × 109cm2-sec−1. The addition of water causes dramatic and systematic reduction of both E and D0 such that at 6 wt% H2O the values are ~25 kcal-mol−1 and 10−5 cm2-sec−1, respectively. At 1300°C, the diffusivity of P increases by a factor of 50 over the first 2% of water added to the melt, but rises by a factor of only two between 2 and 6%, perhaps reflecting the effect of a concentration-dependent mechanism of H2O solution. Calcium diffusion gradients do not conform well to simple diffusion theory because the release of calcium at the dissolving crystal surface is linked to the transport rate of phosphorus in the melt, which is typically two orders of magnitude slower than Ca. Calcium chemical diffusion rates calculated from the observed gradients are about 50 times slower than calcium tracer diffusion.
The origin of different kinds of granitic rocks is examined within the framework of experimental studies of melting of metamorphic rocks, and of reaction between basaltic magmas and metamorphic rocks. Among the types of granitic rocks considered in this chapter, only peraluminous leucogranites represent pure crustal melts. They form by dehydration-melting of muscovite-rich metasediments, most likely during the fast adiabatic decompression that results from tectonic collapse of thickened intracontinental orogenic belts. All other granitic rocks discussed here represent hybrid magmas, formed by reaction of basaltic melts with metamorphic rocks of supracrustal origin. These hybrid rocks include Cordilleran granites, formed at or near convergent continental margins, strongly peraluminous 'S-type' granites, alumina-deficient 'A-type' granites, and rhyolites associated with continental flood basalts. The differences among these types of granites reflect differences both in their source materials and in the pressures at which mantle-crust interactions take place. In turn, these variables are correlated with the tectonic settings in which the magmas form. Hybrid mafic cumulates are also produced by mantle-crust interactions, simultaneously with the granitic melts. These cumulates range from orthopyroxene + plagioclase-rich assemblages at low pressure to clinopyroxene + garnet-rich assemblages at high pressure, and are known to be important constituents of the lower continental crust. With the exception of peraluminous leucogranites, generation of granitic magmas is almost always associated in space and time with growth, rather than just recycling, of the continental crust.
Middle Jurassic to Lower Cretaceous intermediate to silicic plutonic and volcanic rocks of Hong Kong record a transition from calc-alkaline, through high-K calc-alkaline, to transitional shoshonitic compositions with time. Close spatial and temporal associations among comagmatic volcanic-plutonic pairs indicate that magmatism occurred in discrete episodes, mostly of less than one million years duration. Synchronous high-K calc-alkaline and transitional shoshonitic magmatic activity during at least one pulse suggests a relatively rapid transition from a subduction-related to an extension-related tectonic setting. Geochemical signatures indicate that the magmatic suites have a mantle origin with a decreasing crustal contribution from two distinct sources. The earliest mantle-derived magmas interacted strongly with a dominantly Archaean crustal protolith. Younger magmas show evidence for interaction with a dominantly Proterozoic crustal protolith. The strongest mantle influence is shown by magmas which were intruded along the boundary between the two dominant crustal sources. This interface marks a deep crustal discontinuity which promoted the passage of magmas to the surface.
