In the Tarim Basin of northwestern China hydrocarbon deposits have been discovered in parts of the thick strata of Cambrian dolostones. Based on petrographic study, six types of dolostone have been distinguished： Typ...In the Tarim Basin of northwestern China hydrocarbon deposits have been discovered in parts of the thick strata of Cambrian dolostones. Based on petrographic study, six types of dolostone have been distinguished： Type-l, pink mud-bearing silty crystalline dolostone （PMSD）; Type-2, gypsum- and salt-bearing fine crystalline dolostone （GSFD）; Type-3, fine crystalline dolostone with dolomite crystals with cloudy core and clear rim （CCFD）; Type-4, deep gray mud-bearing silty crystalline dolostone （GMSD）; Type-5, euhedral coarse crystalline dolostone （ECD）; and Type-6, xenotopic coarse crystalline dolostone （XCD）. Applying petrographic and geochemical methods, the genesis of the dolostones is studied in this paper. Normally, Type-I dolostone shows U- and Mo- depleted characteristics, reflecting a more oxidized formation environment; High ~80 and the purple color are consistent with formation of Sabkha dolostones on a supratidal flat. Types 2, 3, 4 dolostones show strata formation, similar REE patterns and 87Sr/86Sr ratios with contemporaneous limestones, suggesting a penecontemporaneous origin from seawater. Types 5 and 6 dolostones commonly occur as interbedded rocks, indicating secondary genesis after diagenesis. Type-6 dolostone has the highest order degree （OD） values （average 0.86）, the lowest oxygen isotope values and positive Eu anomalies, which are consistent with previously reported hydrothermal dolostones. Differently, Type-5 shows euhedral texture, higher δ80 value, similar REE characteristic and 87Sr/86Sr ratios in comparison with contemporaneous limestones, suggesting that this type might have been dolomitized by down- transferring evaporated seawater during shallow burial stage. Dolostone fluid sources, formation environments and crystallizing dynamics are summarized and possible genetic models for the six types are proposed.展开更多
Jinshan gold deposit is located in northeastern Jiangxi,South China,which is related to the ductile shear zone.It has a gold reserve of more than 200 tons,with 80%of gold occurring in pyrite. The LREE of gold-bearing ...Jinshan gold deposit is located in northeastern Jiangxi,South China,which is related to the ductile shear zone.It has a gold reserve of more than 200 tons,with 80%of gold occurring in pyrite. The LREE of gold-bearing pyrite is as higher as 171.664 ppm on average,with relatively higher light rare earth elements（LREE;159.556 ppm） and lower HREE（12.108 ppm）.TheΣLREE/ΣHREE ratio is 12.612 and（La/Yb）_N is 11.765.These indicate that pyrite is rich in LREE.The（La/Sm）_N ratio is 3.758 and that of（Gd/Yb）_N is 1.695.These are obvious LREE fractionations.The rare earth element（REE） distribution patterns show obvious Eu anomaly with averageδEu values of 0.664,andδCe anomalies of 1.044.REE characteristics are similar to those of wall rocks（regional metamorphic rocks）,but different from those of the Dexing granodiorite porphyry and Damaoshan biotite granite.These features indicate that the ore-forming materials in the Jinshan gold deposit derived from the wall rocks, and the ore-forming fluids derived from metamorphic water.The Co/Ni ratio（average value 0.38） of pyrite suggests that the Jinshan gold deposit formed under a medium-low temperature.It is inferred from the values of high-field strength elements,LREE,Hf/Sm,Nb/La,and Th/La of the pyrite that the ore-forming fluids of the Jinshan gold deposit derived from metamorphic water with Cl〉F.展开更多
赣南草桃背铀矿床位于会昌地区河草坑铀矿田,是一个大型火山岩容矿的铀矿床,成因机制存在较大争议。矿床内发育有大面积的花岗岩和火山岩,矿体主要赋存在火山隐爆角砾岩和震碎花岗岩中。文章以该矿床内花岗岩、火山岩及矿石中广泛发育...赣南草桃背铀矿床位于会昌地区河草坑铀矿田,是一个大型火山岩容矿的铀矿床,成因机制存在较大争议。矿床内发育有大面积的花岗岩和火山岩,矿体主要赋存在火山隐爆角砾岩和震碎花岗岩中。文章以该矿床内花岗岩、火山岩及矿石中广泛发育的蚀变矿物绿泥石为研究对象,采用电子探针及LA-ICP-MS原位微区分析技术对其矿物化学组成进行分析。分析结果表明,花岗岩和矿石中的绿泥石主要为富铁的鲕绿泥石和蠕绿泥石,火山岩中的绿泥石则为相对富镁的密绿泥石。各类绿泥石阳离子置换关系主要以Fe、Mg置换为主,同时存在一定的Tschermak(TK)和二八—三八面体(DT)替换机制。矿石中绿泥石具有相对较高含量的Ti、Li、Be、B、Zn、Ga、Ge、Sn、Cs、U、Rb和Ba等元素,并显示出较低的Th/U比值。矿石中绿泥石的结晶温度区间为201~269℃,平均值为242℃,属于中低温热液作用的产物。矿石中绿泥石的氧逸度(log f O_(2))变化于-48.4~-41.2之间,平均值为-44.5,硫逸度(log f S_(2))变化于-7.5~+2.8之间,平均值为-1.8,草桃背铀矿床的铀矿化主要形成于低氧逸度、高硫逸度的环境。展开更多
