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SHRIMP zircon U-Pb dating for volcanic rocks of the Daying Formation from Baofeng basin in eastern Qinling, China and its implications
Abstract
The Baofeng basin is one of the most important volcanic basins in the northern part of the East Qinling, where there mainly occur volcanic rocks of the Early Cretaceous Daying Formation. The lavas consist predominantly of trachyandesite and basaltic trachyandesite, which is geochemically equivalent to shonshointic rocks. They are characterized by higher Na2 O + K2 O (8.04%-9.37%) and enrichment in large ion lithopbile elements (LILE) and light rare earth elements (LREE) and depletion in high field strength elements (HSFE). Initial Nd-Sr isotopic compositions are (143Nd /144Nd)i =0.5117-0.5118 and (87Sr/ 86Sr)i = 0.7067-0.7079. On the basis of summarizing the geological and geochemical characteristics of volcanic rocks, precise dating was performed of the trachyandesite and basaltic trachyandesite of the Daying Formation by using the SHRIMP U-Pb zircon dating technique. The results indicate that: zircon grains in basaltic trachyandesite are fine in size and scarce; the cathodoluminescence images show no distinct zoning structure; the SHRIMP concordia ages at two analyzed spots of the zircon grains are 148.5-132.4Ma, which probably represent the ages of relict zircons. However, there exist a large number of zircons in trachyandesite and the cathodoluminescence images of most zircons show distinct zoning structure, implying that they are magmatic zircons. The weighted average value of the SHRIMP concordia ages at 14 analyzed spots of the zircon grains is 117 ± 2Ma (MSWD = 1.3). These age data combined with the regional data indicate that volcanic rocks in the Baofeng basin in the East Qinling formed during the Early Cretaceous (117 ± 2Ma), synchronously with the formation of the A-type Donggou granite porphyry and Taishanmiao granite batholith, as well as the newly found Donggou and Tangjiaping porphyry Mo deposits. There occurred subducted components in the deep process of intracontinental orogeny in the East Qinling orogenic belt and its adjacent regions. The orogenic process terminated in the late Early Cretaceous, and lithospheric extension contributed to lower crustal delamination. All these provide important information for the understanding of the tectonic evolution of the Qiling-Dabie orogenic belt.
... The Yangtze Block was subducted to the depth of >120 km beneath the NCC, generating the HP-UHP mineral assemblages, and creating one of the largest HP-UHP metamorphic terrains in the Earth (e.g., Ernst and Liou, 1995;Cong, 1996;Zheng et al., 2005Zheng et al., , 2019. Besides deep subduction, the Yangtze crust might have interacted with the overlying NCC lithospheric mantle, which are marked by the widespread development of Early Cretaceous post-collisional igneous rocks, i.e., granitoids, mafic-ultramafic plutons and volcanic rocks (e.g., Jahn et al., 1999;Li et al., 1993;Fan et al., 2004;Wang et al., 2005;Xie et al., 2006Xie et al., , 2007Zhao et al., 2007Zhao et al., , 2017aZhao et al., , 2017bHuang et al., 2008Huang et al., , 2010He et al., 2013;Meng et al., 2018). Those rocks along the collisional orogen, particularly the intermediate-mafic rocks with enriched isotopic compositions, are keys for probing whether the deeply subducted Yangtze crust has been exhuming to the surface or is still retained in the mantle (e.g., Li et al., 1993;Zhao et al., 2015;Soret et al., 2021). ...
... In this paper, we present mineral chemistry, zircon U-Pb and in situ Hf-O isotopic data, and whole-rock elemental and Sr-Nd-Pb isotopic results for the post-collisional volcanic rocks from the Eastern and Western regions of the North Huaiyang belt (NHB) of the Dabie Orogen, and the Daying Formation of the East Qinling Orogen . These data, in combination with the data previously reported (e.g., Xie et al., 1996Xie et al., , 2006Xie et al., , 2007Xue et al., 1997;Qiu et al., 2002;Fan et al., 2004;Wang et al., 2005;Gao et al., 2016;Zhang et al., 2019aZhang et al., , 2019bZhang et al., , 2020, constrain the retention style of the recycling Yangtze continental materials in the continental subduction zone together with its reworking process into the NCC, additionally highlighting the decoupling process between surface and deep lithospheric boundaries of the Yangtze Block and the NCC. ...
... 143-130 Ma and ca. 130-120 Ma (e.g., Fan et al., 2004;Wang et al., 2005;Xie et al., 2006Xie et al., , 2007Zhao and Zheng, 2009;Li et al., 2013). The former group is considered to have adakite-like geochemical affinities, indicative of its derivation from over-thickened (>50 km) crust. ...
Mineral chemistry, zircon U-Pb geochronology, and elemental and Sr-Nd-Pb-Hf-O isotopic data for the Early Cretaceous volcanic rocks from the Dabie and East Qinling orogens of China constrain the reworking history of the Yangtze crustal materials in the North China lithosphere. These data provide new insights into the recycling of the deep-subducted crustal materials into the mantle and the transformation process from continental subduction to collision. Our data show that the Early Cretaceous volcanic rocks are characterized by shoshonitic and high-K calc-alkaline basaltic trachyandesite, trachyandesite, and trachyte. They synchronously erupted at ca. 135−120 Ma, and have zircon in situ εHf(t) values ranging from −29.0 to −17.2, and δ18O values from 4.89‰ to 6.84‰. These samples share similar “crust-like” geochemical signatures, whole-rock enrichment in the large-ion lithophile elements, depletion in high field strength elements (Nb/La = 0.12−0.38), highly enriched Sr-Nd isotopic compositions, as well as (206Pb/204Pb)i = 15.97−17.59, (207Pb/204Pb)i = 15.33−15.54, and (208Pb/204Pb)i = 36.87−38.59. The εNd(t) values range from −24.4 to −15.6, −21.8 to −13.1, and −16.8 to −10.9 for the eastern and western North Huaiyang belts of the Dabie Orogen and the East Qinling Orogen, respectively. Such geochemical similarities suggest that the northward deep-subduction of Yangtze crustal materials have been reworked into the North China Craton enriched lithospheric mantle. The spatial pattern of the Early Cretaceous volcanic rocks suggests that the Yangtze subduction extended northward beyond the southern margin of the North China Craton and ran into its interior. The surface and deep lithospheric boundaries are decoupled between the Yangtze Block and North China Craton. The Early Cretaceous igneous rocks in the Dabie−East Qinling Orogen were induced in response to the post-collisional unrooting of the Triassic over-thickened lithosphere coupling.
... Timescale from Cohen et al. (2013). Data sources: Xie et al. (2007); Jiang et al. (2010) in the Songxian County of the Henan Province, China. The Taishanmiao Batholith mainly consists of three distinct phases, including medium-to coarse-grained syenogranite, fine-to medium-grained syenogranite, and fine-grained porphyritic syenogranite (Ye et al. 2008;Gao et al. 2014a). ...
... Although granites in the Taishanmiao Batholith have geochemical affinities with A1-type granites that may have been generated in an anorogenic rift (Eby 1992), they are not temporally and spatially associated with the maficultramafic intrusion. Furthermore, there is no Early Cretaceous anorogenic rifting in the study area, but widespread fault-bounded extensional basins are present ( Fig. 1c; Xie et al. 2007;Jiang et al. 2010). Thus, the A1-A2 subdivision of Eby (1992) does not apply to the aluminous granites in the Taishanmiao Batholith (Bonin 2007). ...
... In addition, geological, geophysical, and geochemical data suggest that the NCC has undergone large-scale lithospheric thinning that led to the removal of a ~120-km-thick lithospheric root during the Mesozoic (Fan and Menzies 1992;Menzies et al. 1993;Griffin et al. 1998;Gao et al. 2002;Hu et al. 2000;Deng et al. 2004;Chen et al. 2008). This period is characterized by the widespread development of Early Cretaceous fault-bounded extensional basins such as the Sanmenxia, Lushi-Luoyang, Songxian-Yichuan, Linru, and Baofeng ( Fig. 1c; Xie et al. 2007;Jiang et al. 2010). The Baofeng Basin contains basaltic andesite with a SHRIMP zircon U-Pb age of 117 ± 2 Ma ( Fig. 2 Wang et al. (2007). ...
Late Mesozoic magmatic rocks from the Taishanmiao Batholith were collected for LA–ICP–MS dating, Sr–Nd–Hf isotope systematics, and whole-rock major and trace element geochemistry to help understand the nature of collisional and extensional events along the southern margin of the North China Craton. The batholith consists of three texturally distinguishable phases of a 125 ± 1 Ma medium- to coarse-grained syenogranite, a 121 ± 1 Ma fine- to medium-grained syenogranite, and a 113 ± 1 Ma porphyritic monzogranite. Most of the units in the batholith are syenogranitic in composition with high levels of silica (70–78 wt% SiO2), alkalis (8.0–8.6 wt% Na2O + K2O), Fe* (FeOT/(FeOT + MgO) = 0.76–0.90), and depletion in CaO (0.34–1.37 wt%), MgO (0.12–0.52 wt%), TiO2 (0.09–0.40 wt%), and A/CNK (Al2O3/(Na2O + K2O + CaO)) molar ratios of 1.00–1.11. All samples have high proportions of Ga, Nb, Zr, Ga/Al, and REE, and depletions in Ba, Sr, Eu, and compatible elements, indicating that the batholith consists of A-type granites. The zircon saturation temperature for these units yields a mean value of 890 °C, and zircons with Early Cretaceous magmatic ages have εNd(t) values of −14.0 to −12.0, εHf(t) values ranging from −18.7 to −2.1, and corresponding Hf model ages of 2339–1282 Ma. These geochemical and isotopic characteristics allowed us to conclude that the primary magma for the Taishanmiao Batholith originated from partial melting of Precambrian crustal rocks in the medium-lower crust. However, the high Nb and Ta contents and low normalized Nb/Ta values for the Taishanmiao granites are due to fractionation in Nb- and Ta-rich amphibole (or biotite). It is further proposed that these aluminous A-type granites were generated in an extensional tectonic setting during the Early Cretaceous, which was induced by lithospheric thinning and asthenospheric upwelling beneath eastern China toward the Paleo-Pacific Plate.
... Moreover, sporadic mafic magmatic rocks in the Xiong'ershan and its adjacent areas have been reported in the last decade (Gao & Zhao, 2017). The mafic magmatic rocks which have zircon U-Pb ages of 148-117 Ma, with peak ages of 129-117 Ma (Xie et al. 2007;Bao et al. 2009;Gao et al. 2014), are contemporary with the Huashan monzogranites, indicating that crust-mantle interaction likely occurred in this region. High SiO 2 samples of the Huashan pluton have more evolved isotopic compositions (Fig. 12), indicative of the mixing between a mafic melt and a crustal component (Van der Meer et al. 2018). ...
... More than half of the samples from the Huashan monzogranites exhibit geochemical characteristics of A-type granites (Fig. 8). The emplacement of the Huashan A-type granites is coeval with the exhumation of metamorphic core complexes and the formation of several deformed basins (Xie et al. 2007;Mao et al. 2010), indicative of lithospheric extension that dominated the southern margin of the NCC during the emplacement of the Huashan magmas. This, in combination with the lack of magmatic rocks derived from the delaminated lower crust, implies that the crustal thinning during the late Mesozoic in the Xiong'ershan area may be caused by the lithospheric extension, similar to that of SE China ). ...
