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Are discrimination diagrams always indicative of correct tectonic settings of granites? Some crucial questions on granite study (3)

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Abstract

It is commonly accepted that the geochemistry of granitic rocks can be used to discriminate the tectonic settings when they formed. However, more and more evidence shows that the tectonic settings of some granitic intrusions cannot be well constrained if only based on their geochemical characteristics. Basically the discrimination diagrams for the tectonic settings of granites were created on the same theory as those for basalts. This study reviews the origin of the discrimination diagrams of basalts and the work of Pearce et al. (1984b) and barbalin (1999) on the discrimination diagrams of granites, and suggests that the geochemistry of granites is actually related to the nature and tectonic setting of their parental magmas rather than the granites themselves. On the basis of distribution of granites around the world, there are three types of granites: (1) granites in oceanic crust and oceanic margin which are derived from basaltic magmas (MORB, IAT and OIB) with apparent mantle contribution (high positive εNd (t) values and low initial Sr ratios); (2) collision-related granites in the continental margin including syn-collisional and post-collisional granites. They both are related to tectonic (deformation) events in the shallower depth, rather than tectonic setting. The difference between them is not only the geochemistry of rocks but also the assemblages of rocks. For example, adakites and low-Sr and -Yb leucogranites formed during collision whereas low-Sr and high-Yb granites and typical A-type granites with very low Sr and High Yb formed during the extension period after the collision and associated with within-plate basalts; (3) granites within the continent crust are usually generated by crustal anaxesis induced by the heat from the mantle such that the geochemistry of the granites is closely dependant on the composition of source rocks and the depth where the source rocks melted, again not related to the tectonic setting when granites formed. It is shown in this study that the proportion of above three types of granites in the present world is approximately ∼ 10%, ∼ 20% and ∼ 70%, respectively. Therefore, about 70% granites that occurred within the continental crust are not necessary to discriminate the tectonic setting when they formed. The discrimination diagrams for the tectonic settings of granites are only suitable to the granites originally formed in the oceanic crust. It is probably a misleading idea to discriminate the tectonic setting for the granites in the continental crust.

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... 2.45-2.2 Ga granitoids are widespread, including those in the São Francisco and Amazon cratons of South America (Teixeira et al., 1996;Vasquez et al., 2008;dos Santos et al., 2009), the Congo and West Africa cratons in West Africa (Gasquet et al., 2003), the Rae and Churchill Provinces and Trans-Hudson orogen of Canada (Hartlaub et al., 2007;Ashton et al., 2009;Berman et al., 2013;Pehrsson et al., 2013;Partin et al., 2014), the North China Craton (Diwu et al., 2007(Diwu et al., , 2014(Diwu et al., , 2018Dong et al., 2007;Zhao et al., 2008;Dan et al., 2012;Huang et al., 2012;Yu et al., 2013;Yang and Santosh, 2015), and Tarim Craton in China (Guo et al., 2003;Zhang et al., 2007aZhang et al., , 2007bLu et al., 2008;Zhang et al., 2013;Yang et al., 2017). Furthermore, in-situ U-Pb dating and Hf-O isotope analyses on both detrital and magmatic zircon grains from these regions suggest the presence of a 2.45-2.2 ...
... Additionally, on the Nb vs.Y and Rb vs. (Y + Nb) Nd isotopic ratios of the Delingha granitoids are calculated using crystallization age of 2367 Ma , and the Hudesheng granitoids from the northern Hudesheng Hill and Liudaoban area are calculated using crystallization age of 2368 Ma and 2390 Ma, respectively. (continued on next page) discrimination diagrams (Pearce et al., 1984), they plot in multiple source fields, overlapping the volcanic arc and within-plate and volcanic arc + syn-collisional fields ( Fig. 13a-b), together suggesting a post-collisional setting for our granitoids (Pearce, 1996;Zhang et al., 2007aZhang et al., , 2007b. Therefore, this evidence suggests that the Delingha and Hudesheng granitoids were most likely emplaced in a post-collisional tectonic setting similar to the coeval Mohe and Quanjishan granitoids He et al., 2018). ...
... 2.4-2.3 Ga granitic magmatism is widely developed in the Quanji Massif (Hao, 2005;Gong et al., 2012Gong et al., , 2014He et al., 2018) and the Tarim and North China cratons (Guo et al., 2003;Diwu et al., 2007Diwu et al., , 2014Diwu et al., , 2018Zhang et al., 2007aZhang et al., , 2007bLu et al., 2008;Zhao et al., 2008;Dan et al., 2012;Huang et al., 2012;Yu et al., 2013;Yang and Santosh, 2015;Yang et al., 2017). (2) the upper Dakendaban subgroup in the Quanji Massif developed a suit of khondalite series gneisses at 2.2-1.9 ...
Article
The global plate tectonic regime in the early Paleoproterozoic period is highly debated. Granitoids bear key information to address such a debate. Petrological, geochemical and geochronological studies are conducted on two post-collisional granitoid plutons in the Quanji Massif, northwestern China, to investigate the tectonic regime during this period. The granitoids are composed of syenogranite, monzogranite, and granodiorite, with minor tonalite, which intruded into the Delingha and Hudesheng regions at ca. 2.39–2.37 Ga. These plutons are high-K I-type granitoids with variable Ga/Al ratios, showing some characteristics of A2-type granitoids. They are characterized by enrichment of LILEs and LREEs and depletion of Sr, P, Ti, and Eu. They show depleted Nd and Hf isotope signatures with whole rock εNd(t) = +0.7 to +4.8 and zircon εHf(t) = −1.0 to +7.8, indicating a juvenile crustal growth event at ca. 2.44–2.37 Ga. Our new results together with other coeval post-collisional granitoids in the Quanji Massif suggest that the protracted post-collisional magmatism at ca. 2.39–2.34 Ga occurred just after a short interval of subduction and generation of juvenile magmas before or around ca. 2.4 Ga. Collectively, the formation of these ca. 2.4–2.3 Ga granitoids in the Quanji Massif may correlate with coeval granitoids in the Tarim and North China cratons, and is broadly coeval with magmatism in several other cratons in the West African and Canadian Shields. Thus, the globally well-documented magmatism at early Paleoproterozoic, or Siderian, provides further information for filling up the age gap of the so-called plate tectonic “shutdown” in the early Paleoproterozoic period worldwide. The geologic record therefore suggests no Siderian shutdown of plate tectonics, but instead, continuous global subduction and generation of juvenile magmas from the Archean through the Paleoproterozoic.
... Although there are many controversies about the granite tectonic discrimination diagram used to define tectonic context, it can still be used to identify plate margin types (Zhang et al., 2007). In the Ta versus Yb and Rb versus (Y + Nb) diagrams (Figure 11a,b), the Bailongshan pluton is plotted in the field of volcanic arc granite, whereas the field is occasionally interpreted as the post-collision zone. ...
... Traditional tectonic discrimination diagrams are usually difficult to distinguish among post-collision, collision, and subduction settings (Han, 2007;Wu, Li, Yang, & Zheng, 2007;Zhang et al., 2007). Pearce (1996a) thought that the post-collisional granites are mostly A-type granites, while the Bailongshan granite does not show this feature. ...
Article
Multiple branch oceans existed in the Paleo-Asian Ocean (PAO), but their closure times are in dispute and unclear, which constrains our understanding of the final closure time of the PAO and the tectonic evolution of the Central Asian Orogenic Belt (CAOB). This study focuses on the Permian plutons of the northern Alxa, which is located in the middle segment of the southern CAOB that recorded the final subduction history of the PAO. We performed the 1:50000 mapping, whole-rock geochem-istry, geochronology, and Sr-Nd-Hf isotopic analysis and compiled the Sr-Nd-Hf isotopic compositions and whole-rock geochemical data of igneous rocks from the northern Alxa. LA-ICP-MS zircon U-Pb dating reveals the study plutons emplaced in the Early Permian (285-296 Ma). Whole-rock geochemical data show the intrusion belongs to medium-K calc-alkaline peraluminous highly fractionated I-type granite, enriched in Rb, K, Th, Pb, and depleted in Nb, Ta, Ti, Sr, and P elements, which suggest a subduction arc-related setting and metaluminous to weak peraluminous parental magma. The weak negative ε Nd (t) (from À2.3 to À1.2), relatively high I Sr (0.704772-0.708037) and depleted mantle model ages T DM (1.14-1.49 Ga), combining with weak negative to slightly positive ε Hf (t) (from À2.0 to +4.1) and crustal model ages T DM C (1.18-1.43 Ga), indicate that the parental magma might originate from remelting of the Mesoproterozoic lower crust and mixing with mantle-derived materials. The field occurrence, deformation, and geochemical features, integrating with the compiled data and regional geology, show that the igneous rocks formed before or after the late Early Permian show different features in terms of deformation , zircon saturation temperatures, crustal thickness, potassium contents, and ε Hf (t) values. This might relate to the closure of the Yagan branch ocean of the PAO in northern Alxa.
... The research on the Triassic granites in the QOB shows that these granites have recorded complete orogenic processes from syn-collision [29], post-collision [30][31][32][33], post-orogenic [26,34] to anorogenic environment (this study). The petrogenesis can reflect different tectonic environments and geodynamic backgrounds [46,77,95,[125][126][127]. In this paper, we suggest that the Shimen pluton is A 1type granite, indicating the continental rifts or intraplate environments [53]. ...
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The North China Block and the South China Block collided in the Middle Triassic, but there is still a lack of consensus regarding the end of collisional orogeny and the closure time of the Paleo-Tethys. In this paper, we report zircon U–Pb ages and geochemistry for the Shimen pluton in the northern margin of the West Qinling Orogenic Belt to investigate its genesis and tectonic environment. The new findings allow to constrain the end time of the Triassic orogeny in the Qinling Orogenic Belt and the closure time of the Paleo-Tethys. The weighted average 206Pb/238U ages of the Shimen pluton are 218.6 ± 1.5 Ma and 221.0 ± 1.7 Ma. Thus, we suggest that the Shimen pluton crystallized at the 218.6 Ma and 221.0 Ma and was formed during the Late Triassic (Norian). The Shimen pluton is mainly syenogranite and has alkaline dark minerals aegirine–augite. It is composed of 73.45 to 77.80 wt.% SiO2, 8.28 to 9.76 wt.% alkali, and 11.35 to 13.58 wt.% Al2O3, with A/CNK ranging from 0.91 to 1.02 and 10,000 Ga/Al ranging from 2.39 to 3.15. These findings indicate that the Shimen pluton is typical A-type granite. The plutons have low rare earth element contents, ranging from 73.92 to 203.58 ppm, with a moderate negative Eu anomaly. All the samples are enriched in large-ion lithophile elements, such as Rb, Nd, Th and U, and light rare earth elements, and are depleted in high field strength elements, such as Nb, P, Zr, Ba, and Sr. The depletion of Ba, Sr, and Zr may be related to the fractionation and evolution of the granite. According to the petrological and geochemical characteristics, the Shimen pluton is an A1-type granite formed in an anorogenic extensional environment. Combined with its tectonic characteristics and petrogenesis, the Shimen pluton was probably formed by the partial melting of the crust under high temperature and low pressure in the intraplate environment after the subduction of the South China Block beneath the North China Block. This observation indicates that the Triassic orogeny in the Qinling Orogenic Belt had ended and the Paleo-Tethys-Mianlve Ocean had also closed by the Late Triassic (Norian).
... and low Zr/Hf ratios (35-44) indicate that the granodiorites belong to the highly fractionated I-type granite (Wu et al. 2003). As shown in previous studies, the geochemistry of granites is actually associated with the nature and tectonic environment of their parental magma rather than the granites themselves (Zhang et al. 2007). Therefore, the tectonic setting of the post-collisional granites cannot be simply differentiated using the trace-element discrimination diagrams without conducting comprehensive research in regional geology (Han 2007). ...