The Xiangshan uranium ore field is the largest volcanic rock hosted uranium deposit in China.The host rock is a volcanic intrusive complex,including rhyodacite,porphyroclastic lava and late stage sub-volcanic rocks.In...The Xiangshan uranium ore field is the largest volcanic rock hosted uranium deposit in China.The host rock is a volcanic intrusive complex,including rhyodacite,porphyroclastic lava and late stage sub-volcanic rocks.In this study,zircons from an early stage rhyodacite and a late stage rhyodacite porphyry were dated by SHRIMP and LA-ICP-MS U-Pb methods,and their Hf isotopic compositions were measured by LA-MC-ICP-MS.206Pb/238U ages of 135.1±1.7 and 134.8±1.1 Ma were obtained for the rhyodacite and rhyodacitic porphyry,respectively.These accurate ages indicate that the Xiangshan volcanic-intrusive complex formed in the Early Cretaceous rather than in the Late Jurassic,as concluded in some previous studies.By the Early Cretaceous,the tectonic setting of the area has evolved into a back-arc extensional setting,possibly related to subduction of the paleo-Pacific plate.The close ages of the(early) eruptive rhyodacite and the(late) hypabyssal rhyodacitic porphyry shows that the Xiangshan volcanism was intensive and concentrated in a short time.Zircons from the rhyodacite show negative εHf(t) values of-5.7 to-8.5,with Hf depleted mantle model ages between 1550 and 1720 Ma,whereas zircons from the rhyodacitic porphyry yield εHf(t) values of-6.9 to-10.1 and Hf model ages between 1621 and 1823 Ma.These zircon Hf model ages are similar to the whole rock Nd model ages(1486 to 1911 Ma).Combined with other geochemical characteristics,the Xiangshan rhyodacite and rhyodacitic porphyry may have been derived from partial melting of the Paleo-Mesoproterozoic metamorphic rocks from the Xiangshan basement,without any significant addition of mantle-derived magma.Contribution of basement of this age is also supported by finding a Paleoproterozoic xenocrystic zircon core in the rhyodacite sample.展开更多
The Qitianling granite batholith (QGB) is located in the southern Hunan Province, middle part of the Nanling Range, South China. Its total exposure area is about 520 km2. Based on our 25 single grain zircon U-Pb age d...The Qitianling granite batholith (QGB) is located in the southern Hunan Province, middle part of the Nanling Range, South China. Its total exposure area is about 520 km2. Based on our 25 single grain zircon U-Pb age data and 7 published data as well as the geological, petrological, and space distribution characteristics, we conclude that QGB is an Early Yanshanian (Jurassic) multi-staged composite pluton. Its formation process can be subdivided into three major stages. The first stage, emplaced at 163―160 Ma with a peak at about 161 Ma, is mainly composed of hornblende-biotite monzonitic granites and locally biotite granites, and distributed in the eastern, northern, and western peripheral parts of the pluton. The second stage, emplaced at 157―153 Ma with a peak at 157―156 Ma, is mainly composed of biotite granites and locally containing hornblende, and distributed in the middle and southeastern parts of the pluton. The third stage, emplaced at 150―146 Ma with a peak at about 149 Ma, is mainly composed of fine-grained (locally porphyritic) biotite granites, and distributed in the middle-southern part of the pluton. Each stage can be further disintegrated into several granite bodies. The first two intrusive stages comprise the major phase of QGB, and the third intrusive stage comprises the additional phase. Many second stage fine-grained granite bosses and dykes intruded into the first stage host granites with clear chilling margin-baking phenomena at their intrusive contacts. They were emplaced in the open fracture space of the earlier