The Xiong’ershan area is the third largest gold-producing district in China. The Late Jurassic to Early Cretaceous magmatism in the Xiong’ershan area can be divided into two episodes: early (165–150 Ma) and late (138–113 Ma). Laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) zircon U–Pb dating yields ages of 160.7 ± 0.6 Ma and 127.2 ± 1.0 Ma for the Wuzhangshan and Huashan monzogranites in the Xiong’ershan area, respectively, representing the two magmatic episodes. The Wuzhangshan monzogranites exhibit adakite-like geochemical features (e.g. high Sr/Y ratios, low Yb and Y contents). Their Sr–Nd–Hf isotopic compositions are consistent with those of the amphibolites of the Taihua Group, indicating that the Wuzhangshan monzogranites were formed from partial melting of the Taihua Group metamorphic rocks. Compared to the Wuzhangshan rocks, the Huashan monzogranites have higher MgO, Cr, Co and Ni contents, but lower Sr/Y and Fe ³⁺ /Fe ²⁺ . All the samples from the Huashan monzogranites plot in the area between the Taihua Group amphibolite rocks and the mantle rocks in the ( ⁸⁷ Sr/ ⁸⁶ Sr)t vs ε Nd( t ) and age vs ε Hf( t ) diagrams, suggesting that the Huashan monzogranites were probably generated by mixing of mantle-derived magmas and the Taihua Group metamorphic basement melts. The gold mineralization (136–110 Ma) is coeval with the emplacement of the late-episode magmas, implying that crustal–mantle mixed magma might be a better target for gold mineralization compared to the ancient metamorphic basement melt. The data presented in this study further indicate that the transformation of the lithosphere from thickening to thinning in the Xiong’ershan area probably occurred between ~160 Ma and ~127 Ma, and that the gold mineralization in this area was probably related to lithospheric thinning.
... The term "the Xiong'ershan orogenic gold hydrothermal minerals system" is used to indicate the main molybdenum and gold deposits occurring in the Early Cretaceous of the study area (Zhao et al., 2004;Xie et al., 2007;Hu et al.,2019 ). The main orecontrolling structures are jointly controlled by NE, NNE, and earlier EW structures, where the intersection of faults provides ore-bearing space for gold deposits (Zhao et al., 2002;Li et al., 2014 ). ...
... Here, we use the term "Taishanmiao batholith" to describe the physical intrusive body, and use the term "Taishanmiao granite" to refer to the undifferentiated lithologies that make up the batholith and which are distinguished from each other based mostly on texture. The undifferentiated Taishanmiao granite is classified as an aluminous, high-K, calc-alkaline A-type granite that was generated in a post-orogenic or intraplate extensional setting (Xie et al. 2007;Ye et al. 2008;Wang et al. 2013Wang et al. , 2016. It includes medium-to coarse-grained syenogranite, fine-to medium-grained syenogranite, and porphyritic syenogranite (Ye et al. 2008) (Fig. 2). ...
... Here, we use the term "Taishanmiao batholith" to describe the physical intrusive body, and use the term "Taishanmiao granite" to refer to the undifferentiated lithologies that make up the batholith and which are distinguished from each other based mostly on texture. The undifferentiated Taishanmiao granite is classified as an aluminous, high-K, calc-alkaline A-type granite that was generated in a post-orogenic or intraplate extensional setting (Xie et al. 2007;Ye et al. 2008;Wang et al. 2013Wang et al. , 2016. It includes medium-to coarse-grained syenogranite, fine-to medium-grained syenogranite, and porphyritic syenogranite (Ye et al. 2008) (Fig. 2). ...
The Taishanmiao granitic batholith, located in the Eastern Qinling Orogen in Henan Province, China, contains numerous small (mostly tens of centimeters in maximum dimension) bodies exhibiting textures and mineralogy characteristics of simple quartz and alkali feldspar pegmatites. Analysis of melt inclusions (MI) and fluid inclusions (FI) in pegmatitic quartz, combined with Rhyolite-MELTS modeling of the crystallization of the granite, have been applied to develop a conceptual model of the physical and geochemical processes associated with the formation of the pegmatites. These results allow us to consider the formation of the Taishanmiao pegmatites within the context of varios models that have been proposed for pegmatite formation.
Field observations and geochemical data indicate that the pegmatites represent the latest stage in the crystallization of the Taishanmiao granite and occupy ≤4 vol% of the syenogranite phase of the batholith. Results of Rhyolite-MELTS modeling suggest that the pegmatite-forming melts can be produced through continuous fractional crystallization of the Taishanmiao granitic magma, consistent with the designation of the pegmatites as a miarolitic class, segregation-type pegmatites rather than the more common intrusive-type of pegmatite. The mineral assemblage predicted by Rhyolite-MELTS after ~96% of the original granite-forming melt had crystallized consists of ~51 vol% alkali feldspar, 34 vol% quartz, 14 vol% plagioclase, 0.1 vol% biotite, and 1 vol% magnetite, similar to the alkali feldspar + quartz dominated mineralogy of the pegmatites. Moreover, the modeled residual melt composition following crystallization of ~96% of the original melt is similar to the composition of homogenized MI in quartz within the pegmatite. Rhyolite-MELTS predicts that the granite-forming melt remained volatile-undersaturated during crystallization of the batholith and contained ~6.3 wt% H2O and ~500 ppm CO2 after ~96% crystallization when the pegmatites began to develop. The Rhyolite-MELTS prediction that the melt was volatile-undersaturated at the time the pegmatites began to form, but became volatile-saturated during the early stages of pegmatite formation, is consistent with the presence of some inclusion assemblages consisting of only MI, while others contain co-existing MI and FI. The relationship between halogen (F and Cl) and Na abundances in MI is also consistent with the interpretation that the very earliest stages of pegmatite formation occurred in the presence of a volatile-undersaturated melt and that the melt became volatile saturated as crystallization progressed.
We propose a closed system, isochoric model for the formation of the pegmatites. Accordingly, the Taishanmiao granite crystallized isobarically at ~3.3 kbar, and the pegmatites began to form at ~734 °C and ~ 3.3 kbar, after ~96% of the original granitic melt had crystallized. During the final stages of crystallization of the granite, small pockets of the remaining residual melt became isolated within the enclosing granite and evolved as constant mass (closed), constant volume (isochoric) systems, similar to the manner in which volatile-rich melt inclusions in igneous phenocrysts evolve during post-entrapment crystallization under isochoric conditions. As a result of the negative volume change associated with crystallization, pressure in the pegmatite initially decreases as crystals form, and this leads to volatile exsolution from the melt phase. The changing PTX conditions produce a pressure-induced “liquidus deficit” that is analogous to liquidus undercooling and results in crystal growth as required to return the system to equilibrium PTX conditions. Owing to the complex closed system, isochoric PVTX evolution of the melt-crystal-volatile system, the pressure does not decrease rapidly or monotonically during pegmatite formation but, rather, gradually fluctuates such that at some stages in the evolution of the pegmatite the pressure is decreasing while at other times the pressure increases as the system cools to maintain mass and volume balance. This behavior, in turn, leads to alternating episodes of precipitation and dissolution that serve to coarsen (ripen) the crystals to produce the pegmatitic texture. The evolution of the pegmatitic melt described here is analogous to that which has been well-documented to occur in volatile-rich MI that undergo closed system, isochoric, post-entrapment crystallization.
... Some researchers argue that these are orogenic gold deposits (Zhou et al. , 2015Deng and Wang 2016). However, unlike conventional orogenic Au deposits forming in convergent tectonic settings, the Au deposits formed at about 130 Ma in the southern margin of the NCC would have formed within an extensional tectonic setting (Zhang and Zheng 1999;Xie et al. 2007;Ye et al. 2008;Li et al. 2020). Additionally, pyrite in the Au deposits contains low concentrations of As (Bi et al. 2016;Tian et al. 2019;Liu et al. 2020) compared to typical orogenic Au deposits (Large et al. 2007(Large et al. , 2009Wu et al. 2018). ...
The Qi191 Au deposit is a newly discovered granite-hosted Au deposit in the Qiyugou Au orefield, located in the southern margin of the North China Craton (NCC). Gold ore bodies are hosted by both the Qi191 hornblende monzogranite and the Taihua Complex and are composed of disseminations and sparse veins. K-feldspar, quartz-sericite, and sericite-chlorite alteration formed outward from the center to the periphery of the mineralized granite. Principal ore minerals include galena, pyrite, and chalcopyrite, with minor sphalerite, hematite, pyrrhotite, gold, famatinite, and hessite. The Qi191 Au deposit formed over four successive stages of hydrothermal mineralization: a quartz stage (I), a quartz-pyrite-galena stage (II), an economic quartz-calcite-galena-pyrite stage (III), and a quartz-calcite stage (IV). The Qi191 hornblende monzogranite yields a SIMS zircon U–Pb age of 131.3 ± 1.0 Ma. Hydrothermal sericite yields a plateau ⁴⁰Ar–³⁹Ar age of 132.9 ± 1.4 Ma, which is consistent with the zircon U–Pb age, suggesting that the Qi191 Au deposit is generically related to the hornblende monzogranite. Trace element contents and S isotopic compositions of sulfides indicate that metals and S entered the hydrothermal fluid as pulses. The equilibrium temperature of S and the isotopic compositions of S–Pb reflect a magmatic source. Gold mainly precipitated as native gold in a relatively acidic and reducing environment. This work suggests that further exploration for magmatic hydrothermal gold deposits in the Qiyugou and more widely in the southern margin of the NCC should target hornblende monzogranites such as the Qi191 and Qi189 granitoids.
... The dome has a core of Archean basement rocks, which is surrounded by the Proterozoic volcanic rocks, carbonates, and clastic rocks . The Neoarchean Taihua Group mid-high grade metamorphic rocks, the Mesoproterozoic Xionger Group intermediate-basic-intermediate-acid volcanic rocks, the Mesoproterozoic Guandaokou Group carbonates and clastic rocks, Cambrian carbonate and clasolite, Triassic sandstones, and Paleogene and Quaternary sediments are widely exposed in the Xiaoshan region (Zhao et al., 2004;Diwu et al., 2007Diwu et al., , 2010Xie et al., 2007;Shi et al., 2011). Folds and faults are well-developed in the Xiaoshan region. ...