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The northern Alxa area is the southernmost segment of the southern Central Asian Orogenic Belt. Its tectonic environment during the Middle–Late Devonian is debated: post-collisional or subduction settings. Here, we conducted a combined study associated with the geochemistry, geochronology, and zircon Hf isotope in Middle–Late Devonian (386–375 Ma) granodiorites from the northern Alxa area for understanding their petrogenesis and tectonic setting. Our results show that these granodiorites are peraluminous (ACNK = 1.08–1.12) and (high-potassium) calc-alkaline I-type granites. Moreover, they exhibit light rare-earth elements (LREEs) and large ion lithophile elements (LILEs) enrichment as well as depletions in Nb, Ta, and Ti. The εHf(t) values of these granodiorites could be divided into two groups: the granodiorites with ages of 385–380 Ma yield mostly positive εHf(t) values (+0.2–+5.1) with young two-stage model ages of 1.78–1.33 Ga and few negative εHf(t) values (from − 0.2 to – 9.9) with old two-stage model ages of 2.69–1.80 Ga, suggesting that they were derived from juvenile oceanic crust materials mixed with older continental materials. In contrast, the Late Devonian granodiorite (375 Ma) yielded radiogenic εHf(t) values between − 12.6 and − 2.1 with old two-stage model ages of 2.91–1.98 Ga, indicating its origination of the partial melting of ancient crustal materials. Combined with sedimentary evidences, coeval adakites, and arc-related volcanic rocks in the northern Alxa area and the adjacent South Beishan Orogenic Belt, implying the Zhusileng–Hangwula Tectonic Zone underwent a subduction setting during the Middle–Late Devonian and this subduction was possibly associated with the Enger Us Ophiolitic Belt.
... Moreover, crustal extensional thinning was accompanied by underplating of asthenospheric upwelling mantle material and heating of the crustal materials to form granites by partial melting. The asthenospheric mantle material also provided a high-temperature environment for the formation of A-type granites (Wu, Li, Yang, & Zheng, 2007;Zhang, Pan, Li, Jin, & Jia, 2007). This extensional environment is generally considered to be the remote effect of the subduction and retreat of the ancient Pacific Plate (Chen, 2012;Dai, Zheng, & Zhao, 2016;Dmitrienko et al., 2016;Mercier, Hou, Vergely, & Wang, 2007;Zhang, Dong, & Wei, 2003;Zhu et al., 2017;Zhu, Jiang, Zhang, & Yin, 2012). ...
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The Trans‐North China Orogen (TNCO) is a part of the North China Craton (NCC) and together provides a classic example of lithospheric destruction. The Pandao granites outcropping in the Wutai Mountains area provide a window to investigate the Mesozoic magmatism in the TNCO. Here, this paper presents the new zircon geochronology, whole‐rock geochemistry, and zircon Hf isotope data for the Pandao granites to discuss their petrogenesis and tectonic implications. The Pandao granites are mainly composed of light‐red medium‐ to coarse‐grained biotite granite and light‐grey fine‐ to medium‐grained biotite granite. Zircon U–Pb ages of 110.05 ± 0.67 Ma and 108.35 ± 0.81 Ma suggest that the Pandao granites were crystallized in the Early Cretaceous. The Pandao granites are classified as high‐K calc‐alkaline and weak peraluminous series. The rocks display abundance in large‐ion lithophile elements (LILE) and light rare‐earth elements (LREE) but show depletion in high‐field‐strength elements (HFSE) and heavy rare‐earth elements (HREE), with strong negative Eu anomalies. The classification diagrams indicate that the Pandao granites are A‐type granites and thus belong to the A1 subtype, formed in an intraplate extensional environment. The Pandao granites have homogeneous zircon Hf isotopic compositions. Their zircons have negative εHf(t) values (−19.1 to −17.1) and old Hf isotope crustal model ages (2,250–2,375 Ma), suggesting that the Pandao granites were formed by partial melting of the Paleoproterozoic lower crustal material. Therefore, it is suggested that the Pandao granites were formed under an intraplate extensional tectonic environment of remote effect due to the ancient Pacific Plate subduction and retreat beneath the Eurasian continent. The TNCO was influenced by the subduction and retreat of the ancient Pacific Plate in the late Early Cretaceous. Petrogenesis of the Pandao granites
... Moreover, crustal extensional thinning was accompanied by underplating of asthenospheric upwelling mantle material and heating of the crustal materials to form granites by partial melting. The asthenospheric mantle material also provided a high-temperature environment for the formation of A-type granites (Wu, Li, Yang, & Zheng, 2007;Zhang, Pan, Li, Jin, & Jia, 2007). This extensional environment is generally considered to be the remote effect of the subduction and retreat of the ancient Pacific Plate (Chen, 2012;Dai, Zheng, & Zhao, 2016;Dmitrienko et al., 2016;Mercier, Hou, Vergely, & Wang, 2007;Zhang, Dong, & Wei, 2003;Zhu et al., 2017;Zhu, Jiang, Zhang, & Yin, 2012). ...
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Remote sensing is a robust and useful tool for providing high‐resolution image data and enabling reliable geological mapping during the initial stage of mineral exploration. One of its main applications is the extraction of lineaments and to locate alteration areas to target gold exploration. It has been long used in the Pan‐African belt of Cameroon to identify a hydrothermal alteration and a great number of lineaments associated with mineralizations. The study area located in the Pan‐African belt hosts numerous alluvial gold deposits where the primary mineralization was still largely poorly unknown until now, due to deep weathering. Therefore, remote sensing combined with field data is useful for targeting potential zones of primary gold resources involved in the hydrothermal and lineament systems. In this study, remote sensing data from Landsat 8 imagery were selected to map the distribution of hydrothermal minerals, and gravity data were interpreted for highlighting structural patterns related to the control of high‐potential zone for gold mineralization, generating a mineral prospect map. The lineaments network shows directions ranging from ENE‐WSW to E‐W, with main direction N45° and a secondary striking N275°. Image enhancement/processing techniques included the application of band ratio and principal component analysis that were helpful to demarcate potential alteration zones marked by iron oxide/hydroxides in which haematite and pyrite are used as proximal alterations and hydroxyl‐bearing minerals in which sericite (muscovite) is used as a marker of proximal alteration, while chlorite, epidote, biotite, quartz, and calcite are used as distal alteration zone, as described by field and petrographic data. The identified alteration zones display a high consistency with the known locations of gold occurrences (mining sites) and closely concordant with large‐scale gold mineralization in the study area. This study presents an integrated approach of Landsat 8 imagery with gravity data and field data for discovering primary mineral resources in a deep weathering area. Regional lineaments from satellite images showing the spatial distribution, and high‐potential areas of gold mineralization in Gamba district.
... Ga × 10 4 /Al > 2.6, the rhyolite thus has a composition similar to A-type granite (Zhang et al. 2012;Whalen et al. 1987). A-type granites are formed under alkali-rich, anhydrous, and anorogenic conditions by the melting of source rocks under low-pressure and hightemperature conditions (Loiselle and Wones 1979;Zhang et al. 2007). A hydrous and high viscosity A-type granitic magma has a similar density to that of rock-forming minerals and does not undergo crystallization differentiation. ...
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Rhyolites with high Nb–Ta contents were recently discovered in the north Daxingan Mountains, China. We determined the geochemical characteristics and zircon U–Pb ages of these rhyolites to elucidate their tectonic setting of formation and petrogenesis. Zircons from the high Nb–Ta rhyolites are idiomorphic or hypidiomorphic, short prismatic crystals with oscillatory zoning; the zircon trace element has a higher Th/U ratio (> 0.4); zircon rare earth element (REE) content is high (average is 1729 × 10⁻⁶) and indicates heavy REE (HREE) enrichment (average is 1561 × 10⁻⁶) and shows positive Ce (Ce/Ce* = 2.1–103.4) and negative Eu (Eu/Eu* = 0.18–0.64) anomalies typical of crustal magmatic zircons. Their weighted-mean LA–ICP–MS U–Pb age of 155 ± 1 Ma indicates that they formed in the Upper Jurassic. The rhyolites are characterized by high SiO2 and alkali contents and low Fe, Ca, Mg, and Mn contents, and are weakly peraluminous, indicating that they are high-K calc-alkaline rocks. Trace element compositions are characterized by enrichments in Nb, Ta, Zr, Hf, Ce, and Rb and depletions in Sr, Eu, Ba, P, Ti, Co, and Ni, with significant positive Ce (Ce/Ce* = 2.4–2.7) and negative Eu (Eu/Eu* = 0.06) anomalies. Niobium and Ta are hosted in the zircons. In (Na2O+K2O+FeOT+MgO+TiO2) vs (Na2O+K2O)/(FeOT+MgO+TiO2) and (Al2O3+FeOT+MgO+TiO2) vs Al2O3/(FeOT+MgO+TiO2) mineral characterization diagrams, data for the samples plot in the metamorphic greywacke or basic argillaceous rock fields, indicating that the magma originated from partial melting of crustal material. εSr(t) values cover a wide range (− 18.2 to + 102.9), whereas εNd(t) values have a narrow range (1.9–2.0) with T2DM model ages of 789–785 Ma, indicating that the source was the Neoproterozoic Xinghua Ferry Group crustal basement. This crustal rock suite comprises a volcanic–sedimentary formation of metamorphosed mafic volcanic and terrigenous clastic rocks derived from a mixture of mantle and crustal materials. Residual phases in the source region include Ca-rich plagioclase, amphibole, orthopyroxene, and zircon + garnet. Together with the positive Ce anomalies and low-Sr/high-Yb characteristics of the rhyolites, this indicates that the source rocks melted at relatively shallow depths (< 30 km), low pressures (< 0.8 GPa), and high O2 fugacity. Ga × 104/Al > 2.6, Ta vs Yb, and (Rb/30) vs Hf vs (3Ta) discrimination diagrams, data for the samples plot in the A-type rhyolite and intraplate granite fields, whereas in the Nb vs Y vs Ce diagram, the data plot in the A1-type field. It is concluded that an extensional tectonic setting, resulting from closure of the Mongolia–Okhotsk Ocean at the end-Triassic, or northward subduction of oceanic lithosphere under the Siberian Plate, caused underplating of mantle-derived basaltic magma and partial melting of metamorphic crustal rocks.
... Documented metamorphic condition in Liuling Group also show a grade much lower than that in the SGSZ in response to the Late Triassic orogeny, highlighting the significance of the extra heat brought by magma emplacement.8.3 | Regional tectonic implicationsThe tectonic setting of the Qinling Orogen during the Late Triassic has long been a subject of debate. Most works focused on the petrological and geochemical studies of the coeval and cogenetic Late Triassic plutons with a common assumption that geochemistry of granites can be used to discriminate the tectonic setting at the time of their formation.However, it should be noted that the geochemistry of granites is actually related to the nature and tectonic setting of their parental magmas rather than the granites themselves(Zhang, Pan, Li, Jin, & Jia, 2007). It remains to be demonstrated if the geochemistry of granites can discriminate the tectonic setting in which they emplace. ...
... The low Sr and Eu contents of the samples studied here reflect residual plagioclase in the source region and argue against differentiation of mantle-derived magma or partial melting of mafic rocks as a formation mechanism (Wu et al., 2007a). In contrast, the high-K calc-alkaline nature of these rocks and their depletion in Nb, P, Ti, and Sr potentially reflect a source region composed of continental crust (Zhang et al., 2007), supporting the suggestion that the Xierzi pluton has a crustal origin. ...
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The opening, subduction and final closure of the Paleo-Asian Ocean led to the formation of the Central Asian Orogenic Belt. Controversy has long surrounded the timing of final closure of the the Paleo-Asian Ocean. Here we present zircon U–Pb ages and petrological, geochemical and in situ Hf isotope data for the Xierzi biotite monzogranite pluton, Linxi, SE Inner Mongolia. U–Pb dating of zircon by LA–ICP–MS yields a middle Permian emplacement age (268.7 ± 2.3 Ma) for the Xierzi pluton that is dominated by biotite monzogranites with high SiO2 (71.2–72.8 wt.%), alkali (Na2O + K2O = 8.05–8.44 wt.%), Al2O3 (14.4–15.2 wt.%) and Fe2O3T relative to low MgO contents, yielding Fe2O3T/MgO ratios of 2.87–3.44, and plotting within the high-K calc-alkaline field on a SiO2 vs. K2O diagram. The aluminum saturation indexes (A/CNK) of the biotite monzogranites range from 1.06 to 1.19, corresponding to weakly to strongly peraluminous. They are enriched in rare earth elements (REE), high field–strength elements (HFSEs; Zr, Hf), and large ion lithophile elements (LILEs; Rb, U, Th). The LREEs are enriched relative to the HREEs, with a distinct negative Eu anomaly in a chondrite–normalized REE diagram. Geochemically, the Xierzi biotite monzogranite is classified as an aluminous A-type granite, with all samples plotting within the A2-type granite field on a Y/Nb vs. Rb/Nb diagram. Zircon εHf(t) values and two-stage modal ages of the zircons within the pluton range from + 4.80 to + 13.65 and from 983 to 418 Ma, respectively, indicating that the primary magma was generated through partial melting of felsic rocks from juvenile crust. Consequently, these results demonstrate that the Xierzi pluton formed under the post-orogenic extensional setting after arc–continent collision in the middle Permian.