stage consolidated rocks. Their isotopic ages are mostly 2―6 Ma younger than their hosts. Conceivably, the time interval from magma emplacement, through cooling, crystallization, solidification, up to fracturing of the earlier stage granites cannot exceed 2―6 Ma. During the Middle-Late Jurassic in the Qitianling area and neighboring Nanling Range, the coeval granitic and basic-intermediate magmatic activities were widely developed. It indicates that the Early Yanshanian period was the culmination time of magmatic activities in this region. The Nanling Range was under a post-orogenic, intracontinental geotectonic environment with an obvious lithospheric extension and thinning. The crust-mantle interaction played an important role in formation of granitic rocks in this region.展开更多
It is undebated fact that the lithospheric mantle beneath eastern China was considerably thinned during the Mesozoic time. However, it has no adequate evidence for the exact timing when the lithosphere thinning starte...It is undebated fact that the lithospheric mantle beneath eastern China was considerably thinned during the Mesozoic time. However, it has no adequate evidence for the exact timing when the lithosphere thinning started. The Liaodong Peninsula is located in the eastern segment of the North China Craton and is one of the important domains to explore the event of lithosphere thinning. SHRIMP U-Pb zircon dating and geochemical study were carried out for the lamprophyre dike swarm that intruded into the magnesite ore-beds in the Dashiqiao Formation of Paleoproterozoic Liaohe Group at the Huaziyu magnesite ore district, Liaodong Peninsula. The results indicate that these lampro- phyre dikes were intruded in late Jurassic (155±4 Ma) and show some geochemical characteristics of potassic magmas. It is now accepted that the lithosphere thinning took place in the late Mesozoic, and the peak thinning stage occurred in early Cretaceous (130―120 Ma). Considering the potassic mafic magmatism marking the onset of the lithospheric thin- ning, we therefore suggest that the studied late Jurassic potassic lamprophyre dike swarm could imply that the late Jurassic is the time that lithosphere thinning started.展开更多
Mesozoic granitoids are widespread in the Qinling-Dabie-Sulu orogenic belt. Precise U-Pb dating on these granitoids can reveal the evolution of the continental collision orogen and thus provide information on the natu...Mesozoic granitoids are widespread in the Qinling-Dabie-Sulu orogenic belt. Precise U-Pb dating on these granitoids can reveal the evolution of the continental collision orogen and thus provide information on the nature of magma sources. This study pre-sents zircon LA-ICP-MS U-Pb dating and whole-rock geochemical analyses for two intrusions at Changba and Huangzhuguan in western Qinling. Zircon U-Pb ages for central and marginal phases of the Huangzhuguang intrusion are 214±1 Ma and 213±3 Ma, respectively. Zircons from the Changba intrusion yield a dominant cluster with an U-Pb age of 213±2 Ma. Collectively, these ages are younger than ages of 220 to 240 Ma for ultrahigh-pressure metamorphism due to the continental collision between the South China Block and the North China Block, corresponding to syn-exhumation magmatism. Some inherited zircons occur in the Changba intrusion, yielding a weighted mean of 206Pb/238U ages at 757±14 Ma. This indicates that the Changba intrusion has the crustal source of mid-Neoproterozoic ages and a tectonic affinity to the South China Block. Geochemically, the two intrusuons are both rich in LILE and LREE but depleted in HFSE and HREE, similar to arc-type igneous rocks. The Huangzhuguang intrusion exhibits linear correlations between SiO2 and the other major oxides, implying chemical evolution from a cognate magma source. It contains mafic enclaves, suggesting possible mixing of felsic-mafic magmas. The Changba granite is rich in Si and K but poor in Fe and Mg as well as has a high value of Fe, suggesting strong differentiation of granitic magma. Therefore, the two intrusions were derived from the Late Triassic anatexis of the continental crust of different compositions in the northern margin of South China Block. This process may be coupled with exhumation of the subducted continental crust in the stage of late collision.展开更多