The Xiaoshan gold deposit of western Henan Province is located in the southern margin of the North China craton (NCC). Ore bodies of the deposit occur as veins in the fault zones. The ore-forming process can be divided into three stages: an early quartz–pyrite stage, a middle quartz–polymetallic sulfide stage, and a late quartz–carbonate stage. The LA-ICP-MS zircon U–Pb age of the Houhe granite, which is related to Au mineralization of the Xiaoshan deposit, was 139–137 Ma. The initial ¹⁷⁶Hf/¹⁷⁷Hf values of the Houhe intrusion are 0.282126–0.282368, with the εHf(t), TDM1(Hf), and TDM2(Hf) values being −19.9 to −11.3, 1242–1583 Ma, and 1912–2454 Ma, respectively, indicating that the Houhe granite is formed by partial melting of the lower crust and that the source is mainly the Taihua Group. Two types of fluid inclusions (FIs) have been distinguished in various quartz veins including liquid-rich (WL-type) and H2O–CO2 (C-type) inclusions. The FIs in the quartz of the quartz–pyrite stage comprise WL- and C-type inclusions; their homogenization temperatures, salinities, and densities vary from 300 °C to 393 °C, 1.6 wt% to 11.0 wt% NaCleqv., and 0.57 to 0.82 g/cm³, respectively. The FIs in the quartz of the quartz–polymetallic sulfide stage consist of WL- and C-type inclusions; their homogenization temperatures, salinities, and densities range from 261 °C to 298 °C, 1.1 wt% to 11.8 wt% NaCleqv., and 0.74 to 0.89 g/cm³, respectively. Only WL-type inclusions are observed in the quartz of the quartz–carbonate stage; their homogenization temperatures, salinities, and densities vary from 193 °C to 258 °C, 2.2 wt% to 12.7 wt% NaCleqv., and 0.87 to 0.97 g/cm³, respectively. The ore-forming fluids of the Xiaoshan gold deposit are generally characterized by medium temperature, moderate–low salinity, low density and belonging to an H2O–NaCl ± CO2 system. The δ¹⁸OH2O values calculated for ore-bearing quartz vary from 0.7‰ to 4.5‰, and the δDV-SMOW values from bulk extraction of fluid inclusion water vary from −69.5‰ to −47.8‰, suggesting that the ore-forming fluid consists dominantly of magmatic water. The δ³⁴SV-CDT values range from 0.37‰ to 3.9‰. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb values of the ore minerals are in the ranges of 17.391–17.728, 15.420–15.577, and 37.420–37.923, respectively. The data from the S and Pb isotopic systems indicate that the ore-forming materials came from granitic magma. The Xiaoshan gold deposit is a mesothermal lode-gold deposit of magmatic-hydrothermal origin. Fluid phase separation and temperature decrease were the dominant mechanisms for the deposition of ore-forming materials.
... They are composed of syenogranite, monzogranite and quartz syenite. Mafic intrusions, volcanic rocks and lamprophyre dykes are also observed in the region (Gao et al., 2014a;Xie et al., 2007;Gao, 2012). Moreover, the late Mesozoic igneous rocks tend to be younger eastward in this region (Table A1). ...
... There are also sporadic mafic rocks which are contemporary with granites and ore deposits. Zircon U-Pb age dates suggest that the mafic rocks were formed at about 148 to 117 Ma, with peak ages of 129-117 Ma ( Xie et al., 2007;Bao et al., 2009;Zhao et al., 2010b;Gao et al., 2014a). As shown in Appendix Table S1 and Figure 2, the Late Meso- zoic magmatisms in the southern margin of the NCC can be divided into two episodes: Late Jurassic to Early Cretaceous and Late Early Cretaceous. ...
Late Mesozoic granitic magmatism (158–112 Ma) are widespread in the southern margin of the North China Craton (NCC), contemporary with many world-class Mo-Au-Ag-Pb-Zn polymetallic deposits. There are abrupt changes in the elements and isotopic compositions of these granites at about 127 Ma. The early stage (158–128 Ma) granites show slightly or no negative Eu anomalies, large ion lithophile elements enriched and heavy REE depleted (such as Y and Yb), belonging to typical I-type granite. The late stage (126–112 Ma) granites are characterized by A-type and/or highly fractionated I-type granite, with higher contents of SiO2, K2O, Y, Yb and Rb/Sr ratio and lower contents of Sr, δEu value and Sr/Y ratio than that of the early-stage granites. Moreover, the whole rock Nd and Hf isotopic compositions of the granites younger than 127 Ma show more depleted than those of the older one. The two stages of Late Mesozoic granites were derived from a source region of the ancient basement of the southern margin of the NCC incorporated the mantle material. The late stage (126–112 Ma) granites contain more fractions of mantle material with depleted isotopic composition than the early ones. The granites record evidence for a strong crust-mantle interaction. They formed in an intracontinental extensional setting which was related to lithospheric thinning and asthenospheric upwelling in this region, which was possibly caused by westward subduction of the Paleo-Pacific plate. 127 Ma is an critical period of the transformation of the tectonic regime.
... Detailed 40 Ar/ 39 Ar dating of metamorphic core complex reveals that lithospheric extension occurred during the 135-123 Ma interval in the Xiaoqinling region . SHRIMP zircon U-Pb ages of Taishanmiao aluminous A-type granite in the Xiong'ershan region and Early Cretaceous medium-basic volcanic rocks in the Songxian-Baofeng region are 115 Ma (Ye et al., 2008) and 117 ± 2Ma (Xie et al., 2007), respectively. In addition, geophysical data show that there is a prominent mantle uplift with a NW trend underneath the Xiaoqinling region (Yuan, 1998), where the thickness of continental crust is less 8-14 km than that of the surrounding areas (e.g. ...
Appinites are commonly derived from a mantle source and are potentially significant in constraining the tectonic nature and evolution of ancient orogens, yet they have received little attention because of their limited outcrop. Here we investigate the newly identified appinitic rocks from the Laoniushan complex in the eastern Qinling Orogen. The appinites are composed of coarse-grained hornblendite, medium- and fine-grained hornblende-gabbro, and diorite porphyrite in the field occurrence. Winthin the appinitic rocks, the hornblendite displays features of cumulates. This study presents LA-ICP-MS zircon U-Pb data, mineral chemistry and whole rock geochemistry of the appinites. Zircons in the mafic to ultramafic rocks yield a U-Pb age of 152 ± 1Ma. The geochemistry of the rocks displays: lower SiO2, higher Fe2O3T and MgO contents, relatively flat chondrite normalized REE patterns with slight enrichment in light REE and a minor negative Eu anomaly; enrichment in large-ion lithophile elements(LILE, e.g. Rb, Ba, Sr and P), and depletion in high field strength elements(HFSE, e.g. Nb, Zr, Hf and Ti). Such geochemical features, together with crust-like bulk Sr-Nd isotopic compositions(initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7057–0.7072, εNd(t) = −17.2 to −9), suggest that the Laoniushan appinites likely originated from an ancient metasomatised mantle, followed by fractional crystallization in the petrogenetic process. The studied appinites were most likely generated in an intracontinental extensional environment in the Late Mesozoic.
... In the Songxian Figure 9. Evolution of the Qinling Orogen from Cambrian to Jurassic. Values are from previous research (Chen et al. 2004a(Chen et al. , b, 2006(Chen et al. , 2009Gong et al. 2009;Guo et al. 2010;He et al. 2007;Hu et al. 2004;Jiang et al. 2009Jiang et al. , 2010Jin et al. 2005;Lerch et al. 1995;Li et al. 1993Li et al. , 1996Li et al. , 1999Li et al. , 2000Li et al. , 2006Li et al. , 2008Liu et al. 2003Liu et al. , 2007Liu et al. , 2009aLiu et al. , 2014Lu et al. 1999bLu et al. , 2003Pei et al. 2005Pei et al. , 2007aQin et al. 2005Qin et al. , 2007Qin et al. , 2008aQin et al. , b, 2009aSun 1991;Sun et al. 1996Sun et al. , 2000Sun et al. , 2002Su et al. 2004;Tian 2003;Wang et al. 2005Wang et al. , 2006Wang et al. , 2008Wang et al. , 2009aWen et al. 2008;Wu et al. 2009;Xie et al. 2007;Xu et al. 2007Xu et al. , 2008Xue et al. 1996;Yan et al. 2007Yan et al. , 2008Yang et al. 1999Yang et al. , 2003Yang et al. , 2009Zhang et al. 1999Zhang et al. , 2001bZhang et al. , 2002Zhang et al. , 2006Zhang et al. , 2009aZhu et al. 2013). County, the Dazhuang-Zhonghu anticline, formed in the Xiong'er Group, was intruded by the Mogou syenite, and the Mogou syenite did not experience deformation. ...
... In the Songxian Figure 9. Evolution of the Qinling Orogen from Cambrian to Jurassic. Values are from previous research (Chen et al. 2004a(Chen et al. , b, 2006(Chen et al. , 2009Gong et al. 2009;Guo et al. 2010;He et al. 2007;Hu et al. 2004;Jiang et al. 2009Jiang et al. , 2010Jin et al. 2005;Lerch et al. 1995;Li et al. 1993Li et al. , 1996Li et al. , 1999Li et al. , 2000Li et al. , 2006Li et al. , 2008Liu et al. 2003Liu et al. , 2007Liu et al. , 2009aLiu et al. , 2014Lu et al. 1999bLu et al. , 2003Pei et al. 2005Pei et al. , 2007aQin et al. 2005Qin et al. , 2007Qin et al. , 2008aQin et al. , b, 2009aSun 1991;Sun et al. 1996Sun et al. , 2000Sun et al. , 2002Su et al. 2004;Tian 2003;Wang et al. 2005Wang et al. , 2006Wang et al. , 2008Wang et al. , 2009aWen et al. 2008;Wu et al. 2009;Xie et al. 2007;Xu et al. 2007Xu et al. , 2008Xue et al. 1996;Yan et al. 2007Yan et al. , 2008Yang et al. 1999Yang et al. , 2003Yang et al. , 2009Zhang et al. 1999Zhang et al. , 2001bZhang et al. , 2002Zhang et al. , 2006Zhang et al. , 2009aZhu et al. 2013). County, the Dazhuang-Zhonghu anticline, formed in the Xiong'er Group, was intruded by the Mogou syenite, and the Mogou syenite did not experience deformation. ...
The Mogou syenite intruded into the Mesoproterozoic Xiong’er Group is the main lithostratigraphic unit, along the southern margin of the North China Craton (NCC). This paper reports zircon LA-ICP-MS data, whole-rock major and trace element compositions of late Triassic magmatic rocks in the Mogou syenite, in order to constrain the formation age of the Mogou syenite, research the origin and evolution of the magma and analyse the geodynamic setting of the Qinling Orogen (QO) in Late Triassic. These rocks consist of medium- to coarse-grained syenite and fine-grained quartz syenite. Zircon U–Pb dating yields a crystallization age of 226.5±2.7 Ma. The syenites are characterized by high SiO2 (63.49–72.17%), alkali (K2O+Na2O of 11.18–15.38%) and potassium (K2O/Na2O of 2.88–28.11), are peralkaline or metaluminous (molar A/CNK of 0.87–1.02) and belong to shoshonite series. The syenites have ΣREE of 33.01–191.30 ppm, LREE/HREE of 14–20, (La/Yb)N of 11–24, with LREE-rich distribution pattern and obvious differentiation between HREE and LREE. Eu anomalies are positive for the medium- to coarse-grained syenite and weakly negative for the fine-grained quartz syenite. In addition, the syenites are enriched in large-ion lithophile elements (Ba, K, Sr, and Pb) but depleted in high strength field elements (Ti, Ta, Nb, Zr, and Hf), and have high differentiation indices of 91.69–97.06. These geochemical features indicate that the primary magma of the Mogou syenite most likely originated from a mantle source with minor crustal component, and underwent a fractional crystallization process during its emplacement. The late Triassic A-type Moguo syenite along the southern margin of the NCC was generated in the late stage of the syn-collision event of QO, recording a transition period from compression to extension at around 227 Ma.