... However, these diagrams are often ambiguous and sometimes debated (Pearce, 1996;Forster et al., 1997). The geochemical compositions of granites are more related to the nature of their source rocks, the temperatures and pressures of partial melting, and hydrothermal fluids than to the environment in which they form (Wu et al., 2007;Zhang et al., 2007). Hence, it is necessary to discriminate tectonic settings by analyzing the late Mesozoic granitic rock assemblages in the Cathaysia Block. ...
Article
The Cathaysia Block is the southeastern part of the South China Block in Southeast (SE) China, and it hosts voluminous late Mesozoic I-, S-, and A-type granitoids, as well as minor highly fractionated granites. We present here zircon U–Pb age data and Nd–Hf isotopic data for the Dayang and Juzhou granites, together with new petrological and geochemical analyses. The Dayang pluton consists of fine-grained two-mica monzonitic granites in which the plagioclases exhibit zoning and poikilitic textures. In contrast, the Juzhou pluton consists of medium- to coarse-grained biotite K-feldspar granites that lack zoning and poikilitic textures. The emplacement ages are 143 ± 2.3 Ma for the Dayang pluton and 133 ± 2.1 Ma for the Juzhou pluton according to zircon U–Pb isotope analyses. The Dayang and Juzhou granites are both metaluminous and belong to the shoshonitic series. The Dayang granite exhibits very flat REE patterns, showing the tetrad effect, and the spidergrams show striking negative Ba, Sr, Nb, and Ti anomalies and a positive Ta anomaly. In contrast, the Juzhou granite has sloping REE patterns, but like the Dayang granite it also has striking negative Ba, Sr, Nb, Ta, and Ti anomalies. Petrographic and geochemical evidence indicates that the Dayang granite is a highly fractionated I-type granite and that the Juzhou granite is a typical I-type granite. The tetrad effect in the Dayang granite can be interpreted in terms of melt–rock interactions at a late stage of magma evolution, whereas the main mechanism during the evolution of the Juzhou magma was fractionation of plagioclase, biotite, hornblende, apatite, zircon, and allanite. Nd–Hf isotope data suggest that the Dayang and Juzhou granites were both formed from mixtures of Paleoproterozoic basement rock and juvenile material (underplating basalts or Mayuan Group amphibolites), with the Juzhou granite having a greater contribution from juvenile material than the Dayang granite. Our new data, together with existing data, suggest that the tectonic setting of the early Yanshanian (∼143 Ma) highly fractionated I-type Dayang granite was a back-arc that formed in response to the westward subduction of the Paleo-Pacific Plate, and that the late Yanshanian (∼133 Ma) Juzhou granite formed in a continental arc setting in response to roll-back of the Paleo-Pacific Plate towards the coastline. The Mo mineralization in the Makeng ore area was probably the result of the exsolution of molybdenite from the Dayang granitic magmas due to extensive fractional crystallization.
... Although controversy still exists in relation to the petrogenesis of A-type granites, it is generally accepted that the occurrence of A-type granite is commonly associated with an extensional environment, either in post-orogenic or anorogenic settings [18][19][20][22][23][24][27][28][29][30]. ...
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Previous studies have shown that the Archean basement is widely distributed throughout the Yangtze craton. To date, however, Archean basement terrains have not been found, except for a few Kongling high-grade metamorphic terrains in the Huangling dome that have been confirmed to be of Archean age. To further understand the basement component and crustal evolution of the Yangtze craton, we carried out a petrological, geochronological and geochemical study of the Jinshan K-feldspar granite emplaced within the Yangpo Group, located in Huji Town, Zhongxiang City, Hubei Province. Results indicate that the zircon SHRIMP U-Pb age of the Jinshan granite is 2655±9 Ma, placing it within the middle Neoarchean. Chemically, this pluton yields abundant silica and alkalis and is depleted in Ca, Mg and Ti. Furthermore, it is enriched in Rb, Th, Ga, Y and Zr, depleted in Sr, Ba, Nb and Ta, and especially lacking in Eu. High ratios of FeO*/MgO (32.0 to 58.7) and 104×Ga/Al (3.19 to 3.41) were also found. The pluton exhibits characteristics typical of A-type granites with crustal source magmas. Moreover, the meta-sedimentary rock association of the Yangpo Group, into which the pluton intruded, clearly shows relatively stable depositional environments of a shallow shelf sequence. Therefore, before the middle Neoarchean, the Yangtze craton contained mature continental crust. This breakthrough discovery opens a new window on the study of the formation and evolution of the Yangtze craton basement.
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Based on the discussions for the four footstones of granite study ( magma mixing, fractional crystallization, tectonic environment and source of magma) , three major misunderstanding aspects in the granite research are discussed in this paper; (1) improperly imitate the theory for the study of basalts and neglect the complexity of granite; (2) inappropriately interpret the origin of continental granites with the theory of the plate tectonics, which successfully explains the magmatism associated with the plate boundary, but cannot resolve the continental geology problems; (3 ) Overrate geochemical study of granite but ignore the basic geological background. The authors point out that the application of geochemistry in the research of granite should be restricted. The three major misunderstanding aspects resulted from insufficient study on geological background, superficial understanding for the plate tectonics theory and under-evaluation of the complexity of granites. Moreover, this article criticizes the slavish thought in academia and indicates that it makes a huge obstacle to innovative thinking, which is urgent to be solved at present.
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This paper reports new zircon U-Pb age and Hf-isotope, and whole-rock major and trace element data from K-feldspar granites located in Gangmacuo area of Longmu Co-Shuanghu-Lancang River suture zone, central Qiangtang, northern Tibetan Plateau. Combined with high Th/U ratios (0.58-1.05), the zircons from K-feldspar granites show no euhedral crystals and have clear oscillatory zones, indicating a magmatic origin. Zircon LA-ICP-MS dating for K-feldspar granites yields a weighted mean age of 352. 4 ±2. 4Ma, suggesting that the crystallization age of Gangmacuo K-feldspar granites is Early Carboniferous. Petrological and geochemical study show that the intrusion is characterized by high silicon (SiO2 =74. 17%-77. 88%) and low aluminum (Al2O3 = 10. 50%-11. 98%), depleted magnesium (MgO = 0. 23%-0. 36%) and abundant alkali (Na2O + K2O = 5. 74%-7. 24%, Na2O > K2O, K2O/Na2O =0. 53 ∼0. 71, A/CNK =0. 87-1. 06). Enrichment of Zr, Hf, Rb, Th, U and REE, depletion of Sr, Eu, P and Ti with high lOOOOGa/Al (3. 12 ∼4. 14), indicate that the K-feldspar granites are aluminous A-type granite and further classified to A2 type. Their zircons have positive ϵHf(t) values (+4.40-+12. 14) and old second stage Hf mode ages (tDM2 =549 ∼981Ma), indicating that they were generated by mixing of crust and mantle. Gangmacuo K-feldspar granites were formed in a back-arc extensional environment of an active continental margin which was resulted from the subduction of Paleo-Tethys Plate beneath the Qiangbei-Qamdo plate.
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How to study granite in the 21st century is still a problem. The authors hold that the current theory for granite study and the popular parlance and terms should be checked up and reflected. Furthermore, we should absorb the essence, reject the dross and abandon those which are unscientific, unrealistic and unreasonable. For example, we should discard the theory of fractional crystallization, lessen the significance of mixing of granite, restrict the application of the theory of tectonic environment and cancel the terms such as mantle-derived, crust-derived and mixed crust-mande-derived in the study of granite. The authors propose two main questions to be solved in the future research on granite : (1) what is granite? (2) why is it like this? This paper puts forward the following five aspects as the key points for the future work : (1) to carry out case study for typical granite; ( 2 ) to combine the research of granite with that for the ancient crust; (3) to combine the research of physical character of granite with geochemistry of granite ; (4) to carry out comprehensive research on experimental work ; and (5) to establish new theory of "Continental Tectonics". Due to the severe problems existing in current granite research, current theory of granite nearly fall apart, the mansion of granite braced by four footstones is about to collapse. Therefore, the authors suggest a debate should be held for granite study, aiming to clear up thoughts, to recognize problems, to define the direction and to find a way out. The authors advocate to advance the study of granite by strengthening learning, quickening the pace of introducing talents, overcoming the emotion of impatience and boasting and exaggeration in the style of learning and abandoning herd and slavish thought. We should further stop evaluating individuals by SCI and improve related policy.
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The Baoxinggou gold deposit, located in the south margin of upper Heilongjiang Foreland Basin, is discovered rencently and of great ore prospecting potential. Gold ore bodies are mainly hosted inner the shallow intrusion and its contact zone with sandstone formation. The deposit is closely related to the quartz diorite and diorite-porphyrite. Geochemical studies show that the two types of magmatic rocks associated with gold mineralization belong to high-K and calc-alkaline rocks and they are rich in REE, with no obvious Eu anomaly, while with significantly negative Nb, Ta, T, Zr, Yb, Y and positive Rb, Th, K. The LA-ICP-MS U-Pb isotopic dating of single zircons from the quartz diorite indicates that the age of (124.92±1.3) Ma represents the crystallization age of diorite. In light of the age of granite-aplite varying from 150 Ma to 450 Ma, it can be guessed that its diagenesis age is about 156 Ma. The Baoxinggou gold deposit was resulted from the tectonic-magmatic-hydrothermal process associated with the tectonic stress field transition from compression to extension in early Cretaceous period and the gold mineralization took place in 107.5-124.92 Ma. Meanwhile, it can be concluded that there existed gold mineralization companying with the Cretaceous magmatism in the south part of Da Hinggan Mountains. Therefore, the districts developing the Late Yanshanian (Early Cretaceous epoch) magmatic activities, especially the shallow intrusions are important ore-prospecting targets for further exploration.
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The Danghe reservoir intrusive rocks are compositionally comparable to the tonalite-trondjemite-granodiorite association (TTG), which of the principle component is the granodiorite, and have the SiO2 contents ranging from 57.02 to 72.75%. The SHRIMP U-Pb age of zircons from TTG is 440 ± 12Ma (MSWD = 2.5), which is the intrusive age of TTG magma. The concentrations of TiO2, Al2O3, MgO, CaO, FeO and P 2O5 decrease with the increase in the SiO2 contents, showing negative correlations, and implying a magmatic differentiation model for the origin of the TTG controlled by hornblende and plagioclase fractionation. The TTG have lower contents of Σ REE with strong fractionation of LREE/HREE ((La/Yb)N =4. 70-58. 88) and no or slightly Eu anomaly. On Cl chondrite-normalized REE fractional diagram, the subparallel patterns indicate a cogenetic relation for all samples. The concentrations of large ion lithophile elements (LILE) such as Rb, K, Th, Sr are enriched, whereas the high field strength elements (HFSE) (e. g. Nb and Ta) as well as P and Ti contents are depleted. On Rb/30-Hf-3 × Ta and Rb/30-Hf-0. 25 × Nb diagrams,all plots set in the area from island arc type to post collision type. All of these imply that the TTG is of the calc-alkaline association forming in a volcanic arc. The regional geological setting, tectonic setting and formation times of the TTG, and contrasting analysis of igneous rocks from the south and north fringe of the Dunhuang terrain indicate that TTG rocks didn't form in island arc tectonic environment, but was produced by partial melting of the hornblende-rich lower crust under the condition of plate collisions along the northern and (or) southern margins of Dunhuang block in Late Caledonian (440Ma).