This paper reports the geochemical and zircon U-Pb dating data of the Sinian to Cambrian low-grade metamorphic rocks in the Miaoer Mountain area,Guangxi and the Jinjiling area,Hunan Province.Petrographic and geochemic...This paper reports the geochemical and zircon U-Pb dating data of the Sinian to Cambrian low-grade metamorphic rocks in the Miaoer Mountain area,Guangxi and the Jinjiling area,Hunan Province.Petrographic and geochemical features indicate that protoliths of these metamorphic rocks are clastic sedimentary rocks with medium weathering,which were formed in the passive continental margin.Geochemistry and zircon U-Pb ages indicate that the Sinian and Cambrian sedimentary rocks in the Jinjiling area have similar detritus components,which are characterized by abundant Grenvillian detrital zircons,suggesting a close affinity with the Cathaysia Block.The Cambrian sedimentary rocks in the Miaoer Mountain area have similar geochemistry and zircon geochronology to those in the Jinjiling area,showing an affinity with the Cathaysia Block.However,the Sinian sedimentary rocks in the Miaoer Mountain area show different geochemical features from the Cambrian sedimentary rocks and those in the Jinjiling area,and are characterized by abundant 840–700 Ma detrital zircons and less^2.0 Ga ones,showing a close affinity with the Yangtze Block.These variations suggest that the Jinjiling area continuously accepted the fragments from the Cathaysia from the Sinian to the Cambrian,whereas the provenance of the Miaoer Mountain sedimentary basin changed from the Yangtze Block to the Cathaysia Block during this interval.This change implies a tectonic movement,which caused the further sinking of the basin in the Miaoer Mountain area and northwestward transferring of the basin center before the Middle Cambrian,so that the Miaoer Mountain basin received the detritus from the Cathaysia Block in the Middle Cambrian.This fact also proves that the Yangtze and Cathaysia blocks have converged at least in Middle Cambrian,and the southwestern boundary between them is located between the Miaoer Mountain and Jinjiling areas.展开更多
Adakitic rocks in continental settings are commonly considered to be formed by partial melting of thickened or delaminated lower crust. Investigations on this kind of rocks can provide important information about crus...Adakitic rocks in continental settings are commonly considered to be formed by partial melting of thickened or delaminated lower crust. Investigations on this kind of rocks can provide important information about crustal evolution complementary to information from other rocks. This paper reports adakitic granodiorite of the Lingxi pluton in the interior of the Cathayisa Block. LA-ICP-MS zircon U-Pb dating shows that it was formed in the late Early Cretaceous(100±1 Ma). The granodiorite has geochemical features of adakitic rocks derived from partial melting of the thickened lower crust, e.g., high SiO2(mainly ranging from 64.4 to 68.9 wt.%) and Sr(624–894 ppm) contents, Sr/Y(49.9–60.8) and La/Yb(23.4–42.8) values, low Y(10.3–17.1 ppm), Ni(5.62–11.8 ppm) and MgO(mostly from 0.86 wt.% to 1.57 wt.