... Gao et al., 2010;Y.L. Gao et al., 2010;Guo et al., 2009;Han et al., 2007b;Huang et al., 2010;Li et al., 2009;N. Li et al., 2012b;Liu et al., 2011;Mao et al., 2008Mao et al., , 2010Su et al., 2009;Tang et al., 2013;Wang et al., 2001Wang et al., , 2005Xie et al., 2007;Yang et al., 2004Yang et al., , 2010Yao et al., 2009;Z.Q. Zhang et al., 2006;Zhou et al., 2009;Zhu et al., 1999). ...
The newly-discovered Shiyaogou molybdenum deposit is located in the eastern Qinling metallogenic belt in central China. The deposit contains at least 152,000 t of Mo metal and bears typical porphyry-type features in terms of its concentric alteration zonation, quartz vein-hosted Mo mineralization, veining sequence and the spatial association with concealed granite porphyries. Re-Os isotope analyses of molybdenite from the deposit yield an ore-forming age of 132.3 +/- 2.8 Ma. LA-ICP-MS U-Pb zircon dating of ore-related porphyries yields crystallization ages from 135 Ma to 132 Ma, indicating a temporal link between granitic magmatism and Mo mineralization. A population of captured magmatic zircons indicates another pulse of magmatism at similar to 143 Ma. A barren granite intrusion near the deposit gives a zircon U-Pb age of 148.1 +/- 1.1 Ma. These magmatic activities were concurrent with the emplacement of the nearby Heyu granitic batholith, a largely ore-barren intrusive complex formed from similar to 148 Ma to similar to 127 Ma. Zircon Ce4+/Ce3+ ratios of ore-related porphyries are obviously higher than those of contemporaneous barren granitoids, implying an affinity between Mo mineralization and highly oxidized magmas. Moreover, zircons from these granitoids overall have decreasing Ce4+/Ce3+ ratios from 148 Ma to 132 Ma, reflecting decreasing oxygen fugacities during magma evolution. Available geological, radiometric and stable isotopic evidence suggests that the decrease of magma oxygen fugacity was probably associated with an increase of mantle contribution to granitic magmatism and metallogenesis, which probably gave rise to successive mineralization of Mo and Au in the eastern Qinling. The intense magmatic-metallogenic events in the eastern Qinling during Late Jurassic to Early Cretaceous times are interpreted as a response to the large-scale lithosphere thinning and subsequent asthenosphere upwelling beneath the eastern part of the North China Craton.
... The Late Mesozoic granitoids are undeformed. They show a transition from I-type to I-and A-type, and finally A-type magmatism from early to late stages, and are associated with contemporaneous mafic to intermediate trachyandesitic volcanic rocks at 117 ± 2 Ma (Xie et al., 2007). This suggests that these granitoids were emplaced during an extensional setting. ...
The Qinling Orogen is one of the main orogenic belts in Asia and is
characterized by multi-stage orogenic processes and the development of
voluminous magmatic intrusions. The results of zircon U-Pb dating
indicate that granitoid magmatism in the Qinling Orogen mainly occurred
in four distinct periods: the Neoproterozoic (979-711 Ma), Paleozoic
(507-400 Ma), and Early (252-185 Ma) and Late (158-100 Ma) Mesozoic. The
Neoproterozoic granitic magmatism in the Qinling Orogen is represented
by strongly deformed S-type granites emplaced at 979-911 Ma, weakly
deformed I-type granites at 894-815 Ma, and A-type granites at 759-711
Ma. They can be interpreted as the products of respectively
syn-collisional, post-collisional and extensional setting, in response
to the assembly and breakup of the Rodinia supercontinent. The Paleozoic
magmatism can be temporally classified into three stages of 507-470 Ma,
460-422 Ma and ˜415-400 Ma. They were genetically related to the
subduction of the Shangdan Ocean and subsequent collision of the
southern North China Block and the South Qinling Belt. The 507-470 Ma
magmatism is spatially and temporally related to ultrahigh-pressure
metamorphism in the studied area. The 460-422 Ma magmatism with an
extensive development in the North Qinling Belt is characterized by
I-type granitoids and originated from the lower crust with the
involvement of mantle-derived magma in a collisional setting. The
magmatism with the formation age of ˜415-400 Ma only occurred in
the middle part of the North Qinling Belt and is dominated by I-type
granitoid intrusions, and probably formed in the late-stage of a
collisional setting. Early Mesozoic magmatism in the study area occurred
between 252 and 185 Ma, with the cluster in 225-200 Ma. It took place
predominantly in the western part of the South Qinling Belt. The 250-240
Ma I-type granitoids are of small volume and show high Sr/Y ratios, and
may have been formed in a continental arc setting related to subduction
of the Mianlue Ocean between the South Qinling Belt and the South China
Block. Voluminous late-stage (225-185 Ma) magmatism evolved from early
I-type to later I-A-type granitoids associated with contemporaneous
lamprophyres, representative of a transition from syn- to
post-collisional setting in response to the collision between the North
China and the South China blocks. Late Mesozoic (158-100 Ma) granitoids,
located in the southern margin of the North China Block and the eastern
part of the North Qinling Belt, are characterized by I-type, I- to
A-type, and A-type granitoids that were emplaced in a post-orogenic or
intraplate setting. The first three of the four periods of magmatism
were associated with three important orogenic processes and the last one
with intracontinental process. These suggest that the tectonic evolution
of the Qinling Orogen is very complicated.
This study presents new geochronological and geochemical data for Early Cretaceous volcanic rocks in the southern margin of the North China Craton (NCC), to discuss the crust–mantle interaction. The studied rocks include pyroxene andesites from Daying Formation, hornblende andesites and andesites from Jiudian Formation, and rhyolites from a hitherto unnamed Formation. These rocks formed in Early Cretaceous (138–120 Ma), with enrichment in light rare earth elements (REE), depletion in heavy REE and arc-like trace elements characteristics. Pyroxene andesites show low SiO2 contents and enriched Sr–Nd–Pb–Hf isotopic compositions, with orthopyroxene phenocryst and Paleoproterozoic (2320–1829 Ma) inherited zircons, suggesting that they originated from lithospheric mantle after metasomatism with NCC lower crustal materials. Hornblende andesites have low SiO2 contents and high Mg# (Mg# = 100Mg/(Mg + Fe²⁺)) values, indicating a lithospheric-mantle origin. Considering the distinct whole-rock Sr isotopic compositions we divide them into two groups. Among them, the low (⁸⁷Sr/⁸⁶Sr)i andesites possess amount inherited Neoarchean to Neoproterozoic (2548–845 Ma) zircons, indicating the origin of lithospheric mantle with addition of Yangtze Craton (YC) and NCC materials. In comparison, the high (⁸⁷Sr/⁸⁶Sr)i andesites, with abundant Neoarchean–Paleozoic inherited zircons (3499–261 Ma), are formed by partial melting of lithospheric mantle with incorporation of NCC supracrustal rocks and YC materials. Rhyolites have extremely high SiO2 (77.63–82.52 wt.%) and low total Fe2O3, Cr, Ni contents and Mg# values, combined with ancient inherited zircon ages (2316 and 2251 Ma), suggesting an origin of NCC lower continental crust. Considering the presence of resorption texture of quartz phenocryst, we propose a petrogenetic model of ‘crystal mushes’ for rhyolites prior to their eruption. These constraints record the intense crust–mantle interaction in the southern margin of the NCC. Given the regional data and spatial distribution of Early Cretaceous rocks within NCC, we believe that the formation of these rocks is related to the contemporaneous far-field effect of the Paleo-Pacific Plate.
The Qinling Orogen is well known for its huge Mo resource. The porphyry Mo deposits are associated with small granite porphyries, albeit great batholiths accommodate limited Mo mineralization. In this study, detailed geochemical, isotopic, and geochronological studies were carried out to constrain the petrogenesis of the Taishanmiao batholith and time framework of magmatism and associated Mo mineralization at Zhuyuangou. Zircon geochronology reveals gradually decreasing emplacement ages, from 124.8 ± 1.0 Ma for the medium- to coarse-grained syenogranite, through 122.8 ± 1.4 Ma for fine- to medium-grained syenogranite, to 115.1 ± 0.95 Ma for porphyritic syenogranite. The last two phases hosted the Zhuyuangou Mo mineralization with a molybdenite Re-Os isochron age of 117.7 ± 1.0 Ma. Major and trace element features support highly fractionated I-type affinity for Taishanmiao. Zircon Lu-Hf isotopic studies yield εHf(t) values of -19.6 − -8, and two-stage Hf model ages (TDM2) of 1679 − 2408 Ma. It thus supports ancient continental crust, especially Taihua Supergroup and Xiong'er Group, as the primary source. Compared to the source rock, the low Mo concentrations of the batholith are attributed to the massive fluid flux that contributes to the formation of the Zhuyuangou and Donggou deposits. Small porphyries can effectively concentrate exsolved fluids and metal compared with great batholith. Their shallow emplacement depths result in greater temperature/pressure/composition gradient with the wall rocks, higher cooling rate, more fractures or even breccia, and fluid boiling and mixing, facilitating intensive hydrothermal alteration and mineralization.
The composition of the continental crust of the North China Craton (NCC) is more felsic than that of the average bulk crust, which is regarded to be the result of the delamination of the thickened lower crust during Mesozoic. However, whether the thickened continental crust existed and when the delamination event happened along the southern margin of the NCC are still debated. Here, we report geochronological, geochemical and Sr-Nd-Hf-Pb isotopic evidence that granitoids from the Late Jurassic Wuzhangshan pluton and the Early Cretaceous Huashani complex were derived by partial melting of the lower crust with different thickness. Our new data shows that the two lithofacies of the Wuzhangshan pluton were mainly formed between ca. 157 and 156 Ma, whereas the five lithofacies of the Huashani complex were mainly emplaced between ca. 132 and 125 Ma. The Wuzhangshan pluton and the earlier four lithofacies granitoids of the Huashani complex (ca. 160–125 Ma) both display adakitic geochemical features, which are characterized by as high SiO2 (63.26–72.71 wt%), Al2O3 (13.97–16.89 wt%) and Sr (413–1218 ppm) contents, and low Y (6.30–14.98 ppm) and YbN (1.55–4.43), and high Sr/Y (33−112) and (La/Yb)N (11.53–29.72) ratios. They also have high (⁸⁷Sr/⁸⁶Sr)i (0.7066–0.7086), and low εNd(t) (−9.9 to −18.8) and εHf(t) (−11 to −26) values, and two-stage Nd and Hf model ages ranging from 2.4 to 1.7 Ga and 2.7 to 1.7 Ga, respectively. In contrast, the late Early Cretaceous (ca. 125–110 Ma) granitoids have higher SiO2 (71.30–76.78 wt%) and lower Sr (64–333 ppm) contents, and lower Sr/Y (17–29) and (La/Yb)N (13.25–18.36) ratios, and similar εNd(t) (−10 to −16) and relatively higher εHf(t) (−10 to −14) values. These geochemical variations suggest that the ca.160–125 Ma granitoids were most probably produced by partial melting of thickened crust (>45 km) with eclogite, garnet amphibolite or amphibolite residues, whereas that the ca. 125–110 Ma granitoids were formed by partial melting of the thinner crust (<33 km). We thus suggest that the NCC likely underwent a synchronous tectonic transformation at ca. 125 Ma from a compressional setting with thickened crust to an intensive extensional setting with thinner crust at ca. 125 Ma, which demonstrates that the lower crust was most likely delaminated.