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Systematic and comprehensive petrological and geochemical study investigates the formation of eight northeastern Yanshanian granites in the southern margin of North China Block in Xiaoshan Mountain area, China. Petrologically, adamellite is domain. Geochemically, according to major elements, eight granites are silicate and belong to high K-shoshonite series; all granites have similar trace element patterns (including REE), obvious partition of LREE from HREE (La/Yb)N =9.52 ∼ 41. 21 (avg. 26. 16), and weakly abnormal Eu (δ5Eu =0. 82 ∼ 1. 35), and enrich in Rb, Ba, Th, K, Pb, Hf and Y, and deplete in Ta, Nb, Zr, P and Ti. High Sr (392. 8×10-6 ∼. 9×10-6, avg. 678. 8×10-6), low Y (8. 12×10-6 -21.34 × 10-6, avg. 14. 86×10-6) and Yb (0. 503 × 10-6 ∼ 1. 756 × 10-6, avg. 1. 26 × 10 -6) characterize a garnet-bearing magma source in a thick lower crust under North China Block. Initial isotopic Sr ratio (ISr = 0. 70645 ∼ 0. 71022, avg. 0. 70828) and initial epsilon Nd (εnd(t) = - 19. 7 ∼ -3.4, avg. - 14.6) disclose a crustal magma source. The proterozoic Nd depleted mantle model ages (t2DM) of the granites are concentrated in 1827 ∼2372Ma, and present high radiogenic Pb and initial Pb ratios (206 Pb/204 Pb = 17. 728 ∼ 18. 720 (avg. 17. 905); 207Pb/204Pb = 15. 444 ∼ 15. 656 (avg. 15. 544); 208Pb/204Pb = 37. 519 ∼ 38. 707 (avg. 38.187). All isotopic data suggest that the batholith was problebly formed by partial melting of the South Qinling crystalline basement with the participatioin of Taihua Group, Xiong' er Group and mantle materials. When extension environment substitute compression conditions caused by subduction of Yangtze Block with Qinling micro-block under North China Block, delamination of thick garnet-bearing crust and asthenosphere upwelling provide heat to melt the thick bottom crust, and forward, to form magma. This suggests that the crystalline basement under South Qinlin Orogen through detachment in north direction into the crystalline basement of Xiaoshan Mountain area, and the north of Xiaoshan Mountain area probably s the northern margin of East Qinling Orogen.
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The lithology, LA-ICP-MS zircon U-Pb age, major and trace element geochemistry, Sr-Nd-Pb isotope compositions of Dabula pluton from the middle part of the Gangdise granitoid belt are systematically studied in the paper. Results suggest that the rock type is adamellite, and the pluton is composed of middle-fine grained border facies and middle-coarse grained central facies. The two facies yield weighted mean ages of 230.6±4.3 Ma-228.2±3.5 Ma, with the corresponding age of Late Triassic. The rocks are relatively rich in SiO2 and kalium, with SiO2 content between 71.79%-77.27%; relatively high K(w(K2O)=4. 06%-5. 26%) and low Ti(w(TiO2)=0.06%-0.40%) contents; A/CNK varies from 1.16-1.19, displaying strong peraluminous characteristics and sharp negative Eu anomalies (δEu=0.06-0.35). Trace element ratio spider diagram displays apparent enrichments of Rb, Th elements, and marked depletions of Ba, Nb, Sr, P and Ti The Sr-Nd isotopes show the granite bodies have the high initial 87Sr/86Sr ratios (0.712 7-0.720 1) and the negative εNd(t) (-10.6) results. Pb isotope feature shows the enrichment of radiogenic Pb. It is concluded that the Dabula pluton was formed by partial melting of mature crustal materials from Gangdise resulted from the underplating of the subduction-related basaltic magmas during the post-collisional extention in the dynamic background associated with the southward Bangong-Nujiang Tethyan seafloor subduction triggered by the collision between the northern Australia and Lhasa terranes.
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Pashtok intrusive sequence, located in the southwestern margin of the Tarim Block and the northern margin of the West Kunlun orogenic belt, is composed of quartz diorite and quartz monzonitic diorite, which is similar to TTG combination. It was formed in the late Mesoproterozoic. Large quantities of data prove that the strata and rocks located in the northern edge of West Kunlun orogenic belt still retain the records of orogenic events, plate subduction and convergence at the end of Mesoproterozoic, and they confirm that late Mesoproterozoic southwestern Tarim paleoplate belongs to the present active continental margin. Tarim ancient block was a part of Rodinia supercontinent. The early Paleozoic closure of the Kuda Ocean was closely related to the subduction between Kuda crust and Tarim ancient block. At the beginning of geochemical analysis, main elements and trace elements of Pashtok intrusive sequence were studied in detail. A discussion was made concerning the genesis of the intrusive sequence, the tectonic setting and the geodynamic model between plates around the intrusive sequence. The research shows that Pashtok intrusive sequence is of the I-type metaluminous high-K calc-alkaline granodiorite series, which belongs to continental arc granitoids of active continental margin. The intrusive sequence is divided into two intrusive periods. Both periods of intrusive rocks experienced comagmatic evolution, and the parental magma was a mixture of crustal materials and mantle magma, which gradually evolved into more acidic magma. Based on physical environment and tectonic setting in the course of magmatic evolution, the authors have reached the conclusion that the subduction of the Kuda Ocean crust brought about the closure of the Kuda Ocean and the convergence between Tarim ancient block and Qaidam ancient block which were pieced together and gradually became a part of the Rodinia supercontinent.
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Tuchushan copper-iron deposit is hosted in volcanic rocks of the Upper Carboniferous Dikan'er Formation. The genesis of mineral deposit is belong to volcanic hydrothermal metasomatic type. Basalts in ore district are rich in aluminum and sodium, but loss potassium, phosphorus and titanium, indicating that the rocks are calc-alkaline volcanic rock series. The rocks are characterized by enrichment of light rare earth elements, large ion lithophile elements and loss of high filed strength elements. The characteristics of trace elements show that genesis of rocks are related to fluid of subduction, primary magmas have experienced crystallization differentiation of olivine or clinopyroxene and chromite, also suggest basalts and iron orebodies were formed in back-arc basin environment. LA-ICP-MS U-Pb zircon dating on diorite and moyite from ore district yielded concordant ages of 326.2±1.6 Ma and 318.2±2.5 Ma, indicating that they were formed in Carboniferous. Based on the contacting relation of diorite, moyite, iron orebody and volcanic rocks, we hereby confine the age of iron mineralization is similar to the forming time of volcanic rocks of the Dikan'er Formation, which is before the age of diorite(326 Ma), but the age of copper mineralization is appreciably later than 326 Ma.
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Geochronology and petro-geochemistry of garnet-bearing granite from Wulan area of Urad Zhongqi are investigated, and its forming age and tectonic background are discussed. The age of garnet-bearing granite dated by SHRIMP U-Pb of zircons is (256.4±2.2) Ma, indicating that it was formed at the Later Permian. Petro-geochemical characteristics show that the granite is weakly peraluminous I-type ones formed in post-collision tectonic settings and its sources is the greywacke in upper continental crust. The partial melting degree of the source rocks is lower. The granite was derived from interaction of an I-type granitic liquid as a result of strong with hydrothermal fluid during the magmatic stage. According to tectonic settings, forming conditions and emplacement times of garnet-bearing granites, it is referred that the suturing between the Northern margin of North China plateform and the Southern margin of Siberian plate in research area took place before 256.4 Ma.
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Granite samples from Ritu County-Lameila pass area of western Tibet were selected for a detailed geochronological and geochemical analysis to study its petrogenesis. All samples were metaluminous (0. 76 < A/CNK < 1. 0), and had Q + Or + Ab + An + Di (or C) + Hy in CIPW composition. They also showed in right steeply-deviation from LREE to HREE with obviously negative Eu anomaly (δEu =0. 56 ∼0. 99), enriched in Rb, Pb, Th and depleted in K, Ba, especially HFSE (Nb, Ta, Ti). LA-ICP-MS U-Pb zircon ages for moyite, monzogranite and granodiorite are 79. 4 ± 0. 4Ma, 81.0 ±0. 5Ma and 81.3 ±0. 5Ma respectively, indicating that they were all formed in Late Cretaceous. Zircon Hf isotopic compositions of two samples showed positive Hf isotopic initial ratio and had 547. 5 ∼658. OMa, 523. 4 ∼710. 2Ma of two stage model ages respectively. Petrochemistry indicated that primitive material of the granite came from hornblende-enriched lower crust, which were formed by old crust and neo-crust mixing and partial melting in the condition of 700 ∼ 800°C, < 8kbar and fluid-enriched. Such conclusion agreed with the petrology evidence that mafic microgranular enclaves (MME) were found in its outcroup. The granite were formed by granitic magmatisms during the oceanic crust orogenic subducting, as the magmatic response to the Bangong-Nujiang Tethys evolution.
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The Yamansubei granite pluton (YB) is situated in the Late paleozioic Qoltag orogenic belt, eastern Tianshan, Northwest China. Petrographically, the pluton is a K-feldspar granite, which is composed of quartz (25% ∼ 40%), perthite (40% ∼ 70%), plagioclase (10% ∼25%, An = 18 ∼30) and biotite (2% ∼5%). Zircon LA-ICP-MS U-Pb dating shows that the YB pluton intruded at 227. 9 ±0.47Ma of Middle Triassic. Geochemically, the pluton is characterized by high silicon (SiO2 =74.27% ∼75.99%), alkali (ALK =7. 64% ∼8. 29%), potassium (K20 =4. 37% ∼4. 79%) and metaluminous, whereas low titanium, calcium, iron and magnesium, which attributes to high K calc-alkaline granite. The pluton is also rich in LILE (K, Rb, Cs etc.), HFSE (Th, U, Zr, Hf etc.) and depleted in Ba, Sr, Nb, Ta and Y. Chondrite-normalised REE distribution pattern for the YB pluton displays right skewed shapes with slightly negative Eu anomalies (δ5Eu = 0. 48 ∼ 0. 86), indicating fractionation of LREE, and fractionation between LREE and HREE. Zircon in-situ Hf isotopic composition analysis yields the ε hf(t) and tDU2 values of the YB pluton varying + 8. 60 ∼ + 11. 31 and 538 ∼711 Ma, respectively. Integrated studies in petrography, geochemistry and Hf isotope and regional tectonic evolution indicate that the YB pluton resulted from mantle magma underpalting induced partial melting of the Neoproterozoic new-born lower crust. After comprehensive study on previous contributions, this paper suggests that the geodynamic source for Indosinian magmatism n eastern Tianshan could attribute o he nfluence of the Paleo-Tethys ocean ectonic regime.
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The geochemical characteristics and classification of granitic rocks is dependent on the composition of source magmas of granitic rocks. It is illustrated in this paper that it is uncorrect to classify the granitic rocks into mantle-derived, crustal-derived and mixed mantle-crustal derived, because granitic rocks are impossible derived directly from the mantle. Therefore, it is meaningless to calculate the proportion of mantle and crustal component according to a mantle-crust mixing model. The composition of source magmas are the most important factor to control the geochemical character of resulted grantic rocks, degrees of partial melting, pressure, temperatures and fluids are all supplementary, whereas magma mixing and fractional crystallization are two less important factors to the formation of granitic rocks. We propose to classify the source rocks of the granitic rocks into three sources: B source (basalt-derived), C source (continent-derived) and BC source. B source means the source rocks are oceanic crustal rocks derived from highly depleted mantle, C source means they are continental crustal rocks, and BC source means they are between B and C sources, which may have derived from subcontinental enriched mantle and may include intermediate-mafic igneous rocks by partial melting of the metasomatized mantle, or mafic rocks contaminated with continental crust. It is also summarized the diversity of the source rocks of the granitic rocks in China, and indicated that the distribution of grantic rocks in China may be controlled by the compositon of the source rocks.
Article
The geometry and timing of amalgamation of the North China Craton (NCC) have been controversial. The research on the Precambrian Sushui Complex in Zhongtiao Mountain, located in the Trans-North China Orogen, can provide important information for Early Precambrian geological evolution of the NCC. This paper concentrates mainly on the Henglingguan, Xiezhou and Yanzhuang granites, which are the representative components of cala-alkaline granitoids of the Precambrian Sushui Complex. Henglingguan and Xiezhou biotite adamellites are similar in the petrography and geochemistry feature, and have almost same forming age (with a zircon age of 2609 ±31Ma and 2620 ± 14Ma), suggesting that the two granitoid instrusions are the product of the same magmatism Yanzhuang K-feldspar granite was formed in Paleoproterozoic (2351 ± 37Ma). In-situ zircon Lu-Hf isotopic analyses for three granitoids show that their εHf(t) range from -2. 3 to +4.8, +4.4 to +7.6 and -1.8 to + 7.8, and the corresponding two-stage model age are 2791 ∼3222Ma, 2628 ∼2823Ma and 2408 ∼2996Ma, respectively. Through multidisciplinary analysis lithology, litho-chemistry and Hf isotopic of the granites and combined with the setting regional structure, we suggested that the two Neoarchean granitic rocks belong to high-potassium cala-alkaline I-type granites and probably the result of the partial melting of ∼ 2. 7Ga TTG rocks and mafic lower crust. The Paleoproterozoic Yanzhuang granite is typically characterized by low Sr and Yb and similar to Himalayan-type granitic rocks, which is related to the partial melt of continental crust caused by crustal thickening. The paper, based on previous and the author's own research results, summarizes that there is no obvious episodic character of crustal growth of the central North China Craton in the long period of 1.0Ga between ∼2. 8Ga and ∼ 1.8Ga, but displays a feature of small frequency persistent pulsing growth, indicating that the eastern and western NCC and the Trans-North China Orogen should be a unified continental block in the Late Archaean.