%) contents and weak Eu anomaly. It has initial 87Sr/86 Sr ratios of 0.7086–0.7091, εNd(t) values of.6.2 to.5.9 and zircon εHf(t) values mostly of.10.1 to.7.6. Based on the geochemical characteristics and simple modelling, it is suggested that the most likely generation mechanism of the Lingxi granodiorite is partial melting of a thickened Proterozoic lower continental crust at a pressure ≥12 kbar(or crust thickness ≥40km), leaving a garnet-bearing amphibolite residue. Combining our results and previous studies of the tectonic evolution of the Cathaysia Block, we propose that the crust was thickened to over 40 km by a compressive event occurring during the late Early Cretaceous, which is supported by the observation that there is an angular unconformity between the Upper Cretaceous Series and the early Lower Cretaceous or the Jurassic rocks. After this event, the Cathaysia Block experienced a lithospheric extension and thinning probably driven by the high-angle paleo-Pacific subduction. With the attenuation of lithosphere, the lower crust was heated to partial melting by upwelling asthenospheric materials, resulting in generation of the Lingxi granodiorite and other coeval granitoids in the Cathaysia Block. This study provides new information on the crustal evolution of the Cathaysia Block during the Early Cretaceous.展开更多
文摘In the Tarim Basin of northwestern China hydrocarbon deposits have been discovered in parts of the thick strata of Cambrian dolostones. Based on petrographic study, six types of dolostone have been distinguished： Type-l, pink mud-bearing silty crystalline dolostone （PMSD）; Type-2, gypsum- and salt-bearing fine crystalline dolostone （GSFD）; Type-3, fine crystalline dolostone with dolomite crystals with cloudy core and clear rim （CCFD）; Type-4, deep gray mud-bearing silty crystalline dolostone （GMSD）; Type-5, euhedral coarse crystalline dolostone （ECD）; and Type-6, xenotopic coarse crystalline dolostone （XCD）. Applying petrographic and geochemical methods, the genesis of the dolostones is studied in this paper. Normally, Type-I dolostone shows U- and Mo- depleted characteristics, reflecting a more oxidized formation environment; High ~80 and the purple color are consistent with formation of Sabkha dolostones on a supratidal flat. Types 2, 3, 4 dolostones show strata formation, similar REE patterns and 87Sr/86Sr ratios with contemporaneous limestones, suggesting a penecontemporaneous origin from seawater. Types 5 and 6 dolostones commonly occur as interbedded rocks, indicating secondary genesis after diagenesis. Type-6 dolostone has the highest order degree （OD） values （average 0.86）, the lowest oxygen isotope values and positive Eu anomalies, which are consistent with previously reported hydrothermal dolostones. Differently, Type-5 shows euhedral texture, higher δ80 value, similar REE characteristic and 87Sr/86Sr ratios in comparison with contemporaneous limestones, suggesting that this type might have been dolomitized by down- transferring evaporated seawater during shallow burial stage. Dolostone fluid sources, formation environments and crystallizing dynamics are summarized and possible genetic models for the six types are proposed.
基金supported by the National Natural Science Foundation of China(No. 40373025)
文摘Jinshan gold deposit is located in northeastern Jiangxi,South China,which is related to the ductile shear zone.It has a gold reserve of more than 200 tons,with 80%of gold occurring in pyrite. The LREE of gold-bearing pyrite is as higher as 171.664 ppm on average,with relatively higher light rare earth elements（LREE;159.556 ppm） and lower HREE（12.108 ppm）.TheΣLREE/ΣHREE ratio is 12.612 and（La/Yb）_N is 11.765.These indicate that pyrite is rich in LREE.The（La/Sm）_N ratio is 3.758 and that of（Gd/Yb）_N is 1.695.These are obvious LREE fractionations.The rare earth element（REE） distribution patterns show obvious Eu anomaly with averageδEu values of 0.664,andδCe anomalies of 1.044.REE characteristics are similar to those of wall rocks（regional metamorphic rocks）,but different from those of the Dexing granodiorite porphyry and Damaoshan biotite granite.These features indicate that the ore-forming materials in the Jinshan gold deposit derived from the wall rocks, and the ore-forming fluids derived from metamorphic water.The Co/Ni ratio（average value 0.38） of pyrite suggests that the Jinshan gold deposit formed under a medium-low temperature.It is inferred from the values of high-field strength elements,LREE,Hf/Sm,Nb/La,and Th/La of the pyrite that the ore-forming fluids of the Jinshan gold deposit derived from metamorphic water with Cl〉F.