The Qianfanling quartz–vein type molybdenum deposit is located in the East Qinling–Dabie Mo belt. The Mo mineralization in the deposit is mainly present as structurally controlled Mo-bearing quartz veins. Based on chromatography analysis of the Qianfanling Mo deposit, the gas compositions of fluid inclusions in quartz dominantly contain H2O and relatively high concentrations of CO2 (11.019 to 18.554 mol%). Cations and anions of liquid compositions of fluid inclusions are mainly Na⁺ and Ca2 +, and SO42 − and Cl⁻, respectively. The characteristics of gas and liquid compositions of fluid inclusions in quartz indicate that the Qianfanling Mo deposit formed under conditions of high oxygen fugacity. Analytical results of He-Ar isotopes of fluid inclusions in pyrite from the Qianfanling Mo deposit show that ³He/⁴He ratios vary from 0.02438 to 0.0796 Ra (Ra is the ³He/⁴He ratio of air = 1.40 × 10− 6), with F⁴He values of 252,509–885,224. The calculated Hemantle values range from 0.18 to 0.87%, with an average of 0.39%. ⁴⁰Ar/³⁶Ar ratios are 694.4–1473.4, with the estimated ⁴⁰Ar* (radiogenic ⁴⁰Ar) proportion of 79.9%. These He-Ar isotopic data indicate that most of the He in the fluid inclusions was derived from the crust, with negligible atmospheric He and only a very small input from the mantle probably. Furthermore, the Ar in the ore-forming fluid was mainly derived from the crustal source, with minor atmospheric Ar involved. 17 Re–Os model ages of 15 molybdenite samples from the 4 quartz-vein type Mo deposits (Qianfanling, Daxigou, Maogou, and Zhifang) range from 215.0 to 248.2 Ma, with a mean value of 235.6 Ma, which is close to the age of peak collision between the North China and Yangtze Cratons. The Re contents in molybdenite from the 4 deposits range from 0.19 to 39.16 ppm, with an average of 17.41 ppm, indicating a crust-dominated source. Re contents in molybdenite from Mesozoic Mo deposits in different ages and types in the East Qinling–Dabie Mo belt show different characteristics of ore sources. Combined with gas and liquid compositions and He-Ar isotopic characteristics of fluid inclusions, as well as Re contents in molybdenite of this paper and previous studies, we suggest that the ore-forming fluid and material of the Qianfanling Mo deposit were mainly derived from crustal magmatic source. This magmatic source could have formed by partial melting of deep crustal rocks under a certain condition during the collision between the North China and Yangtze Cratons.
Two contrasting models have been proposed to explain the large-scale gold mineralization in the Xiaoqinling gold district, which is the second largest gold producer in China. The first model advocates that gold deposits in the Xiaoqinling district formed in the Triassic in relation to continental collision of the Qinling Orogen, whereas the second indicates that gold mineralization occurred in the Early Cretaceous and was genetically related to lithospheric thinning of the North China Craton. In this paper, we present high precision monazite U-Th-Pb ages of the Qinnan gold deposit in the north metallogenic belt of the Xiaoqinling district, by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Petrographic observations show the monazite occurs as euhedral grains and invariably intergrown with quartz and pyrite, suggesting that monazites are of hydrothermal origin and precipitate with gold-bearing minerals coevally. In the 206Pb/238U-207Pb/235U and 206Pb/238U-208Pb/232Th diagrams, all of the 13 analyses are concordant or nearly concordant, with weighted mean 206Pb/238U and 208Pb/232Th ages of 120.9±0.9 Ma (MSWD=1.0) and 122.6±1.9 Ma (MSWD=2.6), respectively. These ages are reproducible with existing geochronological data for major gold deposits in the Xiaoqinling district, demonstrating that the U-Th-Pb isotopes of the monazite have remained closed since its precipitation, and thus the U-Th-Pb ages can be reliably interpreted as the mineralizing age of the Qinnan gold deposit. A synthesis of available age constraints suggests that gold mineralization throughout the Xiaoqinling district occurred with a relatively short time interval in the Early Cretaceous and was likely related to the lithospheric thinning of the North China Craton at that time.
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.
Zhangshiying intrusive complex (ZIC) , consisting of K-feldspar granite, granite porphyry and quartz porphyry, is located in the southern margin of the North China Craton. The mafic enclaves are merely in K-feldspar granite and have textures indicating magma mixing. We obtained their SHRIMP zircon U-Pb ages of 107. 3 ± 2.4Ma, 106. 7 ± 2. 5Ma and 101 ± 3Ma for K-feldspar granite, granite porphyry and quartz porphyry, respectively. According to zircon Hf isotope analyses, their εHf(t) values are -15.96 - -20.80, - 18.97 - -22. 18 and -23. 41 - -27. 95 and their corresponding Hf two-stage model ages (tDM2) are 1880-2018 Ma, 1925-2080Ma and 2144 - 2330 Ma One exception is the zircon inherited Archean information with 207Pb/206Pb age of 2. 6 Ga, -0. 71 of εHf(t) and 3. 0 Ga of tDM2. These data suggest that the source of ZIC is mainly either juvenile crust formed at 1.9 - 2.3Ga or an enriched mantle source with the contribution of Archean continental crust. On integration of regional research results, we suggest that ZIC could form at a regional lithospheric extensional setting which is confined to the existing NE-SW or NW-SE fault zones and is triggered by the drifting direction of the Pacific plate changing to southwest ward.
Zircon U-Pb dating for three large granitoid batholiths, geochemical and Hf isotope analyses for two of them have been carried out in the different blocks of the Qinling. Zircon dating by LA-ICPMS for the Lantian granitoid in the southern margin of the North China Block gives an age of 133 ± IMa. The granitoids show whole-rock εNd(0 = -11.8 ∼ -18. 3, zircon εHf(t) = -37. 7 ∼ -5. 7. The Muhuguan and the Mangling granitoids in the North Qinling orogen yield zircon LA-ICPMS and SHRIMP age of 150 ± IMa and 149 ± 2Ma, respectively. The Muhuguan granitoids show whole-rock εNd (0 = - 7. 6 ∼ -11.4 and zircon εHf(t) = - 7. 3 ∼ - 17. 4. Combined with statistical analyses of 26 zircon ages and 3 Ar-Ar ages of granitoid plutons collected from the literature, the Late Mesozoic magmatism in the Qinling can be divided into two stages. The first-stage magmatism (160 ∼130 Ma) widely occurred in the southern margin of the North China Block, the North Qinling and occasionally in the South Qinling, and has features of I-type granitoids. The second-stage magmatism (120 ∼100 Ma) characterized by transition of I-to A-type and A-type granitoids took place in the North Qinling and the eastern part of the southern margin of the North China Block. The first-stage magmatism occurred in a transition from contractional to extensional settings, while the second-stage in the extension setting. The formation of the grantoids are interpreted as partial melting of old crust, mixed with juvenile mantle component. The second-stage granitoids contain more juvenile component than the first-stage. The isotope data of the granitoids show that juvenile compositions of the basement increas from the southern margin of the North China Block to the Shangdan suture.
The Zhangshiying pluton is located in the southern margin of the North China Craton. Zircon LA-ICP-MS U-Pb dating for Zhangshiying quartz syenite pluton yield 122.8 ± 1.5Ma. Those quartz syenite are alkaline rocks with SiO2 = 66.04% ∼ 67.80%, Na2O + K2O = 9.03% ∼ 10.97%, K2O = 4.40% ∼ 6.37%, K2O/Na2O > 1, A/CNK = 1.26 ∼ 1.58, and A/NK = 1.63 ∼ 1.79. Mg# ranges from 12.9 to 39.4. They are enriched in LREE and depleted in HREE. There is obvious partition of LREE from HREE with (La/Yb)N = 15.48 ∼ 21.12, and with slightly negative Eu-abnormal (δEu = 0.54 ∼ 0.99), and it is enriched in LILE e. g. Rb, K, Th, U, and depleted in HFSE, e. g. Nb, Ta, P and Ti. The εHf(t) for quartz syenite ranges from -17.6 to -13.9, with tDM2 of 1.7 ∼ 1.9Ga. Its magma zirconium saturation temperature ranges between 940 ∼ 1000°C. Geochemical and isotopic composition indicate that the source of the Zhangshiying pluton is mainly old lower crust and minor juvenile composition. The juvenile composition is mainly from mantle. The pluton was generated under an extensional setting. The extensional setting induced mantle magma upwelling, providing heat and causing the partial melting of the bottom crust. The age of pluton is at the same time of the lithospheric thinning and destruction of the North China craton. Zhang Shiying pluton is the response of this geological event.
Lode gold deposit is the most economically important type of gold deposits worldwide. It commonly occurs in Archean to Paleoproterozoioc metamorphic terrains and has close spatial relationship with mafic dykes. However, it is still in dispute whether mafic dykes and gold mineralization are genetically related. In this paper, we present LA-ICPMS zircon U-Pb and biotite 40Ar-39Ar ages of four mafic dikes samples collected from three major gold deposits (Dongchuang, Dahu and Qiangma deposits) in the Xiaoqinling gold district, southern margin of the North China craton. Four samples yielded a consistent zircon LA-ICPMS age (207Pb/206Pb 1819 ± 10 Ma) and a slightly younger 40Ar/39Ar plateau age (1719.0 ± 21.0 Ma) of biotite. Our results suggest that: (1) numerous mafic dikes in the Xiaoqinling gold district formed in Paleoproterozoioc, in an extensional setting after the collision between the eastern and western blocks of the North China craton at ca. 1.85 Ga; (2) previous whole rock K-Ar and Rb-Sr ages (187.6-75.9 Ma) which are significantly younger than our ages represent thermal perturbation (or cooling ages) caused by Mesozoic tectonic-thermal event. Recent geochronological studies show that the majority of gold deposits formed in Early Cretaceous (130-120 Ma) in the Xiaoqinling district. Therefore, the mineralization event had no relationship with Paleoproterozoic mafic dykes although they were spatially associated. However, this study does not rule out the possibility of existence of Mesozoic mafic dykes in the Xiaoqinling area, which may be very important for the understanding of the metallogeny and regional tectonic setting.