Deciphering the magmatic history of continental magmatic arcs, in general, and the growth history of individual intrusions, in particular, is key to understanding the complex history of magma generation, segregation, and transport that define the dynamics of crustal growth. We utilize high precision U-Pb geochronology to resolve a detailed magmatic history from two composite intrusions, the 2-4 kbar Mount Stuart batholith and the 7-10 kbar Tenpeak pluton, emplaced in the Cretaceous North Cascades arc. This temporal framework provides a way to evaluate models of pluton growth that explain common features of intrusions such as concentric compositional zoning and internal magmatic contacts. U-Pb zircon crystallization ages were obtained from 12 samples of the Mount Stuart batholith and 8 samples of the Tenpeak intrusion, representing the range of compositional diversity and geographical extent. These dates indicate that the Mount Stuart batholith was constructed over a ∼5.5 m.y. time period that was punctuated by four intervals of high magma flux. The durations of the high-flux periods are short (a few hundred thousand years) relative to the duration of the batholith. The consistent pattern of magmatic fabrics and the lack of distinct contacts in the batholith may be explained by the juxtaposition of melt-rich and mush zones with subtle contacts between mineralogically and texturally similar tonalite and time-transgressive magma fabrics. In contrast, the Tenpeak intrusion was constructed over a ∼2.6 m.y. time period, with magma influx distributed throughout the intrusive history and texturally distinct magma bodies. The Tenpeak intrusion lacks distinct age domains, which suggests that any magma reservoir was smaller in size and potentially more ephemeral. Although the distinct age domains and discrete compositional and textural phases indicate that pluton growth occurred incrementally, neither pluton bears resemblance to a purely end-member incremental growth model whereby a pluton is constructed from hundreds to thousands of discrete magma pulses that have little, if any, interaction. In particular, ages from the youngest domain of the Mount Stuart batholith indicate that a melt-rich magma reservoir of ≥520 km3 existed over a 170 ± 90 k.y. time span.
The intrusive suite of Yosemite Valley provides an excellent example of coeval mafic and felsic magmatism in a continentalmargin arc. Within the suite, hornblende gabbros and diorites associated with the Cretaceous El Capitan and Taft Granites occur as scattered mafic enclaves, enclave swarms, small pods, synplutonic dikes, and a 2 km2 mafic complex known as the "diorite of the Rockslides." Field evidence suggests that most of the mafic rocks are temporally related to the El Capitan Granite and that significantly less mafic magma accompanied the slightly later intrusion of the Taft Granite. Concordant zircon fractions from the diorite of the Rockslides yield an age of 103 ± 0.15 Ma, which is the same age as the El Capitan Granite. Initial isotopic compositions of the mafic and felsic rocks are similar; the mafic rocks exhibit only slightly higher 87Sr/86Sr, lower 143Nd/144Nd, and higher 206Pb/204Pb ratios than the granites. Because the mafic magmas are only slightly more isotopically evolved than the granites, geochemical variation within the granites is not easily explained in terms of contamination of a depleted-mantle component by partial melts of ancient, high-silica continental crust. Rather, these data are consistent with an interpretation that the El Capitan Granite was derived by partial melting of relatively young mafic sources broadly similar to the mafic rocks of the suite.
Granitoids as categorized by tectonic environment are (1) island arc granitoids (IAG), (2) continental arc granitoids (CAG), (3) continental collision granitoids (CCG), (4) postorogenic granitoids (POG), (5) rift-related granitoids (RRG), (6) continental epeirogenic uplift granitoids (CEUG), and (7) oceanic plagiogranites (OP). Of these, the IAG, CAG, CCG, and POG are considered orogenic granitoids, and the RRG, CEUG, and OP are considered anorogenic granitoids.
The discrimination of granitoids is based on the major-element chemistry. Various discrimination plots are presented which sequentially discriminate the different tectonic environments. OP are separated from all other granitoids on the K2O versus SiO2 plot. Discrimination between group I (IAG + CAG + CCG), group II (RRG + CEUG), and group III (POG) granitoids can be achieved by using plots of Al2O3 versus SiO2, FeO(T)/ [FeO(T) + MgO] versus SiO2, and AFM and ACF ternary diagrams. In the figures, group I and group II plot in individual fields. Identification of group III is different, in that group III does not have a unique field in which it plots. Group III is identified because it consistently displays characteristics of both group I and group II. Further discrimination within group I can be accomplished on the basis of Shand's index. Only CCG have A/CNK [AL2O3/(CaO + Na2O + K2O)] values greater than 1.15. It is not possible to discriminate between IAG and CAG. Further discrimination within group II is done using the TiO2 versus SiO2 plot.
The proposed discrimination scheme is applied to the Proterozoic granitoids of the midcontinent of the United States. It is shown that the Arbuckle granitoids are not anorogenic as previously thought.