文摘赣南草桃背铀矿床位于会昌地区河草坑铀矿田,是一个大型火山岩容矿的铀矿床,成因机制存在较大争议。矿床内发育有大面积的花岗岩和火山岩,矿体主要赋存在火山隐爆角砾岩和震碎花岗岩中。文章以该矿床内花岗岩、火山岩及矿石中广泛发育的蚀变矿物绿泥石为研究对象,采用电子探针及LA-ICP-MS原位微区分析技术对其矿物化学组成进行分析。分析结果表明,花岗岩和矿石中的绿泥石主要为富铁的鲕绿泥石和蠕绿泥石,火山岩中的绿泥石则为相对富镁的密绿泥石。各类绿泥石阳离子置换关系主要以Fe、Mg置换为主,同时存在一定的Tschermak(TK)和二八—三八面体(DT)替换机制。矿石中绿泥石具有相对较高含量的Ti、Li、Be、B、Zn、Ga、Ge、Sn、Cs、U、Rb和Ba等元素,并显示出较低的Th/U比值。矿石中绿泥石的结晶温度区间为201~269℃,平均值为242℃,属于中低温热液作用的产物。矿石中绿泥石的氧逸度(log f O_(2))变化于-48.4~-41.2之间,平均值为-44.5,硫逸度(log f S_(2))变化于-7.5~+2.8之间,平均值为-1.8,草桃背铀矿床的铀矿化主要形成于低氧逸度、高硫逸度的环境。
基金supported by Key Project from the Ministry of Education (Grant No.306007)
文摘The Xiangshan uranium ore field is the largest volcanic rock hosted uranium deposit in China.The host rock is a volcanic intrusive complex,including rhyodacite,porphyroclastic lava and late stage sub-volcanic rocks.In this study,zircons from an early stage rhyodacite and a late stage rhyodacite porphyry were dated by SHRIMP and LA-ICP-MS U-Pb methods,and their Hf isotopic compositions were measured by LA-MC-ICP-MS.206Pb/238U ages of 135.1±1.7 and 134.8±1.1 Ma were obtained for the rhyodacite and rhyodacitic porphyry,respectively.These accurate ages indicate that the Xiangshan volcanic-intrusive complex formed in the Early Cretaceous rather than in the Late Jurassic,as concluded in some previous studies.By the Early Cretaceous,the tectonic setting of the area has evolved into a back-arc extensional setting,possibly related to subduction of the paleo-Pacific plate.The close ages of the(early) eruptive rhyodacite and the(late) hypabyssal rhyodacitic porphyry shows that the Xiangshan volcanism was intensive and concentrated in a short time.Zircons from the rhyodacite show negative εHf(t) values of-5.7 to-8.5,with Hf depleted mantle model ages between 1550 and 1720 Ma,whereas zircons from the rhyodacitic porphyry yield εHf(t) values of-6.9 to-10.1 and Hf model ages between 1621 and 1823 Ma.These zircon Hf model ages are similar to the whole rock Nd model ages(1486 to 1911 Ma).Combined with other geochemical characteristics,the Xiangshan rhyodacite and rhyodacitic porphyry may have been derived from partial melting of the Paleo-Mesoproterozoic metamorphic rocks from the Xiangshan basement,without any significant addition of mantle-derived magma.Contribution of basement of this age is also supported by finding a Paleoproterozoic xenocrystic zircon core in the rhyodacite sample.