Article
In this study, we test the reliability of new classification of granitic rocks based on Sr and Yb concentrations of granitic rocks that we suggested in our previous study (Zhang et al. , 2010a). A large data set of granitic rocks worldwide including those from Qaidam, the North Qilian, the Inner Mongolia, Songpan-Ganze, the Dabei Mountains, the Taihang Mountains, and Paleoproterozoic granitic rocks in the North China and Neoproterozoic granitic rocks in South China Blcok, and those from Turkey, Russia, Brazil, Congo, Bohemia and Italy are used to examine the relationship of the Sr and Yb concentrations of granitic rocks and their formation pressure and depth. The data indicate that the variation of granitic rocks in Sr and Yb concentrations is closely related to the pressure when they formed, which can be used to estimate the variation of the thickness of the crust Such as the Paleoproterozoic granites in the North China by this method to study, not found in central North China Craton occurred collision in the Paleoproterozoic. Therefore, the North China Craton was divided into two blocks need to be further re-understanding. Another example is the the Neoproterozoic granite in South China Block. Our study shows that crust of the South China Block is thin during the Neoproterozoic, and however, it seems irrelevant with rifting-reakup of the supercontinent Rodinia. The data also indicate that the different mineralization associated with granitic rocks is also related to pressure. Cu-Au mineralization is commonly hosted in adakitic-type and Himalaya-type granitic rocks which likely formed from thickened lower crust, whereas W-Sn mineralization is usually hosted in Nanling-type granitic rocks which formed from shallower crust However, such classification based on Sr and Yb concentrations of granitic rocks needs more test in both practice and theory before it can be accepted by confidence.
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The Huashan and Heyu multistage granite complexes occur in the northwestern and southeastern portions of the southern margin of the North China Craton, respectively. The contamination phase monzonitic granite at top of the Hushan complex and the third stage porphyric biotite monzonitic granite of the Heyu complex yield LA-ICP-MS zircon U-Pb weighted average ages of 133. 8 ± 1. 1Ma and 134. 5 ±1. 5Ma, respectively. These two plutons share similar geochemical features. Both of them belong to high K cal-alkaline granites, with SiO2 > 69. 0% , Al2 O3 > 13. 0% , K 2 O + Na2 O > 7. 0% , Na2 O > 3. 2% , and ACNK < 1. 1 ; obvious fractionation between LREE and HREE ; high Sr contents ( mostly Sr >400 x 10 -6 ) and low Y and Yb contents ( Y < 18 x 10 -6 , Yb < 2 x 10 -6 ) ; slightly negative Eu anomaly ( δEu > 0. 67 ) ; and enrichment in LILE and depletion in HFSE ( Nb, Ta and Ti ). This indicates the fractionation of plagioclase + hornblende + garnet + rutile assemblage from the primary magmas or residence during partial melting of magma source. Zircons of two plutons show similar Hf isotopic compositions. The εHf ( t ) values of the Huashan pluton range from -20 to - 18, with tDM2 ages between 2. 1 and 1. 8Ga; while the εHf(t) values of the Heyu pluton from - 17 to - 16, with most tDM2ages of 2.0 -1.7Ga. This suggests that both plutons were formed by partial melting of the thickened lower crust ( 2. 1 - 1.7Ga) . In combination with the regional tectonic evolution, we argue that the crust in the study area was thickened by collisional compression or overthrusting during Jurassic and ealier time, and then the conditions of decompression and geothermal increasing during transition from Jurassic compression to Cretaceous extension result in partial melting of the thickend lower crust, and thereby the collisional transformation-type granites such as the Huashan and Heyu complexes formed at the southern margin of the North China Craton.
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To better understand the Pan-African-early Paleozoic tectonothermal events of the Nyainrong microcontinent and the constraints on its tectonic evolution, here we report the results of zircon LA-ICP-MS U-Pb dating and geochemical features of Amdo gneiss in the Nyainrong microcontinent. The outcrops of Amdo gneiss is about 30 km south of Amdo County in northern Tibet. The field occurrence, mineral composition, textural characteristics, and whole-rock geochemical features of the four gneiss samples indicate the protolith of the gneisses is intermediate-acid intrusive rock. Gneiss zircon trace element tracing and genetic analysis shows that zircon has typical characteristics of magmatic zircon. The 206Pb/238U concordant age of zircon is 505–517 Ma, corresponding to the Middle-Late Cambrian, which is the formation age of the protolith. The samples have characteristics of high silicon, alkali-rich, alkalic rate AR =1.73–3.7, the differentiation index DI = 70.78–90.28; rock aluminum saturation index ranges from 1.02 to 1.05, FeO / MgO ranges from 2.63 to 4.50, 10000 × Ga/Al ranges from 2.12 to 2.41, and P2O5 and Al2O3 content decreased with SiO2 increasing. Th and Y contents have a good positive correlation with Rb content; the genetic type of protolith of the gneiss is the differentiation of subalkaline over aluminum I-type granite. Combined with regional data, the tectonic setting of the Amdo gneiss protolith is closely related to the collision orogenic process. The preliminary view is that the Middle-Late Cambrian magmatic events developed on the microcontinent could be the result of Andean-type orogeny along the Gondwana super-continental margin after the end of the Pan-African orogeny.
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We compiled and analyzed 56 U–Pb zircon ages, including 38 ages obtained during the present study and 18 from other sources, for granitic rocks of the Chaihe–Moguqi region, central Great Xing’an Range, China. Magmatism in this area can be divided into eight stages: Early Devonian (399±3 Ma), Late Devonian (365–358 Ma), Late Carboniferous (322–299 Ma), Early Permian (295–282 Ma), Late Triassic (231–227 Ma), Early–Middle Jurassic (179–172 Ma), Late Jurassic (152–149 Ma) and Early Cretaceous (137–120 Ma). Granites were mainly emplaced during the latter four (Mesozoic) stages of magmatism. Paleozoic granites formed during several stages, and were associated with oceanic subduction and the amalgamation of crustal blocks in the eastern fragment of the Central Asian Orogenic Belt. The Late Triassic is an important period with respect to the change from the Paleo-Asian Ocean tectonic regime to the circum-Pacific tectonic regime. The formation of Late Triassic granites may have been related to post-orogenic lithospheric extension after closure of the Paleo-Asian Ocean. Early–Middle Jurassic granites resulted from either subduction of the Paleo-Pacific Plate or subduction during the amalgamation of the Jiamusi Massif and the Songliao terrane, whereas the intrusion of Late Jurassic–Early Cretaceous granites was associated with both the subduction of the Paleo-Pacific Plate and the closure of the Mongol–Okhotsk Ocean.
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The most commonly used tectonic discrimination diagrams for granites were introduced by Pearce et al. [Pearce, J.A., Harris, N.B.W., Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J. Petrol. 25, 956–983.]. Since then, many studies have shown that some granites defy classification or their geochemical assignment does not fit with the geodynamic environment in which they are thought to have formed. In this paper we evaluate the performance of the Pearce et al. tectonic discrimination method, specifically, the most widely-used Rb-(Y + Nb) diagram, using a new data base of over 250 occurrences worldwide, the tectonic settings of which are fairly well known. We conclude that a correlation of geochemistry and tectonic position exists, but that ambiguities and misclassifications arise from one or both of the following factors. First, complex or polyphase orogeny can mix source rocks of different tectonic provenance. This is common in continental arcs and collisional settings, which can be closely associated in space and time with extensional regimes. Second, differentiation can produce compositional trends which cross field boundaries, especially the VAG to WPG boundary. One can minimize this problem by using less felsic, noncumulate members of cogenetic series.We demonstrate the inherent weaknesses of trace element tectonic discrimination diagrams. Such diagrams are of little use if applied alone, but they can be valuable in combination with other methods such as dating and geologic assessment.
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The South Tien Shan Collisional belt consists of two segments divided by the largest in the Central Asia Talas-Fergan diagonal dextral strike-slip fault. The narrow eastern (Kokshal) segment, where the Late Carboniferous collision was followed by continental subduction of the Precambrian Tarim platform beneath the Caledonian Kazakh continent, is characterized by overthickened (60–65km) crust and ultrahigh-pressure (coesite eclogite) regional metamorphism. The Permian postcollisional magmatism is represented there by a small volume of relatively ‘cool’ two-mica peraluminous leucogranites, moderate volumes of K-rich calk-alkaline (transitional from I- to S-type) granites and several large plutons of A-type rapakivi granites. The latest could have formed by a two-step process involving: (1) syncollisional submersion of the Tarim platform granulitic basement into the mantle accompanied by considerable heating of the source rocks, (2) ‘rapid’ postcollisional exhumation accompanied by decompression and extensive high-degree of melting. In the western (Alay) segment, which is much wider, the collision was not followed by continental subduction. The thickness of crust decreases westward dramatically (from 60 to 45km). The Alay segment is characterized by the high-temperature/low-pressure regional metamorphism and significant volume of Permian shoshonitic/ultrapotassic magmatism, connected with postcollisional strike-slip tectonics. The shoshonitic magmatism is indicative of lithosphere delamination and input of mantle-derived melts and heat into the crust. In contrast to the Kokshal segment, the postcollisional granitic magmatism in the Alay segment is represented by relatively ‘hot’ cordierite- and sillimanite-bearing strongly peraluminous (S-type) granites, locally rooted in the high-temperature/low-pressure metamorphic rocks and large volume of calc-alkaline (I-type) granitoides of mixed (crust-mantle) origin.
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The Camaquã Basin comprises a volcano-sedimentary succession, located in southernmost Brazil, and represents a molasse basin formed at the post-collisional stage of the Brasiliano/Pan-African orogenic cycle in the Neoproterozoic III to Ordovician period. This basin is one of the most well-preserved ancient volcano-sedimentary sequences undeformed and unmetamorphic in the world, dominantly developed on a continental setting under subaerial conditions. It is composed of five major stratigraphic units, four of them with a distinct volcanic character from the bottom to the top, as: (1) Maricá; (2) Bom Jardim; (3) Acampamento Velho; (4) Santa Bárbara; and (5) Guaritas Allogroups. A concise sight of geochemical and isotopic rock data is presented, as well as stratigraphic correlation and description of rock structures and textures that lead to the identification of their genetic processes, the aim of this paper, indicating a relation with a coeval plutonism, and volcanism that evolved from high-K calc-alkaline to shoshonitic and ended with a silica-saturated sodic alkaline magmatism, with a crustal component represented by peraluminous granites. Volcanic deposits from bottom to top are made mostly of volcanogenic sedimentary deposits, succeeded by basic to intermediate lava and pyroclastic flows of shoshonitic affinity, followed by intermediate and acid lava flows and ignimbrites of sodic alkaline affinity. The last volcanic event is represented by basalt pahoehoe flows, probably of mildly alkaline sodic affinity.
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Many I-type granitoid magmas are generated through partial melting of older metaigneous rocks, and the compositions of such melts are broadly calc-alkaline and metaluminous. These melts are granitic to tonalitic, and result from thermal extremes in their lower crustal source regions. Data on the experimental partial melting of common crustal rocks suggest that high-K, I-type granitoid magmas can be derived only from the partial melting of hydrous, calc-alkaline to high-K calc-alkaline, mafic to intermediate metamorphic rocks in the crust. Because of their low K2O contents, metabasaltic rocks of all kinds are unsuitable as sources, and models that propose mixing of mantle-derived basaltic magmas and crustal melts are also inadequate. There is no requirement that I- type calc-alkaline magmatism be related in any way to subduction processes.
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Permian and Lower Triassic igneous rocks from La Pampa province, central Argentina, are part of the Choiyoi Group, whose extension in Argentina exceeds 500,000 km². In La Pampa, the distribution of these outcrops occurs along a NW–SE belt that cuts obliquely across the N–S structures of the Lower Paleozoic rocks. The basement of the Choiyoi Group in western La Pampa consists of Mesoproterozoic to Lower Paleozoic rocks that form part of the exotic Cuyania terrane. In central La Pampa, the basement consists of Lower Paleozoic igneous and metamorphic rocks affected by the Lower Paleozoic Famatinian orogeny.