New ion microprobe U/Pb dates from zircon in deformed orthogneiss and migmatite and an undeformed granite in Mabja Dome are the first to constrain the timing of peak metamorphism, and onset and duration of mid-crustal ductile extension, in southern Tibet at 35.0 ± 0.8 Ma and ˜12 19 million years. The structural, metamorphic, and intrusive his tories in mid-crustal rocks exposed in these north Himalayan gneiss domes are similar to those recorded in the Greater Himalayan sequence, suggesting that middle crust was continuous from beneath southern Tibet southward to the high Himalaya. Strain compatibility indicates that 35 Ma ductile extension in mid-crustal rocks of southern Tibet was accommodated to the south at shallow crustal levels via normal slip along the southern Tibetan detachment system, the oldest age estimate for slip along this normal fault zone, and extrusion of its footwall. If gravitational collapse is an additional important process driving extension, then southernmost Tibet may have been at or near maximum elevation by late Eocene early Oligocene time.
The early continental crust is composed dominantly of Archean tonalite-trondhjemite-granodiorite (TTG) gneisses and is generally explained as the product of melting of metabasalts in the subducted crust. However, whether the melting occurs in shallow-level amphibolite facies or in relatively deep eclogite facies is debated. Here I present experimental partition coefficients (Ds) for 27 trace elements between garnet/amphibole and tonalitic melts. They are used together with published mineral/melt trace element Ds to model the melting of metabasalt in order to delimit the conditions for TTG production. The results clearly show that model melts with trace element characteristics that completely mimic the TTG are in equilibrium with rutile-bearing anhydrous and hydrous (amphibole bearing) eclogitic residues, but not rutile-free, amphibole-dominated residues. Rutile appears to be a necessary residual phase to account for the characteristic negative Nb-Ta anomaly in the TTG. These results thus suggest that the early continental material was produced under eclogite facies conditions. Based on the modeling results obtained using appropriate partition coefficients and the Archean geotherm, the preferred process for the TTG production is the melting of rutile-bearing hydrous eclogite, triggered by the release of H2O from the progressive breakdown of amphibole. The pressure-temperature (P-T) conditions for TTG production via this process are constrained to 1.5 2.5 GPa (˜50 80 km) and 850 1050 °C by the P-T stability boundaries of amphibole and rutile in the basalt system.
It has been long recognized from Nd and Sr isotopes that depleted mantle sources consist of recycled oceanic materials, but difficulty was encountered in identifying this signature by means of oxygen isotopes because of significant postemplacement hydrothermal alteration. Zircon is expected to preserve this signature because it is resistant to high- temperature hydrothermal alteration. This effect is illustrated by a combined Sm-Nd and oxygen isotope study of whole-rock and mineral samples from a Mesozoic A-type granite at Nianzishan in northeastern China. The Sm-Nd isotope results show positive εNd(t) values of +0.86 to +4.27 with young Nd model ages of 569 846 Ma, manifesting a significant input of newly mantle derived material. The zircon delta18O values of 3.120/00 4.190/00 are significantly lower than the delta18O value of 5.30/00 ± 0.30/00 for the normal mantle zircon and thus appear to require remelting of hydrothermally altered oceanic crust. The combined Nd-O isotope studies not only provide compelling evidence for geochemical recycling of young juvenile crust by plate subduction, but also demonstrate that the granitic magmas can result from partial melting of mantle-derived rocks that were subjected to seawater- hydrothermal alteration before magma generation. Disequilibrium oxygen isotope fractionations are observed between common rock-forming minerals with significantly lower delta18O values for alkali feldspar than seawater, corresponding to meteoric-hydrothermal alteration after magma crystallization.
We propose a flat-slab subduction model for Mesozoic South China based on both new sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon data and a synthesis of existing structural, geochronological, and sedimentary facies results. This model not only explains the development of a broad (˜1300-km-wide) intracontinental orogen that migrated from the coastal region into the continental interior between ca. 250 Ma and 190 Ma, but can also account for the puzzling chain of events that followed: the formation of a shallow-marine basin in the wake of the migrating foreland fold-and-thrust belt, and the development of one of the world's largest Basin and Range style magmatic provinces after the orogeny. The South China record may serve as an example of the multiple effects of flat-slab subduction, including migrating orogenesis and foreland flexure, synorogenic sagging behind the active orogen, postdelamination lithospheric rebound, and the development of a Basin and Range style broad magmatic province.