基金Supported by National Natural Science Foundation of China (Grants Nos. 40730423, 40373014)Funds from the Ministry of Land and Resources (Grant No. 1212010632100)
文摘The Qitianling granite batholith (QGB) is located in the southern Hunan Province, middle part of the Nanling Range, South China. Its total exposure area is about 520 km2. Based on our 25 single grain zircon U-Pb age data and 7 published data as well as the geological, petrological, and space distribution characteristics, we conclude that QGB is an Early Yanshanian (Jurassic) multi-staged composite pluton. Its formation process can be subdivided into three major stages. The first stage, emplaced at 163―160 Ma with a peak at about 161 Ma, is mainly composed of hornblende-biotite monzonitic granites and locally biotite granites, and distributed in the eastern, northern, and western peripheral parts of the pluton. The second stage, emplaced at 157―153 Ma with a peak at 157―156 Ma, is mainly composed of biotite granites and locally containing hornblende, and distributed in the middle and southeastern parts of the pluton. The third stage, emplaced at 150―146 Ma with a peak at about 149 Ma, is mainly composed of fine-grained (locally porphyritic) biotite granites, and distributed in the middle-southern part of the pluton. Each stage can be further disintegrated into several granite bodies. The first two intrusive stages comprise the major phase of QGB, and the third intrusive stage comprises the additional phase. Many second stage fine-grained granite bosses and dykes intruded into the first stage host granites with clear chilling margin-baking phenomena at their intrusive contacts. They were emplaced in the open fracture space of the earlier stage consolidated rocks. Their isotopic ages are mostly 2―6 Ma younger than their hosts. Conceivably, the time interval from magma emplacement, through cooling, crystallization, solidification, up to fracturing of the earlier stage granites cannot exceed 2―6 Ma. During the Middle-Late Jurassic in the Qitianling area and neighboring Nanling Range, the coeval granitic and basic-intermediate magmatic activities were widely developed. It indicates that the Early Yanshanian period was the culmination time of magmatic activities in this region. The Nanling Range was under a post-orogenic, intracontinental geotectonic environment with an obvious lithospheric extension and thinning. The crust-mantle interaction played an important role in formation of granitic rocks in this region.
基金This work was financially supported by the National Natural Science Foundation of China (Grant No.40221301).
文摘It is undebated fact that the lithospheric mantle beneath eastern China was considerably thinned during the Mesozoic time. However, it has no adequate evidence for the exact timing when the lithosphere thinning started. The Liaodong Peninsula is located in the eastern segment of the North China Craton and is one of the important domains to explore the event of lithosphere thinning. SHRIMP U-Pb zircon dating and geochemical study were carried out for the lamprophyre dike swarm that intruded into the magnesite ore-beds in the Dashiqiao Formation of Paleoproterozoic Liaohe Group at the Huaziyu magnesite ore district, Liaodong Peninsula. The results indicate that these lampro- phyre dikes were intruded in late Jurassic (155±4 Ma) and show some geochemical characteristics of potassic magmas. It is now accepted that the lithosphere thinning took place in the late Mesozoic, and the peak thinning stage occurred in early Cretaceous (130―120 Ma). Considering the potassic mafic magmatism marking the onset of the lithospheric thin- ning, we therefore suggest that the studied late Jurassic potassic lamprophyre dike swarm could imply that the late Jurassic is the time that lithosphere thinning started.