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Here we present the results of dehydration melting, melt morphology and fluid migration based on the dehydration melting experiments on natural biotite plagioclase gneiss performed at the pressure of 1.0–1.4 GPa, and at the temperature of 770–1028°C. Experimental results demonstrate that: (i) most of melt tends to be distributed along mineral boundaries forming “melt films" even the amount of melt is less than 5 vol%; melt connectivity is controlled not only by melt topology but also by melt fraction; (ii) dehydration melting involves a series of subprocesses including subsolidus dehydration reaction, fluid migration, vapor-present melting and vapor-absent melting; (iii) experiments produce peraluminous granitic melt whose composition is similar to that of High Himalayan leucogranites (HHLG) and the residual phase assemblage is Pl+Qz+ Gat+Bio+Opx±Cpx+Ilm/Rut±Kfs and can be comparable with granulites observed in Himalayas. The experiments provide the evidence that biotite plagioclase gneiss is one of source rocks of HHLG and dehydration melting is an important way to form HHLG and the granulites. Additionally, experimental results provide constraints on determining the P-T conditions of Himalayan crustal anatexis.
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New analyses of 131 samples of A-type (alkaline or anorogenic) granites substantiate previously recognized chemical features, namely high SiO2, Na2O+K2O, Fe/Mg, Ga/Al, Zr, Nb, Ga, Y and Ce, and low CaO and Sr. Good discrimination can be obtained between A-type granites and most orogenic granites (M-, I and S-types) on plots employing Ga/Al, various major element ratios and Y, Ce, Nb and Zr. These discrimination diagrams are thought to be relatively insensitive to moderate degrees of alteration. A-type granites generally do not exhibit evidence of being strongly differentiated, and within individual suites can show a transition from strongly alkaline varieties toward subalkaline compositions. Highly fractionated, felsic I- and S-type granites can have Ga/Al ratios and some major and trace element values which overlap those of typical A-type granites.A-type granites probably result mainly from partial melting of F and/or Cl enriched dry, granulitic residue remaining in the lower crust after extraction of an orogenic granite. Such melts are only moderately and locally modified by metasomatism or crystal fractionation. A-type melts occurred world-wide throughout geological time in a variety of tectonic settings and do not necessarily indicate an anorogenic or rifting environment.
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The post-collisional Saraycık granodiorite intruded into a late Paleocene to early Eocene nappe pile that formed during collision of the Pontides in the North and the Anatolide-Tauride platform in the South, leading to the formation of the İzmir-Ankara-Erzincan suture. A relatively shallow pluton intrusion depth (∼5 to 8km) was estimated from Al-in-hornblende geobarometry and contact metamorphic assemblages. The emplacement age is tightly constrained to ∼52Ma by two Ar–Ar plateau and total fusion ages on biotite. The main mass of the pluton consists of metaluminous to peraluminous biotite granodiorite and hornblende-biotite granodiorite. In addition, up to 10-m thick dacitic and <25-cm thick aplitic dikes occur. Granodiorites and dacites show many close compositional similarities to high-silica adakites from supra-subduction zone settings, but tend to be slightly more felsic and to have a higher aluminium saturation index. Chondrite-normalized (cn) rare earth element patterns are characterized by high ratios of (La/Yb)cn, concave-upward shapes of the HREE and a lack of significant Eu anomalies. In conjunction with relatively high abundances of Ba and Sr as well as low abundances of Y, HREE and Sc, these patterns suggest a feldspar-poor, garnetamphibole-rich fractionating mineral assemblage (residue). All samples have very similar Nd–Sr isotopic characteristics, regardless of rock type. Initial εNd values range from −0.3 to −1.2 and initial 87Sr/86Sr ratios from 0.70491 to 0.70529. It is suggested that the magmas formed by partial melting of mafic lower crust at elevated pressures (∼1 to 2GPa).
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Here we present an insight into the genesis of Himalayan granulitic lower crust based on the experimental studies on the dehydration melting of natural biotite plagioclase gneiss performed at the temperatures of 770–980°C and the pressures of 1.0–1.4 GPa. The experiments produce peraluminous granitic melt and residual phase assemblage (Pl+Qz+Gat+Bio+Opx±Cpx+Ilm/Rut±Kfs). The residual mineral assemblage is similar to those of granulites observed at the eastern and western Himalayan syntaxises, and the chemical compositions of characteristic minerals-garnet and pyroxene in the residual phase and the granulite are identical. Additionally, the modeled wave velocities of the residual phase assemblage are comparable well with those of the top part of lower crust beneath Himalayas. Hence, we suggest that (1) the top part of lower crust beneath Himalayas is probably made up of garnet-bearing intermediate granulite; (2) the formations of granulite and leucogranites in Himalayas are interrelated as the results of crustal anatexis; and (3) dehydration melting of biotite-plagioclase gneiss is an important process to form granulitic lower crust, to reconstitute and adjust the crustal texture. Moreover, experimental results can provide constraints on determining the P-T conditions of Himalayan crustal anatexis.
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In NW Iran, about 30 subvolcanic porphyritic dacitic to rhyodacitic domes (1–5 km2) are intruded into a variety of rock sequences from Permian to Early Miocene in age. These subvolcanic domes occur along the North Tabriz, North Misho and Darediz dextral faults in the northern part of the Urumieh-Dokhtar magmatic arc (UDMA) of Iran. The UDMA contains intrusive and extrusive rocks of Eocene-Quaternary age. Geochemical data indicate that the subalkalic dacitic to rhyodacitic rocks have an adakitic composition with Na2O/K2O > 1, high Sr (346–737 ppm), Mg# = 0.48 and low Y (10–20 ppm) and HREE. Fractionated REE patterns, (Ce/Yb)N = 9–76, absence of negative Eu anomaly, low content of Y, Nb, Ti, and high Sr/Y (20–58) and (Ce/Yb)N ratios suggest that the source was probably amphibole-eclogite or garnet-eclogite, possibly generated during subduction of the Neo-Tethyan oceanic slab beneath the Central Iran microplate. The adakitic volcanism was followed by eruption of alkaline magmas including ultrapotassic, shoshonitic, and lamprophyric volcanic rocks. Slab melting occurred after cessation of subduction, possibility from the detached slab. Transtensional tectonics accompanied by a locally extensional stress regime may account for magma genesis and ascent.
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Strongly peraluminous (SP) granites have formed as a result of post-collisional processes in various orogens. In `high-pressure' collisions such as the European Alps and Himalayas, post-collisional exhumation of overthickened crust (>50 km), heated by radiogenic decay of K, U and Th during syn-collisional thickening, produced small- to moderate-volume, cool (<875°C) SP granite melts with high Al2O3/TiO2 ratios. In `high-temperature' collisions such as the Hercynides and Lachlan Fold Belt (LFB), there was less syn-collisional crustal thickening (≤50 km). Crustal anatexis was related to post-collisional lithospheric delamination and upwelling of hot asthenosphere, forming large-volume, hot (≥875°C) SP granite melts with low Al2O3/TiO2 ratios. Both clay-rich, plagioclase-poor (<5%) pelitic rocks and clay-poor, plagioclase-rich (>25%) psammitic rocks have been partially melted in high-pressure and high-temperature collisional orogens, with the pelite-derived SP granites tending to have lower CaO/Na2O ratios (<0.3) than their psammite-derived counterparts. The predominance of pelite-derived SP granites in the Himalayas and psammite-derived SP granites in the LFB suggests that mature continental platforms made up more of the accreted crust in the Himalayan collision than in the LFB.
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Analyses for Ti, Zr, Y, Nb and Sr in over 200 basaltic rocks from different tectonic settings have been used to construct diagrams in which these settings can usually be identified. Basalts erupted within plates (ocean island and continental basalts) can be identified using a Ti-Zr-Y diagram, ocean-floor basalts, and low-potassium tholeiites and calc-alkali basalts from island arcs can be identified using a Ti-Zr diagram (for altered samples) and a Ti-Zr-Sr diagram (for fresh samples). Y/Nb is suggested as a parameter for indicating whether a basalt is of tholeiitic or alkalic nature. Analyses of dykes and pillow lavas from the Troodos Massif of Cyprus are plotted on these diagrams and appear to the tholeiitic ocean-floor rocks.
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Abundant high-K calc-alkaline (HKCA) magmatism appears to be post-collisional and often shifts to shoshonitic or alkaline–peralkaline compositions in the final stages of orogeny. The nature and the causes of this transition are studied on the basis of 308 major element and of 86 unpublished trace element (including REE) analyses of the Pan-African granitoids from the Tuareg shield (Adrar des Iforas, Mali and Aı̈r, Niger). This database covers a wide variety of magmas from subduction-related to intraplate-type including abundant HKCA batholiths. Literature data from geodynamically well-constrained cases are also included. In addition to a conventional geochemical approach of the studied magmatism, the sliding normalization method is proposed. This tool aims at comparing magmatic series: each studied rock is normalized to the interpolated composition of the reference series that has the same SiO2 content as the sample. This method amplifies differences in sources and in fractionation processes and allows comparison of rocks from basic to acid composition. Two distinct juvenile sources are proposed: a previously enriched phlogopite-K richterite bearing lithospheric mantle or a lower juvenile crustal equivalent for HKCA-shoshonitic magmas, and a lowest lithospheric-upper asthenospheric OIB-type mantle for alkaline-peralkaline magmatism. The first source is melted only shortly after its generation when the lithosphere was still hot, which restricts HKCA magmatism mainly to post-collisional settings. The second asthenospheric/lowest lithosphere source is by definition close to its melting temperature and can generate magma ubiquitously both in space and time. The main melting triggers are lithospheric major structures which are not only operative in a post-collisional setting but also in other environments such as intraplate setting. Geochemistry thus gives indications about the nature of the source and on geotectonic settings. However, the latter is a second rank information, which is partly model-dependant. The post-collisional period differs from other settings by a propensity to generate large amounts of magma of various kinds, among which HKCA magmatism is volumetrically the most prominent.
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It has been well accepted that basaltic magmas are derived from the mantle and most granites are generated by fractional crystallization of basaltic magmas. The evidence for the fractional crystallization of basaltic magmas and even andesitic magmas is there is corresponding cumulate, however, cumulate is rarely associated with Si-rich granitic magmas, therefore, it is unlikely that fractional crystallization happened in granitic magmas. This is probably because (1) granitic magmas have higher viscosity such that the crystallization of minerals ( i. e., plagioclase) have been obstructed to form euhedral crystals and high density minerals (i. e., hornblende) have been blocked to settle down; and (2) primary minerals (e. g., plagioclase) have similar density to that of granitic magmas. We argue that plagioclase is unlikely fractionally crystallized from granitic magmas although the process was reported in numerous literature on the basis of the Harker diagrams. We argue that the Harker diagrams are more suitable for basalts rather than granites because granites are totally different from basalts in magma source and composition. We also argue that the Bowen reaction series, the principal of the Harker diagrams, is unlikely happened to form a continuous sequence from basalt through andesite and dacite to rhyolite. The unit and super-unit mapping method of granites is not suitable for regional geological mapping. Mesozoic granites in East China and Tanncherfi granites in Morocco are taken as two examples to indicate the fractional crystallization of granitic magmas is impossible. Variable compositions of granites mainly result from the composition of magma source, temperature, pressure, volatile composition, degrees of partial melting, mixing and differentiation process. Among them the composition of magma sources is the most crucial to the diverse compositions of granite whereas fractional crystallization of granitic magmas has rarely effect on the compositional variation of granites. Therefore, the role of fractional crystallization in the evolution of granite should be appreciated in a limited scale rather than a large scale.
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How granitic magmas are mixed has become an interesting topic in recent years. Many workers are trying to interpret the variation of the composition of mixing granites with the different proportion of several end-members. However, we argue that such kind of mixing process for granites is uncommon and is unlikely happened in the field on the basis of a comparison of the properties of granite and basalt. The ability or competence that two granitic magmas can be mixed mainly depends on the viscosity and temperature of the magmas. The viscosity is related to the structure of SiO4 tetrahedron in the rocks. Granites in comparison with basalts have relatively high SiO2 contents and low temperature, and therefore granitic magmas have low competence to be mixed with each other. Unlike the mixing process of basaltic magmas, granitic magmas are more likely mingling, rather than mixing, with each other. There is rare example for the mixing of granitic magmas, the only case is high-Mg andesite or high-Mg adakite formed by mixing of adakite and upwelling mantle. Many workers argued that mafic microgranular enclaves (MME) in granites are the direct evidence for the mixing of granitic magmas. However, we argue that it is an exact evidence for that granitic magmas are difficult to be mixed with these mafic enclaves. Granitic rocks are actually heterogeneous in compositions because they have derived from heterogeneous sources with complicated melting processes. It is thus unlikely to assume the original granites are homogeneous. The variation of compositions of granites and the correlation of elements in Harker diagrams are mainly the results of heterogeneity of sources, rather than mixing or differentiation processes. Usually magmas tend to be homogeneous by mixing in magma conduits on the way upwelling to emplacement and at the same time they may become heterogeneous with the crustal contamination. However, grantic magmas are hard to be as homogeneous as basaltic magmas in this process because of their higher viscosity than that of basaltic magmas. Therefore, not only geochemical compositions of granites but also their physical properties should be considered in the study of the origin of granites, and it should be cautious when using geochemical data to discriminate the origin and tectonic environment of granites.