The Late Cretaceous was a period of extremely voluminous magmatism and rapid crustal growth in the western United States. From approximately 98 to 86 Ma, greater than 4000 km of exposed granodioritic to granitic crust, including the largest composite intrusive suites in the Sierra Nevada batholith, were emplaced in eastern California. Plutons intruded during this period include the highest peaks in the Sierra; we informally refer to this as the Sierra Crest magmatic event. Field, petrologic, geochemical, and geochronologic data indicate that, although they comprise an insignificant volume of exposed rocks (less than 100 km), mafic magmas were intruded contemporaneously with each episode of intermediate and high-silica magmatism in the event. This observation attests to the fundamental importance of high-alumina basaltic magmas during crustal-growth episodes in continental arcs. Geochemical data for suites of coeval plutonic rocks of the Sierra Crest magmatic event, ranging in composition from basalt to high-silica rhyolite, demonstrate that recycling of pre-existing crust locally played a minor role in the growth of new crust. Thus, major chemical and isotopic characteristics of Sierra Crest plutons, such as variable isotopic compositions, were inherited from the mantle source of the high-alumina basalts and are not necessarily the result of interaction with the overlying crust. Consequently, we interpret isotopic boundaries in the western United States, such as the Sr/Sr = 0.706 isopleth, to be largely features of the continental lithospheric mantle. Furthermore, isotopic data demonstrate that enrichment of the lithospheric mantle in the western United States probably occurred in the Precambrian during assembly of the North American craton. Geophysical and xenolith investigations by other workers support the hypothesis presented here that Cretaceous magmatism in the Sierra Nevada may have locally restructured most, if not all, of the crustal column. The timing of Sierra Crest magmatism correlates with voluminous magmatism elsewhere in the Cordilleran arc. We speculate that this intense episode of magmatism may have played a role in the global marine geochemical excursions and extinctions at the Cenomanian-Turonian boundary.
Mesozoic igneous rocks are widespread throughout eastern China, but precise geochronological and petrogenetic constraints were previously lacking. Ten samples from the Liaodong Peninsula in northeastern China were chosen for zircon U–Pb SHRIMP and laser ablation ICP-MS dating. The magmatic ages range from 179±3 to 156±3 Ma. Data compilation indicates that contemporaneous granitic magmatism is widespread throughout eastern China, establishing the Jurassic as an important period of igneous activity in eastern China. Petrographically, these granites can be divided into three groups that underwent a complex history of crystal fractionation. Two end-members of granodioritic and monzogranitic magma are identified. The granodioritic rocks, having lower (87Sr/86Sr)i and higher ɛNd(t) values than those of the monzogranitic rocks, came from the partial melting of juvenile crust, whereas the monzogranitic rocks came from partial melting of the Precambrian basement. It is proposed that Pacific plate subduction resulted in crustal thickening and subsequent lithospheric delamination which resulted in the upwelling of asthenospheric mantle and formation of juvenile crust by underplating of mantle-derived magma in the lower crust. A subsequent underplating and heating event from the asthenosphere partially melted the overlying pre-existing underplated mafic rocks and ancient crust, leading to the formation of granodioritic and monzogranitic magmas. This thickening by subduction could have been a necessary precursor for limited delamination in the Jurassic and more extensive delamination in the Early Cretaceous.