A decade of molybdenum deposits comprising several large-supper large ones have been explored in the past several years. At the same time a lot of vein-type Pb-Zn-Ag deposits are discovered, which are distributed around the porphyry molybdenum deposits, such as the Nannihu giant porphyry molybdenum ore district and the Donggou giant porphyry molybdenum deposit, a new property. These vein-type Pb-Zn-Ag deposits obviously exhibit a close relationship to the porphyry Mo deposits in space and time, sharing one ore system. The porphyry molybdenum deposits in the episodes of 156-137 Ma and 125-114 Ma represented by the Nannihu and the Donggou, are derived from anatexis of the mantle and crust, and the crust only, respectively. Although the two episodes of mineralization have differences of the ore-forming time and sources the mineralization zoning reflects a normal temperature-decreasing process around the high-fractionation granite plutons associated with the all porphyry deposits. On the other hand, the granite plutons are able to act as energy engines, which triggered to form the convective hydrothermal system. The hydrothermal solution in the system leached the ore-substances from the host rocks of carbonate and discharged them in the available structures, for instance, the extension locations of faults, and discordant boundary of the strata. In light of the basic characteristics of the ore system and understanding for the metallogenic process, we propose a new model to describe.
The hydrothermal ore-forming process of the Nannihu-Sandaozhuang Mo (W) deposit can be divided into four stages, i.e., from early to late, (1) skam and hornfels stage, (2) quartz - K-feldspar and retrograde stage, (3) quartz-sulfides stage and (4) quartz-carbonate stage. Based on studies of carbon, hydrogen and oxygen isotopes of different stages, the authors hold that the ore -forming fluid was derived mainly from the magmatic system and subordinately from the carbonate strata with minor organic matter. The atmospheric water probably made very little contribution to the ore -forming process. From early to late stage, the ore -forming process probably experienced immiscibility between melt and volatile-rich hydrothermal fluid, water-rock reaction, and phase separation of CO2 rich gas from ore fluid caused by sudden decompression.
Nannihu-Sandaozhuang molybdenum deposit is the largest porphyry-skarn type deposit in eastern Qinling region. This paper reports new LA-ICP-MS zircon U-Pb age data of granitoids (including granitic veins) and ICP-MS molybdenite Re-Os isotopic age data of the samples from the mining area. The obtained formation ages of granitoids are 145.2 ± 1.5 - 146.7 ± 1.2Ma, and the model ages of 5 molybdenite samples from stockwork in the Nannihu mining area and 5 molybdenite samples from disseminated skarn in the Sandaozhuang mining area are 143.4 ± 2.0 - 146.5 ± 2.3Ma with isochron age of 146.0 ± 1.1Ma. These new geochronological data show explicitly that the granitoids and ores were formed simultaneously, suggesting their genetic relationship. Furthermore, these new data have confirmed the existence of magma-thermal event at about 158Ma revealed by previous dating work, and also for the first time revealed that the eastern Qinling area once underwent another magma-thermal event at about 175Ma. The published weighted average age data are of distinct deviation from the real intrusive age of the Nannihu magmatic body, which results from 1) nonhomogeneous origin of zircons in magmatic rock and 2) unrepresentative dating of zircon group.
The East Qinling–Dabie molybdenum belt, which is located in the nearly east–west trending Qinling–Dabie orogen, East China, is the largest Mo belt in the world and is characterized by extensive Mesozoic mineralization. Although the late Early Cretaceous (ca. 120–100 Ma) is the latest stage of the Mo mineralization in the East Qinling–Dabie Mo belt, this period shows relatively intensive Mo mineralization, such as the world-class Shapinggou Mo deposit. All of the late Early Cretaceous Mo deposits are closely related to granitic magmatism, some of which are typical porphyry-type deposits (e.g., Donggou, Tangjiaping, Shapinggou). Homogenization temperatures of fluid inclusions in the late Early Cretaceous Mo deposits range from 115 to 550 °C, with salinities ranging from 0.02 to 62.1 wt.% NaCl equivalent. Different amount of CO2 have been recognized in the hydrothermal systems of these Mo deposits, indicating H2O–NaCl–CO2 systems. The ore-forming fluids in these Mo deposits were magmatic water mixed with different amount of meteoric water. Except the Donggou deposit, the δ34S values of sulfides from the late Early Cretaceous Mo deposits have a narrow range of 0.4–6.3‰, suggesting a deep magmatic source for the sulfur. The Donggou Mo deposit has slightly higher δ34S values (7.5–9.4‰), reflecting that the ore sulfur was mainly derived from magma, probably with some upper crust materials. Available isotopic data show that the ore-associated magmas of the late Early Cretaceous Mo deposits were mainly derived from crustal materials, probably with some contribution of mantle or newly added crust. The Donggou, Tangjiaping, and Shapinggou Mo deposits are genetically related to A-type magmatism, suggesting that they formed in an extensional setting. The late Early Cretaceous Mo deposits in the East Qinling–Dabie Mo belt formed in an intraplate extensional setting accompanied by the lithospheric thinning and asthenospheric upwelling.
The newly discovered porphyry Cu deposits in the South Qinling Belt (SQB) have not been well researched as compared with the large porphyry Mo province in the southern North China Block (S-NCB), and the origin of granitoids associated with porphyry Cu mineralization in the Zha-Shan district, SQB is poorly constrained. Here, we present detailed zircon U–Pb geochronological, whole rock elemental and Sr–Nd isotopic data for important Late Mesozoic granitoid stocks associated with porphyry Cu deposits in the Zha-Shan district; these data are used to constrain the age and the source of magmas that formed these granitoids, and implication of regional metallogeny. The new zircon LA-ICPMS U–Pb ages presented here indicate that the granitoids related to porphyry Cu system at Chigou, Beishagou, Shuangyuangou and Yuanjiagou developed at 148–144 Ma, 144 Ma, 145–144 Ma and 146 Ma, respectively. These rocks are high-K calc-alkaline I-type granitoids, which are enriched in large ion lithophile elements (e.g., Th, U, and Pb) and light rare earth elements, are depleted in Nb, Ta and Ti, characterizing by wide variations in initial εNd(t) (−3.8 to −9.5), and moderate radiogenic Sr isotopes ((87Sr/86Sr)i = 0.7046 to 0.7093). These features indicate that the magmas that formed the granitoids related to porphyry Cu system in the Zha-Shan district formed as a result of variable degrees of mixing between crustal and metasomatic lithospheric mantle. The new zircon LA-ICPMS U–Pb ages in this study, combined with previous published data, suggest that regional-scale Late Jurassic to Early Cretaceous granitoid stocks, and associated porphyry Cu and Mo systems in both the S-NCB and SQB formed almost contemporaneously, with 147–139 Ma porphyry Mo deposits in the S-NCB and 148–145 Ma porphyry Cu deposits in the SQB. The Cu-related intrusions contained a greater contribution of lithospheric mantle component than the Mo-related intrusions in the East Qinling Orogeny.
The Qianfanling Mo deposit, located in Songxian County, western Henan province, China, is one of the newly discovered quartz-vein type Mo deposits in the East Qinling–Dabie orogenic belt. The deposit consists of molybdenite in quartz veins and disseminated molybdenite in the wall rocks. The alteration types of the wall rocks include silicification, K-feldspar alteration, pyritization, carbonatization, sericitization, epidotization and chloritization. On the basis of field evidence and petrographic analysis, three stages of hydrothermal mineralization could be distinguished: (1) pyrite–barite–quartz stage; (2) molybdenite–quartz stage; (3) quartz–calcite stage.
Lode gold deposits in Precambrian cratons represent the world's major gold source and were mostly generated during formation and stabilization of the cratons. However, there is an extraordinary exception in the North China craton (NCC), where lode gold deposits formed after prolonged stabilization of the craton. Molybdenite Re–Os and hydrothermal sericite and biotite 40Ar/39Ar dating of major gold deposits from the Xiaoqinling district, southern NCC, bracket their emplacement in the range of 154.1±1.1 to 118.9±1.2 Ma (n=23), postdating formation of the craton by more than 1.7 billion years. Fluid inclusions extracted from gold-bearing pyrite have elevated 3He/4He ratios (1.52–0.22 Ra) and mantle-like Ne isotopes (20Ne/22Ne=10.02−9.22 and 21Ne/22Ne=0.033−0.027), indicating presence of mantle-derived fluids in the ore system. Measured δ34S of pyrite and δD and δ18O of hydrothermal micas and fluid inclusion waters in auriferous quartz further confirm a magmatic/mantle source for sulfur and ore fluids. Gold deposits of similar ages also widely occur in the eastern and northern margins of the NCC, which, together with those in the Xiaoqinling district, have a total reserve of ∼2500 t gold, forming the only known giant late Mesozoic gold province in the world's Precambrian cratons. These deposits formed coevally with extensive felsic to mafic magmatism, development of intracontinental rift basins, and exhumation of metamorphic core complexes across the eastern NCC, events interpreted as indicating thinning and destruction of the lithosphere beneath the craton. Rising of asthenosphere coupled with destruction of the lithosphere has generated voluminous mafic and felsic magmas that provided sufficient fluids, sulfur and, by inference, other ore components to form the giant gold provinces.
The Eastern Qinling region, located in the southern margin of the North China Craton, is characterised by widespread granitoid intrusions. Precise geochronological constraints on the age and timing of these intrusions are lacking. In this paper, we report SHRIMP zircon U-Pb ages of 12 representative granitoid plutons, together with one syenite stock, dolerite, diorite and granitic dykes in Eastern Qinling. The results revealed two main magmatic events, which occurred in the Late Jurassic-Early Cretaceous (158 ± 3 to 136 ± 2 Ma) and the Early Cretaceous (134 ± 1 to 108 ± 2 Ma), respectively. The granitoids formed in the early magmatic event are similar in composition and are characterised by a mantle source mixed with variable amounts of crustal components, whereas those resulting from the late-stage magmatism show characters of I-, S- and A-types granite and coexist with coeval dolerite dykes and syenite stocks. A similar magmatic age distribution is also recognised in other belts or regions in East China, and even in other parts of the East Eurasian continental margin. This suggests that the Jurassic-Early Cretaceous magmatism was associated with the subduction of the Izanagi plate at a shallow angle or flat-slab subduction beneath the eastern China continent, whereas the Early Cretaceous magmatism was related to lithospheric thinning, asthenospheric upwelling and partial melting of the lower crust, induced by a change in Izanagi plate motion parallel to the continent margin.
The East Qinling–Dabie orogenic belt accommodates the largest Mo ore district in the world. It contains 8.43 Mt of proven Mo metal reserves which accounts for 66% of the total proven Chinese Mo reserves. The Mo ore district includes 24 deposits and 12 occurrences, with four major types of Mo mineral systems, i.e., porphyry, porphyry-skarn, skarn and hydrothermal veins. The latter can be further subdivided into quartz vein and carbonatite vein types. Although Mo mineralization in the belt began in the Paleoproterozoic (1680 ± 24 to 2044 ± 14 Ma), all economically significant deposits were formed during the Mesozoic. Re/Os dating of molybdenite has shown that there are three episodes of Mo mineralization, i.e., Late Triassic (233–221 Ma), Late Jurassic to Early Cretaceous (148–138 Ma) and Early to middle Cretaceous (131–112 Ma).