Article
Badaling granitoid complex in the Beijing area is a typical example of Yanshanian intrusions. According to the geochemical characteristics and genesis, the complex can be divided into three groups. The first group is composed of gabbro-diorites. The rocks are characteristics of enriched Fe, Ti and P, Th/Ta ≈ 1 (0.7 ∼ 1.2), and low Isr ratios (0.705 on the average) and low εNd (t) values (-8 ∼ -11), probably formed in the interplate environment. The gabbro-diorites are resulted from partial melting of subcontinental lithospheric mantle, probably representing the underplating basaltic magma below the thickened continental crust during Yanshanian period. The second group is the main body of Badaling complexe, consisting of quartz diorite, granodiorite and adamellite. The chemical compositions show that they have SiO2> 57%, K2O> 2. 7%, Na2O/K2O = 0. 9 ∼ 1.7, Al2O3 between 16 and 14%, LREE enrichment with (La/Yb)N = 24 ∼ 41, HREE depletion with Yb<1.32μg/g, no or small negative Eu anomaly (Eu/Eu* = 1.0 ∼ 0.9), high Sr content (354 ∼ 1191μg/g) and low Y content (<16μg/g) with high Sr/Y ratios (45 ∼ 156). Except for relatively high K2O and low Al2O3 content, they are similar to adakite in composition, suggesting that the magma is equilibrated with garnet in the deep and is derived from the bottom of thickened continental crust of North China block during Yanshanian period, probably the products of partial melting of intermediate-basic granulite in the lower crust. The third group is composed of adamellite and quartz monzonite. They belong to A-type granite and are characteristic of Na2O+K2O>9%, Sr and Ba depletion and Rb and LREE enrichment, with relatively large negative Eu anomaly (Eu/Eu* = 0.4 ∼ 0.5). Due to the formation of adakite-like rocks of the second group, the lithospheric delamination occurs and cause asthenosphere to upwell to the bottom of thinned continental crust. The granotoid rocks of the third group is therefore formed by the partial melting of the materials in the crust-mantle transformation belt. The Yanshanian magmatism in Badaling area is not related to the subduction of paleo-Pacific Plate, but to the extension within the continent plate.
Article
The thesis is developed that different types of granites, of different origin, typify different kinds of mobile belts. By using several critical geochemical parameters, it is possible to define an M-type which includes the scanty plagiogranite of the oceanic island arcs, and which grades into the I (Cordilleran)-type representing the voluminous gabbro-quartz diorite-tonalite assemblage of active continental plate edges. The latter is separate, however, from an I (Caledonian)-type representing granodiorite and granite of post-orogenic uplift regimes. In sharper contrast are an S-type, incorporating the peraluminous granite assemblage of encratonic and continental-collision fold belts, and a unique A-type, which includes alkali granites both of the stabilized fold belts and the swells and rifts of the cratons. This close relationship between granite type and geological context occurs because granite, in the widest sense, arises as the end-stage of several processes involving source rocks, each process and source being appropriate to a particular environment. A review of the occurrence of granite in Phanerozoic mobile belts supports this connection, with a particularly clear contrast between the granitic rocks of the Mesozoic Andean, the Upper Palaeozoic Hercynian and the late-Caledonian regimes.-R.A.H.
Article
Rocks with the geochemical characteristics of melts derived directly from subducted lithosphere are present in some modern island and continental arcs where relatively young and hot lithosphere is being subducted. These andesites, dacites, and sodic rhyolites or their intrusive equivalents are usually not associated with parental basaltic magmas. It is shown here that the trace-element geochemistry of these magmas is consistent with a derivation by partial melting of the subducted slab, and in particular that subducting lithosphere younger than 25 Myr seems to be required for slab melting to occur.
Article
In the context of Gondwana North Victoria Land forms the Antarctic conjugate terrain to the East Australian Lachlan Fold Belt, where the distinction between S- and I-type granitoids was first worked out (Chappell &White 1974). Thus the area was considered a good testing ground for the hypothesis when the German Antarctic North Victoria Land Expedition (Ganovex) resumed fieldwork there in 1979/80. It was known at that time that there existed a Cambro/Ordovician and a Devonian/Carboniferous granitoid generation, which, in analogy to Australia, were taken to be related to two different orogenetic events. Our geological, petrographical and geochemical investigations, together with radiometric age dating, revealed that this is true only for the older granite generation. The younger generation is in fact anorogenic. At the same time, it became obvious that the S- and I-type classification did not completely fit the observed field data. At this pointPitcher's (1982) subdivision of the I-types into a »Cordilleran« and a »Caledonian« suite offered a solution to account for the observed irregularities. At about the same time, plate tectonic models based on evidence independent from the granite classification were developed for this part of the active Gondwana margin. This offers the opportunity to crosscheck the tectonic environment derived from the granite classification: The characteristics of the older, Cambro/Ordovician granitoids allow a perfect accommodation in the model of an active margin above a subduction zone as derived from other evidence. For the younger (Devonian/Carboniferous) granitoids, however, the postulated tectonic setting (post-collisional uplift and faulting) could not be verified in North Victoria Land. It is concluded thatPitchers classification is applicable in its petrographic and geochemical aspects but that the tectonic environment postulated for the production of »Caledonian« I-type granitoids may not be the same in all investigated areas.
Article
The two main rocks in the area require separate sources. The approx 572 m.y.-old biotite-hornblende monzogranite shows both LIL-enrichment and HREE-depletion, characteristic of a calc-alkaline magma derived from within a hydrated mantle wedge. The approx 534 m.y.-old aegirine-arfvedsonite peralkaline granite is enriched in high-field-strength (HFS) elements and depleted in Sr, Ba and Eu, and a F-bearing fluid phase is necessary. The resulting metasomatic rock types include an aplitic rim with a high content of HFS elements which is a potential economic source of U, and a red granite with varying abundances of trace elements and strong Eu depletion. The peralkaline granite is typical of an intra-plate tectonic environment with trace elements controlled by volatiles, probably mantle-derived.-R.E.S.
Article
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.
Article
Three southern Nova Scotia plutons crystallized rapidly at ∼375 Ma from magma containing both mantle and crustal components. Isotopic and chemical data suggest that the crustal contribution included both lower crustal material and Cambro-Ordovician turbidites of the host Meguma Group. Despite local evidence of mixing and mingling of magmas, the bulk of the plutons evolved by assimilation and fractional crystallisation. Evolved portions of the plutons have compositions appropriate for development of rare-metal pegmatite fields, but pegmatites are relatively rare and little differentiated. Like parental plutons, pegmatites fall into biotite+plagioclase and muscovite+potassium feldspar assemblages. The latter locally contain Mn-rich garnet+biotite, giving calculated P–T conditions of pegmatite crystallisation of ∼620°C, 0.44 GPa under water-saturated conditions. Host rocks at the time of emplacement experienced P–T conditions varying from
Article
Granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la Roche and his coworkers. This shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas. Possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors. These suggest that most granitoid series result from a two-stage process. First, fractional crystallisation of clinopyroxene, olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition. This magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series.
Article
The mafic granulites in Dinggye, as various scale lense-shaped enclaves within the high Himalayan crystalline rock series, occur along mylonitic foliations at the junction between the Southern Tibetan Detachment System (STDS) and the Xainza-Dinggye normal fault system. The main lithological assemblage comprises garnet plagioclase pyroxenite, garnet two-pyroxene granulite, pyroxene garnet amphibolite and so on. The detailed petrological analyses show that these mafic granulites underwent at least four-stage metamorphic evolution. The first metamorphic stage, the garnet+clinopyroxene+quart mineral assemblage (M1) was probably formed under eclogite facies, the second stage, the plagioclase+clinopyroxene symplectite mineral assemblage (M2) was produced under high-pressure granulite facies by the early decompressive breakdown of M1 mineral assemblage, the third stage, the plagioclase+clinopyroxene+ hypersthene symplectite mineral assemblage (M3) was formed at granulite facies by the late period decompressive breakdown of M1 and M2 mineral assemblages and the final stage, pla-gioclase +hornblende mineral assemblage (M4) was formed by hydrolysis of earlier mineral assemblages during late uplifting. The detailed mineral composition analyses suggest that garnets and clinopyroxenes within M1 and M2 mineral assemblages display similar compositions to the equivalents in the B and C types of eclogites, whereas the M3 clinopyroxenes are akin to these of the same kind of minerals in the granulite. These mineral chemistry features and P-T estimates calculated by mineral thermometers and barometers indicate that the early stage relic porphyroblasts (M1) could be formed at the eclogite facies, the early decompressive breakdown (M2) occurred at the high-pressures granulite facies of 1.35–1.48 GPa and 625–675°C, the M3 mineral assemblage recorded the granulite facies of 0.7–0.95 GPa and 775–900°C and M4 plagioglase+hornblende retrograde mineral assemblage was produced under the amphibolite facies metamorphism with pressure of 0.4 to 0.75 GPa and temperature at between 660 and 700°C These construct P-T paths from crustal subduction overthickening to tectonic uplift tectonothermal evolution. The mineral chemical characteristics and P-T condition at every metamorphic stage of these granulites indicate that these rocks experienced the eclogite facies metamorphism during the early stage. Subsequently, these mafic granulites underwent the three-stage exhumation of the eclogite facies tectonic uplift, isostatic uplift related to the transformation from eclogite/high-pressure granulite to granulite facies and extensional uplift.
Article
Previous studies have demonstrated that partial melting of hydrated basalt at lower crustal to upper mantle pressures is capable of generating up to 40% high-SiO2 liquids. It is apparent that the initiation of melting coincides with the beginning of amphibole dehydration but that a broad reaction interval over which amphibole coexists with high-SiO2 liquids is defined by the wet basalt solidus on one side, and the amphibole-out phase boundary on the other. The phase relations of an alkali-rich tholeiitic metabasalt have been examined in the outer half of this region, up to and beyond the amphibole-out phase boundary. Results indicate that amphibole exerts a strong control over the amount and composition of coexisting liquid over this interval. Physical and chemical criteria suggest that efficient melt segregation leading to TTG plutonism requires more than 20-30% batch melting of a garnet-bearing basaltic protolith, near or beyond the amphibole-out phase boundary. Lower degrees of melting may involve critical melting. -from Author
Article
The Capoas intrusion is a metaluminous, high-K calc-alkaline, I-type biotite granite emplaced within Permian-Jurassic sedimentary rocks of the North Palawan Continental Terrane (NPCT) in the western Philippines. The NPCT is a fragment of the Mesozoic Andean-type margin of southeast China that was separated from the mainland during the late Oligocene-early Miocene opening of the South China Sea. Zircons from the granite have xenocrystic cores, and form a discordant array with a lower intercept age of 15 (+ 3/ −4) Ma. Monazites have concordant 207Pb235U ages with a mean age of 13.4 (± 0.4) Ma. The late middle Miocene age and the location of the pluton in the NPCT uniquely constrain the formation of the Capoas granite in a post-rifting, non-collisional tectonic setting unrelated to any subduction zone. The major and trace element geochemistry of the granite and the presence of apparently Proterozoic xenocrystic zircon indicate that the pluton is composed largely, if not entirely, of older continental crust. The only viable heat source for crustal melting and/or assimilation was widespread basaltic magmatism that occurred in the area following cessation of seafloor spreading in the South China Sea in early Miocene time. The geochemical affinity of the Capoas granite with calc-alkaline magmatic arc and collisional granites is therefore a function of the source rocks that were melted to produce the granite rather than the specific tectonic setting in which the granite was generated. The calc-alkaline source rocks most likely formed in the Mesozoic Andean-type margin of south China and subsequently underwent partial melting in late middle Miocene time in an ‘anorogenic’ setting.