Geochronological, elemental and Sr–Nd–Pb isotopic data of early Cretaceous basic-intermediate rocks from the Dabie Orogen provide new insights into the nature of the late Mesozoic lithospheric mantle beneath the region and its tectonic relationship with neighboring blocks. Basic-intermediate rocks from the North Dabie Complex (NDC) include diabases, lamprophyres and trachyandesites, which have 40Ar/39Ar plateau ages of 127.6–131.8 Ma. Similar rock types from the North Huaiyang Unit (NHY) erupted at nearly the same time (135–116 Ma). Coeval rocks from both tectonic units form a continuous array in Harker diagrams, and exhibit similar geochemical characteristics. Both are significantly enriched in LILEs, and depleted in HFSEs, coupled with very low ɛNd(t), (206Pb/204Pb)i, and prominent positive Δ8/4 and Δ7/4 values, which are similar to those of Mesozoic mafic rocks in the North China Craton (NCC) exterior. These geochemical signatures are inconsistent with crustal contamination during magma ascent, and reflect derivation from an enriched lithospheric mantle source contaminated by the deeply subducted Yangtze crust. The observed geochemical similarities thus suggest that early Cretaceous igneous rocks from the NDC and NHY share a similar continental lithospheric mantle source that is tectonically affiliated to the NCC, although the surface geology of both tectonic units correlates with that of the Yangtze Block. Tectonic decoupling along a suture is proposed to explain the generation of early Cretaceous mafic rocks in the Dabie Orogen. The Wuhe-Shuihou fault likely represents the Mesozoic lithospheric boundary between the Yangtze Block and NCC, despite the fact that the present-day surface suture is situated at the Xiaotian-Mozitan fault or other faults to the north.
Two stages of early Cretaceous post-orogenic granitoids are recognized in the Dabie orogen, eastern China, which recorded processes of extensional collapse of the orogen. The early stage granitoids (∼ 132 Ma) are foliated hornblende quartz monzonites and porphyritic monzogranites. They are of high-K calc-alkaline series and metaluminous to weakly peraluminous, with high K2O and low MgO contents (Mg# values: 32.0–46.0), they contain high Sr, low Y and heavy rare earth elements (HREE), and have high Sr/Y and (La/Yb)N ratios, without clear negative Eu, Sr and Ti anomalies. The early stage deformed granitoids have adakitic geochemical compositions and are equilibrated with residues rich in garnet and poor in anorthite-rich plagioclase, and thus indicate the existence of an over-thickened (>50 km) crustal root beneath the orogen at ∼ 132 Ma. The later stage granitoids (∼ 128 Ma) are undeformed fine-grained monzogranites, fine-grained K-feldspar granites and coarse-grained K-feldspar granite-porphyry. They belong to a peraluminous and high-K calc-alkaline to shoshonite series, and display a flat HREE pattern and have strong negative Eu, Sr and Ti anomalies, with low Sr/Y and (La/Yb)N ratios. The late stage granitoids are equilibrated with residues rich in anorthite-rich plagioclase, hornblende, ilmenite/titanite and poor in garnet, indicating that the crust of the Dabie orogen became thinner (
Geochronological, geochemical, whole-rock Sr–Nd and zircon Hf isotopic analyses have been carried out on two suites of Late Mesozoic mafic to felsic magmatic rocks in the Sulu orogenic belt (east-central China) with the aim of characterizing their petrogenesis and tectonic implications. The Shijiusuo monzogranite has a SHRIMP zircon 206Pb/238U age of 127±2 Ma and an 40Ar/39Ar age on hornblende of 123.5±0.4 Ma. A mafic enclave from this pluton has a SHRIMP zircon 206Pb/238U age of 124±3 Ma and a hornblende 40Ar/39Ar age of 124.2±0.4 Ma, indicating coeval crystallization of the mafic enclaves and host monzogranite. Whole rock 40Ar/39Ar dating gives an emplacement age of 111.2±0.1 Ma for the mafic dikes. The monzogranites have low MgO and Cr, high Na2O, and Sr–Nd–Hf isotopic data (87Sr/86Sr>0.7085, ɛNd(t)=−20.5 and ɛHf(t)=−22.5 to −56.6) consistent with derivation from Late Archean to Paleoproterozoic lower crust with involvement of mantle materials. The presence of coeval mafic enclaves with high ɛNd(t) and ɛHf(t) values indicates magma mixing and involvement of mantle-derived materials in the generation of the Shijiusuo pluton. The mafic dikes that intrude the monzogranite have characteristics of ultrapotassic rocks. Their geochemical features, such as high MgO (Mg# up to 75) and Cr (up to 1233 ppm), low TiO2 (1.11–1.24 wt.