The NWW-striking Qinling Orogen formed in the Triassic by collision between the North China and Yangtze Cratons. Triassic granitoid intrusions, mostly middle- to high-K, calc-alkaline, are widespread in this orogen, but contemporaneous intrusions are rare in the southern margin of the North China Craton, an area commonly considered as the hinterland belt of the orogen. In this paper, we report zircon U–Pb ages, elemental geochemistry, and Sr–Nd–Hf isotope data for the Laoniushan granitoid complex that was emplaced in the southern margin of the North China Craton. Zircon U–Pb dating shows that the complex was emplaced in the late Triassic (228±1 to 215±4Ma), indicating that it is part of the post-collisional magmatism in the Qinling Orogen. The complex consists of, from early to late, biotite monzogranite, quartz diorite, quartz monzonite, and hornblende monzonite, which span a wide compositional range, e.g., SiO2=55.9–70.6wt.%, K2O+Na2O=6.6–10.2wt.%, and Mg# of 24 to 54. The biotite monzogranite has high Al2O3 (15.5–17.4wt.%), Sr (396–1398ppm) and Ba (1284–3993ppm) contents and relatively high La/Yb (mostly 14–30) and Sr/Y (mostly 40–97) ratios, but low Yb (mostly 1.3–1.6ppm) and Y (mostly14–19ppm) contents, features typical of adakitic rocks. The quartz monzonite, hornblende monzonite and quartz diorite have a shoshonitic affinity, with K2O up to 5.58wt.% and K2O/Na2O ratios averaging 1.4. The rocks are characterized by strong LREE/HREE fractionation in chondrite-normalized REE pattern, without obvious Eu anomalies, and show enrichment in large ion lithophile elements but depletion in high field strength elements (Nb, Ta, Ti). The biotite monzogranite (228Ma) has initial 87Sr/86Sr ratios of 0.7061 to 0.7067, εNd(t) values of −9.2 to −12.6, and εHf(t) values of −9.0 to −15.1; whereas the shoshonitic granitoids (mainly 217–215Ma) have similar initial 87Sr/86Sr ratios (0.7065 to 0.7075) but more radiogenic εNd(t) (−12.4 to −17.0) and εHf(t) (−14.1 to −17.0). The Sr–Nd–Hf isotope data indicate that the rocks were likely generated by partial melting of an ancient lower continental crust with heterogeneous compositions, as partly confirmed by the widespread presence of early Paleoproterozoic inherited zircons. Mafic microgranular enclaves (MMEs), characterized by fine-grained igneous textures and an abundance of acicular apatites, are common in the Laoniushan complex. Compared with the host rocks, they have lower SiO2 (48.6–53.7wt.%) and higher Mg# (51–56), Cr (122–393ppm), and Ni (24–79ppm), but equivalent Sr–Nd isotope compositions, indicating that the MMEs likely originated from an ancient enriched lithospheric mantle. The abundance of MMEs in the granitoid intrusions suggest that magma mixing plays an important role in the generation of the Laoniushan complex. Collectively, it is suggested that the Laoniushan complex was a product of post-collisional magmatism related to lithospheric extension following slab break-off. Formation of the adakitic and shoshonitic intrusions in the Laoniushan complex indicates that the Qinling Orogen had evolved into a post-collisional setting by about 230–210Ma.
This contribution reviews the tectonic structure and evolution of the Qinling orogenic belt, which extends east–west nearly 2500 km across Central China and is a giant orogenic belt formed by the convergence and collision between North China and South China Blocks. The principal tectonic elements including metamorphic basement and its Neoproterozoic to Triassic cover, ophiolitic sutures, nature and ages of granitoid belts, provenance studies and tectonometamorphic studies of metamorphic belts allow tracing the polarity of two stages of plate convergence and collision and the further tectonic history. In this review, we present new distribution maps of the Early Paleozoic ophiolites and associated volcanics in the Shangdan suture zone and the Middle Devonian–Middle Triassic ophiolitic melange in the Mianlue suture zone, as well as the maps of granitoid and metamorphic belts displaying various ages (Silurian–Devonian, Triassic, Late Jurassic–Early Cretaceous). These maps allow better constrain the polarity of subduction and collision. We also discuss the significance of the Early Cretaceous Yanshanian events, which represent a linkage between tectonic events in the Tethyan and East China/Pacific realms.
The East Qinling–Dabie molybdenum belt is part of a larger East–West trending metallogenic belt in eastern China. Most of the molybdenum deposits occur as porphyry or porphyry–skarn type, but there are also some vein type deposits. Following systematic Re–Os dating of molybdenite from 13 deposits and comparisons with two previously dated deposits, we have recognized that the molybdenum mineralization in the East Qinling–Dabie belt was developed during hydrothermal activity linked to magmatism and the emplacement of granitoid stocks. Three pulses of granitoid magmatism and Mo mineralization are recognized corresponding to significant tectonic events in the East Qinling–Dabie belt. Vein type deposits dated at 233–221 Ma were formed in detachment fractures, indicating localized extension within the collisional setting of the North China and Yangtze Cratons. I-type and transitional I- and S-type granites and related mineralization dated at 148–138 Ma may have formed part of a continental magmatic arc, with widespread magmatism and back-arc extension caused by subduction of the Izanagi or Paleopacific plate beneath the Eurasian continent in a WNW–ESE direction in the Late Jurassic–Early Cretaceous. Both S-type and transitional S- and I-type granite-associated porphyry molybdenum deposits dated at 131–112 Ma are part of an extensive mineralization event throughout East China that can be ascribed to regional large-scale lithospheric thinning, delamination and thermal erosion.
Geochemistry of two groups of late Mesozoic volcanic rocks from the north Huaiyang belt (NHB) of the northern Dabie orogen provide evidence for their derivation from an enriched lithospheric mantle with subducted continental crustal relicts. The older group (149–137 Ma) consists of trachyandesites and trachydacites, showing light rare earth element (LREE) and large ion lithophile element (LILE) enrichment and strong high field strength element (HFSE, e.g., Nb, Ta, P and Ti) depletion, and highly enriched Sr–Nd isotopic signatures (87Sr/86Sr(i)=0.7082–0.7098 and εNd(t)=−24.4 to −19.1). Despite variations in SiO2, MgO and isotopic ratios, they exhibit slight change in TiO2, P2O5, Nb and LREE, suggesting that source differences rather than magmatic processes could be a major controlling factor for their genesis. The younger group (132–116 Ma) is mainly composed of basaltic trachyandesites and trachyandesites, having more significant LREE and LILE enrichment and HFSE depletion but less enriched Sr–Nd isotopic compositions (87Sr/86Sr(i)=0.7080–0.7084 and εNd(t)=−19.2 to −16.2) than the older group. The time-integrated elemental and isotopic variations from groups 1 to 2 show that less continental crustal materials were involved in producing the younger group of basaltic magmas because of easy exhaustion of the crustal rocks trapped in the lithospheric mantle during generation of the older intermediate-felsic melts. The petrogeneses of the two volcanic suites in the NHB imply that, though some of the subducted continental slices were rapidly exhumed to the crust shortly after collision, a significant volume of the subducted continental crust might be trapped in the lithospheric mantle.
Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≃ Rb ≃ (≃ Tl) ≃ Ba(≃ W) > Th > U ≃ Nb = Ta ≃ K > La > Ce ≃ Pb > Pr(≃ Mo) ≃ Sr > P ≃ Nd (> F) > Zr = Hf ≃ Sm > Eu ≃ Sn (≃ Sb) ≃ Ti > Dy ≃ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the istopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. -from Authors
Trace element abundances in igneous zircons, as determined by electron microprobe and laser-ablation microprobe ICPMS analysis, are shown to be sensitive to source rock type and crystallisation environment. The concentrations of 26 trace elements have been determined for zircons from a wide range of different rock types and reveal distinctive elemental abundances and chondrite-normalised trace element patterns for specific rock types. There is a general trend of increasing trace element abundance in zircons from ultramafic through mafic to granitic rocks. The average content of REE is typically less than 50 ppm in kimberlitic zircons, up to 600-700 ppm in carbonatitic and lamproitic zircons and 2,000 ppm in zircons from mafic rocks, and can reach per cent levels in zircons from granitoids and pegmatites. Relatively flat chondrite-normalised REE patterns with chondrite-normalised Yb/Sm ratios from 3 to 30 characterise zircons from kimberlites and carbonatites, but Yb/Sm is commonly over 100 in zircons from pegmatites. Th/U ratios typically range from 0.1 to 1, but can be 100-1000 in zircons from some carbonatites and nepheline syenite pegmatites. The geochemical signatures characteristic of zircon from some rock types can be recognised in bivariate discriminant diagrams, but multivariate statistical analysis is essential for the discrimination of zircons from most rock types. Classification trees based on recursive partitioning techniques provide a rapid means of relating parent rock type to zircon trace element analysis; zircons from many rock types can be discriminated at confidence levels of 75% or more. These trees allow recognition of the provenance of detrital zircons from heavy mineral concentrates, and significantly enhance the usefulness of zircon in regional crustal studies and as an indicator mineral in mineral exploration.
Geochemical characteristics and the provenance of the Cenozoic basic volcanic rocks from Western Henan province are studied in this paper. Cenozoic basic volcanic rocks in Western Henan are mainly trachybasalts, tephrites and basalts, which enrich in LREE and incompatible trace elements. The trachybasalt is predominant in these rocks. The chemical characteristics of these rocks indicate that these rocks probably represent the primary magma that derived by small degree melting of mantle rocks. The fractional crystallization and crustal rock assimilation during the magma evolution had little effect on the chemical composition of this magma. Based on its isotopic features, the provenances of these volcanic rocks is analyzed. It is believed that Cenozoic basic volcanicrocks of this area are the mixture of the melts from the Depleted Mantle (DM) and the Enriched Mantles (EMI and EMII).
The SHRIMP U-Pb ages are reported for two volcanic rocks from the Longwangshan and the Dawangshan formations respectively, Ningwu area, Jiangsu Province. The Dawangshan formation (NB-01) is dated at (127 3) Ma, and the Longwangshan formation (NL-01), (131 4) Ma. Besides, a few Archean zircons are also found in the Longwangshan formation, which suggests a possible Archean basement in the region.
Recycling of subducted continental crust in continental collision belts is examined by a combined study of zircon U-Pb dating and O isotope analysis of mineral separates from post-collisional granitoids in the Dabie terrane, east-central China. Although zircon U-Pb dating give concordant ages of 121 to 131 Ma for magma crystallization, the occurrence of inherited cores is identified by CL imaging and SHRIMP dating in some zircon grains that yield older ages of 742 to 815 Ma and 222 Ma, respectively, in agreement with Neoproterozoic protolith ages of UHP metaigneous rocks and Triassic tectono-metamorphic event in the Dabie-Sulu orogen. All the samples have zircon δ18O values of 4.14 to 6.11‰, most of which differ from the δ18O value of 5.3±0.3‰ for the normal mantle zircon. This indicates the remelting of continental crust containing both 18O-depleted and enriched components, with significant isotope homogenization relative to the UHP metaigneous rocks. These observations indicate the involvement of the subducted Yangtze plate in the Cretaceous granitoids and thus provide evidence for geochemical recycling of the continental crust that was thickened by the Triassic collision.