Article
Supra-subduction zone (SSZ) ophiolites have the geochemical characteris- tics of island arcs but the structure of oceanic crust and are thought to have formed by sea-floor spreading directly above subducted oceanic lithosphere. They differ from 'MORB' ophiolites not only in their geochemistry but also in the more depleted nature of their mantle sequences, the more common presence of podiform chromite deposits, and the crystallization of clinopyroxene before plagioclase which is reflected in the high abun- dance of wehrlite relative to troctolite in their cumulate sequences. Most of the best- preserved ophiolite complexes in orogenic belts are of this type. Geological reconstructions suggest that most SSZ ophiolites formed during the initial stages of subduction prior to the development of any volcanic arc. Evidence from these ophiolites suggests that the first magma to form in response to intra-oceanic subduction is boninitic in composition, derived by partial melting of hydrated oceanic lithosphere in the 'mantle wedge'. As subduction proceeds, the magma composition changes to island-arc tholeiite, probably because the hydrated asthenosphere of the 'mantle wedge' eventually becomes the dominant mantle source. Other SSZ ophiolites formed in the early stages of back-arc spreading following splitting of a pre-existing arc. Nonetheless the more common mechanism for formation of SSZ ophiolites appears to have been pre-arc rather than back-arc spreading.
Article
The intracratonic Bushveld igneous province that formed 2.1–1.9 Ga ago contains three contrasting suites of siliceous rocks that are demonstrably of magmatic origin. The oldest of these are the high-Mg (HMF) and low-Mg (LMF) felsites, which form interstratified flows in the Rooiberg Group. Bushveld granites intrude the Rooiberg Group and constitute the youngest component of the province.Well-defined interelement variation trends illustrate that the granites do not share the same petrogenetic history as the Rooiberg magmas. Nd isotope measurements indicate that the two eruptive suites probably formed under similar differentiation conditions but from parental magmas that were derived from compositionally different sources. On trace-element discrimination diagrams, the Bushveld granites and LMF are usually correctly assigned to a within-plate setting but, conversely, the HMF are generally misclassified as subduction-related eruptives. It is argued that the trace-element signatures of the granites and felsites do not identify their tectonic setting per se, but rather point to the melting and crystallization histories of the source regions from which their parent magmas were extracted. As such, tectonic discrimination diagrams may provide valuable pointers to processes that have affected igneous source materials in much earlier magmatic cycles.
Article
The granitoid rock dominated central Wabigoon subprovince of the Superior Province records low-K trondhjemite–tonalite–granodiorite (TTG) type magmatic episodes at <2.83–2.74 and 2.722–2.709 Ga, and high-K mafic to felsic plutonism at 2.690–2.685 Ga. High-K units consist of granite to granodiorite dykes and sills, a K-feldspar megacrystic granodiorite suite of sanukitoid affinity and a suite of mafic dykes and intrusions. Initial ϵNd values (−3.1 to +3.3) indicate variable input to all units from light REE-enriched older crustal materials. The δ18O (VSMOW) range of felsic compositions (+7.1 to +8.9%) overlaps closely that of average upper Superior Province crust. The granite/granodiorite units probably received melt components derived from both older tonalitic crust and isotopically juvenile supracrustal material. The thermal flux for partial melting was provided by mafic components of the coeval megacrystic granodiorite suite. This latter suite likely formed by extensive crustal assimilation and fractionation of enriched-mantle-derived high-Mg dioritic magmas in a post-collisional setting, possibly resulting from slab breakoff or broader scale lithospheric delamination. A genetic link is inferred between mafic magmatism and the late- to post-tectonic high-K granitoid magmatism that typically represents the last stabilization event within Superior subprovinces. That crustal recycling processes played a major role in the petrogenesis of central Wabigoon high-K granitoid suites is consistent with other evidence that supports repeated and substantial continental recycling within this subprovince as far back as the Mesoarchean.
Article
NE China is the easternmost part of the Central Asian Orogenic Belt (CAOB). The area is distinguished by widespread occurrence of Phanerozoic granitic rocks. In the companion paper (Part I), we established the Jurassic ages (184-137 Ma) for three granitic plutons: Xinhuatun, Lamashan and Yiershi. We also used geochemical data to argue that 0these rocks are highly fractionated I-type granites. In this paper, we present Sr-Nd-O isotope data of the three plutons and 32 additional samples to delineate the nature of their source, to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the Phanerozoic. Despite their difference in emplacement age, Sr-Nd isotopic analyses reveal that these Jurassic granites have common isotopic characteristics. They all have low initial 87Sr/86Sr ratios (0.7045 ± 0.0015), positive εNd(T) values (+1.3 to +2.8), and young Sm-Nd model ages (720-840 Ma). These characteristics are indicative of juvenile nature for these granites. Other Late Paleozoic to Mesozoic granites in this region also show the same features. Sr-Nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two-stage processes: (1) formation of parental magmas by melting of a relatively juvenile crust, which is probably a mixed lithology formed by pre-existing lower crust intruded or underplated by mantle-derived basaltic magma, and (2) extensive magmatic differentiation of the parental magmas in a slow cooling environment. The widespread distribution of juvenile granitoids in NE China indicates a massive transfer of mantle material to the crust in a post-orogenic tectonic setting. Several recent studies have documented that juvenile granitoids of Paleozoic to Mesozoic ages are ubiquitous in the Central Asian Orogenic Belt, hence suggesting a significant growth of the continental crust in the Phanerozoic.
Article
Experiments were conducted on a natural basalt (with 5 wt.% added H2O) at 1.0–2.5 GPa and 900–1100°C. Experimental products include partial melts (quenched glasses) + residual mineral assemblages of amphibolite or eclogite. Electron microprobe and LAM-ICP-MS were used to determine major and trace element compositions of these quenched melts, respectively. Major element compositions of all the melts are tonalitic-trondhjemitic, similar to adakite. Their trace element characteristics are controlled by coexisting residual minerals. Signatures of adakite such as high Sr/Y, low HREE and negative Nb-Ta anomaly, etc. are present only in the melts coexisting with residual assemblages containing rutile and garnet (rutile-bearing eclogite or rutile-bearing amphibole-eclogite). Garnet leads to HREE depletion in melts, whereas rutile controls Nb and Ta partitioning during the partial melting and causes negative Nb-Ta anomaly in melts. Therefore, in addition to garnet, rutile is also a necessary residual phase during the generation of adakite or TTG magmas to account for the negative Nb-Ta anomaly of the magmas. The depth for the generation of adakite/TTG magmas via melting of metabasalt must be more than about 50 km based on the approximate 1.5 GPa minimum-pressure for rutile stability in the partial melting field of hydrous basalt.
Article
Synthesis experiments were conducted on a natural basalt (with 2 or 5 wt.% H2O added) at 1.0–2.5 GPa and 900–1100 °C to investigate the stability field of rutile and rutile/liquid HFSE partitioning during partial melting of hydrous basalt. The basalt chosen has TiO2 content close to average N-MORB. 100 ppm of Ta, Nb, Hf, Zr, etc., were added to the starting composition in order to improve analytical precision with the LAM-ICP-MS and the electron microprobe.Rutile occurs in the partial melting field of hydrated basalt at pressures higher than approximate 1.5 GPa, depending on H2O content and bulk composition (especially TiO2 and K2O). Its stability increases with increasing pressure and decreasing temperature. H2O helps produce a more mafic melt and so results in dissolution of rutile and shrinkage of the P–T field of rutile crystallization.The rutile/melt partitioning results confirm previous observations [ [22], [27], [12] and [54]], including that rutile is a dominant carrier for Nb and Ta, and that rutile favours Ta over Nb with DNb always lower than DTa for each rutile/melt pair. In addition our experiments demonstrate that both DNb and DTa decrease with increasing H2O content but increase with decreasing temperature.Rutile is a necessary residual phase during the generation of Archean tonalite– trondhjemite–granodiorite (TTG) magmas to account for the negative Nb–Ta anomaly of the magmas. The depth for TTG production via melting of subducted oceanic crust must be more than 45–50 km based on the approximate 1.5 GPa minimum pressure for rutile appearance. Rutile fractionates Nb from Ta and will result in slightly higher Nb/Ta in coexisting liquids. Archean TTG magmas with subchondritic Nb/Ta must, therefore, have been derived from low Nb/Ta source regions [cf. Rapp, R.P., Shimizu, N., Norman, M.D., 2003. Growth of early continental crust by partial melting of eclogite. Nature 425, 605–609] unless alternative magmatic processes have lowered the Nb/Ta ratio. Also rutile-bearing residues should display lower Nb/Ta after TTG liquids are extracted. Hence, the present data do not support the view that subducted rutile-bearing eclogitic oceanic crust is a superchondritic Nb/Ta reservoir on Earth.
Article
High-pressure (HP) leucogranulites of the Bohemian Massif are interpreted as the metamorphosed equivalents of HP leucogranites produced by deep crustal melting. This is supported by their preserved mineral assemblages (Grt-Ky-mesoperthite), bulk rock chemistry, P-T estimates, and garnet and accessory phase trace element abundances. Following melting and peak metamorphism, the leucogranulites have been exhumed from lower crustal depths to their present position at the highest structural level of the Gföhl Nappe. The nearisothermal decompression (ITD) P-T path and available geochronological data imply high exhumation rates.The dry character of the leucogranulites reflects the water-undersaturated conditions that prevailed during formation of the precursor leucogranitic melts and their subsequent recrystallization in the middle and lower crust. Compositions of the leucogranulites are displaced towards the Qz-Or join in the Qz-Ab-Or ternary diagram, which corresponds to experimental results for water undersaturated melting. Trace element and REE abundances in whole rocks, garnets and accessory phases are consistent with muscovite and biotite dehydration melting coupled with K-feldspar fractionation or separation as the principal controls on the chemical evolution of the rocks. The melting reactions and protoliths potentially involved in the generation of these HP leucogranite melts are evaluated in the light of available experimental data for water-saturated and dry melting of crustal rocks.
Article
X-ray fluorescence measurements have been made of Rb, Sr, Y, Zr and Nb in 35 samples of ocean floor basaltic rocks from four different areas of the oceans. Samples include fresh, weathered and metamorphosed basalts and dolerites. Comparison is made with K and Ti determinations on the same samples. Rb, K and, to a lesser extent, Sr are affected by processes of ocean floor weathering and metamorphism, but abundance of Y, Zr, Nb and Ti seem to be unaffected even by severe secondary processes. High correlations are found between Y and Ti, and Zr and Ti for both fresh basalts and the group of samples as a whole. These correlations define a narrow composition space in terms of these elements for ocean floor basaltic rocks even when they are severely altered. It is possible that this property might be used to identify altered ocean floor basalts and to distinguish them from volcanics from other sources.
Article
Granitoids are divided into several types according to their mineral assemblages, their field and petrographical features, and their chemical and isotopic characteristics. This typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field, petrographical and mineralogical criteria. It thus has the advantage of distinguishing the various granitoid types in the field, in most cases. The proposed classification shares many similarities with the twenty most used genetic classifications of granitoids. Both types of peraluminous granitoids are of crustal origin; the «tholeiitic», alkaline and peralkaline granitoids are of mantle origin; and both types of calc-alkaline granitoids are of mixed origin and involve both crustal and mantle materials. Each granitoid type is generated and emplaced in a very specific tectonic setting. Each stage of the Wilson cycle is characterised by typical associations of granitoids. Well-typed and precisely-dated granitoids can then complement structural approaches and indicate on the geodynamic environment. With reference to some case-studies, the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed.
Article
Despite the association of certain characteristic trace-element signatures with particular tectonic environments of eruption, there are accumulating data which would result in significant tectonic misassignments. Ambiguity of signals appears in active arc/back-arc systems of the southwestern Pacific and particularly in some intracontinental plate suites. Given the selective preservation of continental as opposed to oceanic lithosphere, inappropriate paleotectonic inferences are probable using trace-element criteria alone.Strong relative fractionation of the alkalis and alkaline earth elements (AEE) with respect to the rare earth elements (REE) in the majority of arc-related magmas and a number of intraplate continental basalts is strongly suggestive of the involvement of hydrous fluids at some stages in the respective petrogenetic processes occurring in these two tectonic regimes. In contrast, fractionation of high-field-strength elements (HFSE) such as Nb and Ta with respect to the REE in the same suites is most readily explained by the involvement, at some stage in the magma formation process, of high-SiO2 melts. A number of widely applied tectonic discriminants makes use of AEE/HFSE fractionation, but the processes and sources involved in subduction-zone petrogenesis may be duplicated during interaction of mantle-derived basalt with the heterogeneous components of continental lithosphere, both mantle and crust. A significant role for both volatile-dominated fluids and silicate melts is implicated in collision and some intracontinental plate magmatism.