%) and total Fe2O3 (8.33–9.09 wt.%), enrichment in LILEs (e.g., Rb, Ba, Sr) and LREEs, and depletion in HFSE (e.g., Nb, Ta and Ti), together with initial 87Sr/86Sr ratios of 0.7076–0.7078 and negative ɛNd(t) values (−17.6 to −18.2), indicate they were derived from an amphibole-bearing, refractory lithospheric mantle. The Shichang–Fangzi monzodioritic to monzonitic rocks have zircon SHRIMP U–Pb ages of ∼122 Ma. These rocks have Sr, Nd and Hf isotopic compositions (initial 87Sr/86Sr=0.7083–0.7088, ɛNd(t)=−16.5 to −17.7 and ɛHf(t)=−20.4) similar to the mafic dikes in the nearby Shijiusuo pluton, indicating they were derived from a common source. High Rb and HREE, low Sr and Ba and strongly negative Eu anomalies indicate that the monzodioritic magmas resulted from pyroxene- and plagioclase-dominated fractionation of magma derived from an enriched mantle source. Taken together, these features indicate that Early Cretaceous magmatism in the Sulu orogenic belt was not related to Late Triassic subduction or exhumation of the ultrahigh-pressure metamorphic rocks that characterize the Sulu region; instead they resulted from mantle–crust interaction in an extensional setting, most likely induced by widespread removal of lithospheric mantle in the eastern North China Craton during the Early Cretaceous.
NE China is characterized by the massive distribution of Phanerozoic granitoids. Most of them are of I- and A-type granites, whereas S-type granites are rarely documented. The present work deals with the Dongqing pluton, a small granitic body emplaced in the southern Zhangguangcai Range. The pluton comprises a two-mica (±garnet) granite and a garnet-bearing muscovite granite; the latter occurs as veins in the former. The pluton shows a gradational contact with the surrounding host granites. Rb–Sr and Sm–Nd isotope analyses on whole-rocks and minerals reveal that the two types of granites were emplaced synchronously at about 160 Ma. The pluton was emplaced coeval with the surrounding I-type granitic pluton, and had a rapid cooling history. It is characterized by an initial Sr isotopic ratio of ∼0.706, slightly negative εNd(T) values (−0.5 to −1.9) and young depleted-mantle model ages (970–1090 Ma). This suggests that the parent magma originated from partial melting of relatively juvenile crust, which is largely compatible with the general scenario for much of the Phanerozoic granitoids emplaced in the Central Asian Orogenic Belt.
The regional distribution in different depositional facies belts is here regarded as an important criterion for defining and
recognizing the various orders of sequences. The third-order sequence is possibly global in nature, which may be discerned
in different depositional facies belts in one continental margin and can be correlated over long distances, sometimes even
worldwide. Commonly, correlation of subsequence (fourth-order sequence with time interval of 0.5–1.5 Ma) is difficult in different
facies belts, although some of them may also be worldwide in distribution. A subsequence should be able to discern and correlate
within at least one facies belt. The higher-order sequences, including microsequence (fifth-order sequence) and minisequence
(sixth-order sequence), are regional or local in distribution. They may reflect the longer and shorter Milankovitch cycles
respectively. Sequence and subsequence are usually recognizable in different facies belts, while microsequence and minisequence
may be distinguished only in shallow marine deposits, but not in slope and basin facies deposits. A brief discussion is made
on the essential conditions for correct identification of sequences, useful methods of study, and problems meriting special
attention in outcrop sequence stratigraphy.