The suture zone between the Sino-Korean and Yangtse cratons in central China is marked by a series of fault-bounded belts composed of crustal protoliths. In the Dabie Mountains some of these blocks contain coesite and diamond, indicating subduction to minimum depths of ˜120 km as a result of continental collision. The blocks range from the structurally lowest ultrahigh-pressure metamorphic belt and decrease in metamorphic grade upsection to the structurally highest, low greenschist facies belt. Furthermore, the faults mark large contrasts in metamorphic grade. These features form a structure that is similar to metamorphic core complexes, indicating extension as part of the exhumation process. Timing of the continental collision has previously been inferred as Late Triassic, from regional geologic relations, or Caledonian, from 40Ar/39Ar dating of the adjacent and possibly related Qinling metamorphic belt. U-Pb dating of zircon from ultra-high-pressure eclogites yields a metamorphic age of 209 ±2 Ma. This age coincides with later stages of collision and is therefore a minimum estimate of the time of collision.
Single zircons from two Early Cambrian volcanic horizons have been analysed using the SHRIMP ion microprobe. Full details of the analytical procedures and data reduction are given. Zircons from tuff within the Lie de Vin Formation, near Tiout, Morocco, show little spread in U-Pb age and have a mean value of 521 +/- 7 Ma (2-sigma). Those from a bentonite within unit 5 of the Meishucun section near Kunming, southern China, show relatively dispersed U-Pb ages, revealing the presence of both detrital or xenocrystic grains as well as areas within grains that have lost radiogenic Pb. The main population has as mean age of 525 +/- 7 Ma, but a mean Pb-207/Pb-206 age of 539 +/- 34 Ma which is a maximum estimate for the bentonite age. These results conflict with previous Rb-Sr whole rock ages of c. 580 Ma for overlying Cambrian shales at Meishucun, and c. 570 Ma for Atdabanian shales from the E. Yangtse Gorges area.
Geochronological, elemental and Sr–Nd–Pb isotopic data of early Cretaceous basic-intermediate rocks from the Dabie Orogen provide new insights into the nature of the late Mesozoic lithospheric mantle beneath the region and its tectonic relationship with neighboring blocks. Basic-intermediate rocks from the North Dabie Complex (NDC) include diabases, lamprophyres and trachyandesites, which have 40Ar/39Ar plateau ages of 127.6–131.8 Ma. Similar rock types from the North Huaiyang Unit (NHY) erupted at nearly the same time (135–116 Ma). Coeval rocks from both tectonic units form a continuous array in Harker diagrams, and exhibit similar geochemical characteristics. Both are significantly enriched in LILEs, and depleted in HFSEs, coupled with very low ɛNd(t), (206Pb/204Pb)i, and prominent positive Δ8/4 and Δ7/4 values, which are similar to those of Mesozoic mafic rocks in the North China Craton (NCC) exterior. These geochemical signatures are inconsistent with crustal contamination during magma ascent, and reflect derivation from an enriched lithospheric mantle source contaminated by the deeply subducted Yangtze crust. The observed geochemical similarities thus suggest that early Cretaceous igneous rocks from the NDC and NHY share a similar continental lithospheric mantle source that is tectonically affiliated to the NCC, although the surface geology of both tectonic units correlates with that of the Yangtze Block. Tectonic decoupling along a suture is proposed to explain the generation of early Cretaceous mafic rocks in the Dabie Orogen. The Wuhe-Shuihou fault likely represents the Mesozoic lithospheric boundary between the Yangtze Block and NCC, despite the fact that the present-day surface suture is situated at the Xiaotian-Mozitan fault or other faults to the north.
The largest gabbroic intrusion, Jiaoziyan, in the Northern Orthogneiss
of the Dabie Shan was investigated to test whether such intrusions
represent arc-related or collision-related magma, as postulated in
recent regional tectonic models. The Jiaoziyan intrusions comprise a
gabbro-monzogabbro suite and are locally characterized by cumulus
textured anhydrous silicates. Petrologic indications against arc
magmatism include (1) coexisting olivine-plagioclase compositions beyond
the documented range of arc gabbros, and (2) a compositional trend of
Al/Ti ratios in the gabbroic clinopyroxenes that is significantly
different from subduction-related gabbros worldwide. In contrast, the
Jiaoziyan rocks are mineralogically similar to rift gabbros.
Geochronologic and geometric arguments against collision-related
magmatism (i.e., as a consequence of slab breakoff) include the
following: (1) crystallization of the Jiaoziyan mafic intrusion
postdated the Triassic (245 240 Ma) subduction and peak orogenic
metamorphism by ˜100 m.y. and (2) the rocks of the Dabie
Cretaceous magmatic suite, including Jiaoziyan, are on the footwall
plate, rather than on the hanging wall, of the Triassic subduction-zone
framework and do not show a linear distribution. These observations
argue that the voluminous Early Cretaceous magmatism in the Northern
Orthogneiss is geodynamically unrelated to the Qinling-Dabie orogeny,
but was caused by extensional tectonism related to regional magmatic
underplating beneath the thinned Dabie crust.
The Re–Os (rhenium–osmium) chronometer applied to molybdenite (MoS2) is now demonstrated to be remarkably robust, surviving intense deformation and high-grade thermal metamorphism. Successful dating of molybdenite is dependent on proper preparation of the mineral separate and analysis of a critical quantity of molybdenite, unique to each sample, such that recognized spatial decoupling of 187Re parent and 187Os daughter within individual molybdenite crystals is overcome. Highly precise, accurate and reproducible age results are derived through isotope dilution and negative thermal ion mass spectrometry (ID-NTIMS). Spatial decoupling of parent–daughter precludes use of the laser ablation ICP-MS microanalytical technique for Re–Os dating of molybdenite. The use of a reference or control sample is necessary to establish laboratory credibility and for interlaboratory comparisons. The Rb–Sr, K–Ar and 40Ar/39Ar chronometers are susceptible to chemical and thermal disturbance, particularly in terranes that have experienced subsequent episodes of hydrothermal/magmatic activity, and therefore should not be used as a basis for establishing accuracy in Re–Os dating of molybdenite, as has been done in the past. Re–Os ages for molybdenite are almost always in agreement with observed geological relationships and, when available, with zircon and titanite U–Pb ages. For terranes experiencing multiple episodes of metamorphism and deformation, molybdenite is not complicated by overgrowths as is common for some minerals used in U–Pb dating (e.g. zircon, monazite, xenotime), nor are Re and Os mobilized beyond the margins of individual crystals during solid-state recrystallization. Moreover, inheritance of older molybdenite cores, incorporation of common Os, and radiogenic Os loss are exceedingly rare, whereas inheritance, common Pb and Pb loss are common complications in U–Pb dating techniques. Therefore, molybdenite ages may serve as point-in-time markers for age comparisons.
The Jinniu Basin in southeast Hubei, located at the westernmost part of middle-lower valley of the Yangtze River, is one of
the important volcanic basins in East China. Volcanic rocks in the Jinniu Basin are distributed mainly in the Majiashan Formation,
the Lingxiang Formation and the Dasi Formation, consisting of rhyolite, basalt and basaltic andesite, (trachy)-basalt and
basaltic trachy-andesite and (trachy)-andesite and (trachy)-dacite and rhyolite respectively, in which the Dasi volcanism
is volumetrically dominant and widespread. The Dasi volcanic rocks were selected for SHRIMP zircon U-Pb dating to confirm
the timing of volcanism. The results indicate that there exist a large amount of magmatic zircons characterized by high U
and Th contents in the volcanic rocks. The concordia ages for 13 points are 128 ± 1Ma (MSWD = 3.0). On account of the shape
of zircons and Th/U ratios, this age is considered to represent the crystallization time of the Dasi volcanism. The volcanic
rocks in the Dasi, Majiashan and Lingxiang Formations share similar trace element and REE partition patterns as well as Sr-Nd
isotopic compositions. In combination with the regional geology, it is proposed that the southeast Hubei volcanic rocks were
formed mainly during the Early Cretaceous, just like other volcanic basins in middle-lower Yangtze valley. A lithospheric
extension is also suggested for tectonic regime in this region in the Cretaceous Period.
Mesozoic igneous rocks are widespread throughout eastern China, but precise geochronological constraints were previously lacking. Thirty-two samples, including dolerite, diorite and granite, from the Liaodong Peninsula in northeastern China were chosen for zircon U–Pb SHRIMP (5 samples), laser ablation (LA) ICP-MS (22 samples) and TIMS (5 samples) dating. The ages range from 131 ± 2 to 117 ± 7 Ma, which establishes that the Early Cretaceous was a significant period of igneous activity in the Liaodong Peninsula, with a duration of about 10 Ma. A similar magmatic age pattern is identified in other areas of northern and eastern China, and elsewhere in southeast Asia. These rocks were all emplaced in an extensional setting, as indicated by the occurrence of A-type granite, dolerite dyke swarms and metamorphic core complexes. It is proposed that this giant igneous event was related to coeval lithospheric delamination in eastern China, which resulted from Kula-Pacific Plate subduction, possibly aided by major superplume activity associated with global-scale mantle upwelling.
Origin and genesis of post-collisional mafic–ultramafic rocks in the Dabie orogen of China are controversial issues with respect to Mesozoic transport of mass and energy between mantle and crust. Zircon U–Pb age, whole-rock major and trace elements, mineral and whole-rock oxygen isotopes, and carbon concentrations and isotope compositions of apatite and whole-rock were determined for these igneous rocks in order to find a resolution. The results show that the Dabie mafic–ultramafic rocks are characterized by strong LREE enrichment and negative HFSE (Nb, Zr, Ti and P) anomalies but positive anomalies of Pb and Ba. Zircon U–Pb dating yields consistent ages of 122–128 Ma for magma crystallization and 105–116 Ma for post-magmatic hydrothermal alteration. There is a large variation in oxygen isotope ratios of minerals and their host rocks, with whole-rock δ18O values of 1.1–6.6‰, clinopyroxene 3.85–5.7‰, plagioclase 2.8–7.3‰, and zircon 3.85–6.04‰. Most of the zircons have δ18O values different from the normal mantle values. Equilibrium oxygen isotope fractionations between minerals have been preserved for some of the samples, but the others show significant disequilibrium indicating post-magmatic subsolidus alteration. Carbon concentrations and isotope compositions vary widely from 0.03 to 0.18% and from −27.0 to −5.8‰, respectively. Most of the samples are characterized by the low δ13C values typical of organic carbon. A comparison of the element and isotope data for the Cretaceous mafic–ultramafic rocks with the Triassic metamorphosed eclogites from the Dabie–Sulu orogenic belt shows that they share many geochemical and isotopic features in common, pointing to a genetic relationship in protolith origin between them. No signature of depleted mantle has been detected from Nd and Sr isotopes in the post-collisional igneous rocks. Protoliths of the Cretaceous mafic–ultramafic rocks and the Triassic eclogites are thus considered the mid-Neoproterozoic mafic–ultramafic rocks that were the products of rift magmatism along the northern margin of the Yangtze plate. Partial melting of the subducted lithosphere itself is proposed to produce the post-collisional igneous rocks. The Early Cretaceous superplume event is hypothesized to provide a heat source to trigger the melting of a large scale and a short period beneath the collisionally thickened orogen.