Vapor-absent melting experiments are performed on a biotite- and amphibole-bearing, Archean tonalitic gneiss (AGC150) at 10kbar and 875 to 1050°C. The experiments show that, under vapor-absent conditions, intrusion of hot, mantle-derived magmas into the lower crust is necessary to initiate widespread biotite-dehydration melting in rocks with compositions like AGC150. It is proposed that the high thermal stability of biotite in AGC150 suggests that this rock is residual after a previous episode of partial dehydroxylation that left behind somewhat F-enriched biotite. It is shown that dehydration melting of such F-enriched biotite produces F-rich granitic liquids, with compositions within the range of A-type granite, and leaves behind a granulitic residue consisting of orthopyroxene, plagioclase, quartz, titanomagnetite, and magnetite. -from Authors
A‐type granites are a minor, but distinctive, component of the granites of the Lachlan Fold Belt of southeastern Australia. They are felsic rocks with SiO2 contents ranging from 69.7 to 77.1%, with an average of 73.8% (55 analyses). When unfractionated, as evidenced by high Ba contents, they are distinguished from felsic I‐type granites by a greater abundance of high‐field‐strength elements, such as Zr. The Wangrah Suite contains a diverse association of A‐type granites, comprising four main units with coherent geochemical trends overall, but with textural variation from equigranular through to porphyritic. The least felsic granites from the suite (Danswell Creek Granite ∼70% SiO2) have compositional features that suggest that they represent parental magma compositions. The most felsic granites (Dunskeig Granite ∼76% SiO2) were derived from such compositions by fractional crystallisation. The Wangrah Suite granites were emplaced at shallow levels (∼200 MPa), at high zircon saturation temperatures (>830°C) and relatively low water activity. The chemical composition of the Wangrah granites cannot be easily related to the adjacent mafic magmas. The compositionally variable Wangrah Suite differs from the homogeneous A‐type suites, such as the Gabo Suite to the southeast. Its variability is probably related to the efficiency of fractional crystallisation and emplacement along a major fault at shallow levels. We favour a single‐stage petrogenetic scheme where the A‐type magmas were produced by high‐temperature, partial melting of quartzo‐feldspathic crustal rocks. The relatively refractory nature of the source rocks may have been due to limited H2O content, relatively low fO2 and relatively high (TiO2 + FeOtotal)/MgO.
A new compilation of reliable isotope age data indicates that Cretaceous magmatism in SE China occurred in four major episodes during 136–146 Ma, 122–129 Ma, 101–109 Ma and 87–97 Ma. A-type granitic and within-plate basaltic magmatism from 140–90 Ma suggests a dominant extensional environment in the region. Voluminous coeval high-K calc-alkaline rocks, which have geochemical features similar to those formed in continental back-arc and post-collision extension settings, are interpreted to have been generated in response to lithospheric extension. Cretaceous magmatism, NNE-trending wrench faulting and formation of extensional basin systems favour an extensional tectonic regime in SE China at that time, which was probably similar to present-day Basin and Range Province in the western US.
Recent studies on calc-alkaline plutonic rocks reveal field and petrographic relationships with strong implications for the processes involved in their genesis. The presence of magmatic inclusions and magma mingling zones in these rocks supports an origin by magma mixing. Compositional variations and isotopic anomalies are a good test for such petrogenetic models. Most of these calc-alkaline plutonic rocks which characterize both collision-related and active plate margin environments have been classically identified as I (igneous)-type granitoids, following Chappell and White's (1974) classification, and then interpreted as derived from partial melting of older igneous rocks (restite model). If the magma mixing origin of a type of granitoids can be demonstrated from field and petrographic evidence and supported by chemical and isotopic variations, the nomenclature must be changed. In this sense the new category of H (hybrid)-type is tentatively introduced. It includes most of the so-called I-type granitoids and some of the S-type. As deduced from field relationships of magma mingling zones, a mixing model is suggested that can operate on a large scale to explain the petrogenesis of the Hercynian calc-alkaline granodiorites of Iberia. Finally, a modified classification of common plutonic rocks in orogenic environments is proposed.
Samarium-neodymium isotope systematics provide a means of determining the age of the continental crust, where "age' refers to the amount of time the crustal rock material has been isolated from the convecting mantle. This age is referred to as the Sm-Nd model age or the mantle separation age. Methods are presented for treating isotopic data on continental rock materials to obtain meaningful mantle separation ages. The methods are applied to the Precambrian and Mesozoic rocks of the southwestern United States to produce a mass-age distribution for the region, which represents 1% of the global continental mass. The results suggest episodic crustal growth, with short growth periods at circa 2.8, 1.8, and 0.1 Ga. About 90% of the crust was formed by 1.8 Ga, the remaining 10% was added in the Phanerozoic. The mean age of this section of continent is determined to be 1.84 Ga. There is no correlation between crustal thickness and the crust formation age in the area studied; the age-area and age-mass curves are nearly identical. -from Authors
A feature of the S‐type Wilson's Promontory Batholith (Lachlan Fold Belt, Australia) is the presence of microgranitoid enclaves. These enclaves are fine‐grained and have rounded or irregular shapes, with diameters varying from 1 to 150 cm. Most contain megacrysts (mainly quartz, plagioclase and K‐feldspar), many of which show overgrowths of material formed at higher temperature. ‘Double enclaves’ (consisting of either darker microgranitoid enclaves or hornfelsic xenoliths enclosed in another microgranitoid enclave) are common. The enclaves contain similar minerals to the host rocks, but contain more biotite and less garnet. They have poikilitic or equigranular textures, and acicular apatite is common. The major and trace element geochemistry of enclaves and host overlap, but initial Sr and Nd isotopic ratios are different. The available data can most satisfactorily be explained by an origin of the enclaves involving mingling between a more mafic ‘enclave’ magma and a more felsic host magma. Origins for enclaves as fragments of restite or accumulations of early‐formed minerals less satisfactorily fit the available evidence.
A review of the geologic history of the Himalayan-Tibetan orogen suggests that at least 1400 km of north-south shortening has been absorbed by the orogen since the onset of the Indo-Asian collision at about 70 Ma. Significant crustal shortening, which leads to eventual construction of the Cenozoic Tibetan plateau, began more or less synchronously in the Eocene (50–40 Ma) in the Tethyan Himalaya in the south, and in the Kunlun Shan and the Qilian Shan some 1000–1400 km in the north. The Paleozoic and Mesozoic tectonic histories in the Himalayan-Tibetan orogen exerted a strong control over the Cenozoic strain history and strain distribution. The presence of widespread Triassic flysch complex in the Songpan-Ganzi-Hoh Xil and the Qiangtang terranes can be spatially correlated with Cenozoic volcanism and thrusting in central Tibet. The marked difference in seismic properties of the crust and the upper mantle between southern and central Tibet is a manifestation of both Mesozoic and Cenozoic tectonics. The form...
Microdiorite enclaves are arguably the most poorly understood of the more prominent features of granitoids. Surprisingly, they have been invoked as support for completely different models for the origin of chemical zonation in granitoids, depending on whether they are interpreted as restites, syn-plutonic blobs of magma, remobilized cumulate or basified country-rock xenoliths. Here we present the first detailed neodymium and strontium isotope study of suites of such enclaves from two plutons. In both cases we find relatively high σNd values, most readily explained if enclaves represent syn-plutonic injections of mafic magma into the host granitoid magma. As such they are the tangible remains of a very important process of mantle-derived heat and mass transfer operating in the generation of granitic magmas.
Although compositional variation in zoned calc-alkalic plutons is often ascribed to crystal fractionation, diagnostic large-scale field evidence of crystal accumulation in these slowly cooled bodies is generally missing. In many plutons, however, small-scale crystal cumulates have been preserved as layered schlieren and in microcosm may allow an assessment of the importance of crystal fractionation in their host pluton's development. Small, widely separated patches of schlieren in the Tuolumne Intrusive Series, Yosemite National Park, California, formed as cumulates. The data suggest that fractional crystallization did not produce the dominant chemical patterns seen in the Tuolumne and similar Sierra Nevada granites. -from Authors