基金supported by the Chinese Ministry of Science and Technology (2006CB403505)
文摘Mesozoic granitoids are widespread in the Qinling-Dabie-Sulu orogenic belt. Precise U-Pb dating on these granitoids can reveal the evolution of the continental collision orogen and thus provide information on the nature of magma sources. This study pre-sents zircon LA-ICP-MS U-Pb dating and whole-rock geochemical analyses for two intrusions at Changba and Huangzhuguan in western Qinling. Zircon U-Pb ages for central and marginal phases of the Huangzhuguang intrusion are 214±1 Ma and 213±3 Ma, respectively. Zircons from the Changba intrusion yield a dominant cluster with an U-Pb age of 213±2 Ma. Collectively, these ages are younger than ages of 220 to 240 Ma for ultrahigh-pressure metamorphism due to the continental collision between the South China Block and the North China Block, corresponding to syn-exhumation magmatism. Some inherited zircons occur in the Changba intrusion, yielding a weighted mean of 206Pb/238U ages at 757±14 Ma. This indicates that the Changba intrusion has the crustal source of mid-Neoproterozoic ages and a tectonic affinity to the South China Block. Geochemically, the two intrusuons are both rich in LILE and LREE but depleted in HFSE and HREE, similar to arc-type igneous rocks. The Huangzhuguang intrusion exhibits linear correlations between SiO2 and the other major oxides, implying chemical evolution from a cognate magma source. It contains mafic enclaves, suggesting possible mixing of felsic-mafic magmas. The Changba granite is rich in Si and K but poor in Fe and Mg as well as has a high value of Fe, suggesting strong differentiation of granitic magma. Therefore, the two intrusions were derived from the Late Triassic anatexis of the continental crust of different compositions in the northern margin of South China Block. This process may be coupled with exhumation of the subducted continental crust in the stage of late collision.
基金supported by the National Natural Science Foundation of China(Grant No.41272085)research grants from the Bureau of Nuclear Geology of China(YK11)+1 种基金the Major State Basic Research Program(Grant No.2012CB416701)the State Key Laboratory for Mineral Deposits Research(Nanjing University)(Grant No.ZZKT-201106)
文摘This paper reports the geochemical and zircon U-Pb dating data of the Sinian to Cambrian low-grade metamorphic rocks in the Miaoer Mountain area,Guangxi and the Jinjiling area,Hunan Province.Petrographic and geochemical features indicate that protoliths of these metamorphic rocks are clastic sedimentary rocks with medium weathering,which were formed in the passive continental margin.Geochemistry and zircon U-Pb ages indicate that the Sinian and Cambrian sedimentary rocks in the Jinjiling area have similar detritus components,which are characterized by abundant Grenvillian detrital zircons,suggesting a close affinity with the Cathaysia Block.The Cambrian sedimentary rocks in the Miaoer Mountain area have similar geochemistry and zircon geochronology to those in the Jinjiling area,showing an affinity with the Cathaysia Block.However,the Sinian sedimentary rocks in the Miaoer Mountain area show different geochemical features from the Cambrian sedimentary rocks and those in the Jinjiling area,and are characterized by abundant 840–700 Ma detrital zircons and less^2.0 Ga ones,showing a close affinity with the Yangtze Block.These variations suggest that the Jinjiling area continuously accepted the fragments from the Cathaysia from the Sinian to the Cambrian,whereas the provenance of the Miaoer Mountain sedimentary basin changed from the Yangtze Block to the Cathaysia Block during this interval.This change implies a tectonic movement,which caused the further sinking of the basin in the Miaoer Mountain area and northwestward transferring of the basin center before the Middle Cambrian,so that the Miaoer Mountain basin received the detritus from the Cathaysia Block in the Middle Cambrian.This fact also proves that the Yangtze and Cathaysia blocks have converged at least in Middle Cambrian,and the southwestern boundary between them is located between the Miaoer Mountain and Jinjiling areas.
基金supported by the National Basic Research Program of China (Grant No. 2012CB416703)Geological Bureau of China National Nuclear Corporation
文摘Adakitic rocks in continental settings are commonly considered to be formed by partial melting of thickened or delaminated lower crust. Investigations on this kind of rocks can provide important information about crustal evolution complementary to information from other rocks. This paper reports adakitic granodiorite of the Lingxi pluton in the interior of the Cathayisa Block. LA-ICP-MS zircon U-Pb dating shows that it was formed in the late Early Cretaceous(100±1 Ma). The granodiorite has geochemical features of adakitic rocks derived from partial melting of the thickened lower crust, e.g., high SiO2(mainly ranging from 64.4 to 68.9 wt.%) and Sr(624–894 ppm) contents, Sr/Y(49.9–60.8) and La/Yb(23.4–42.8) values, low Y(10.3–17.1 ppm), Ni(5.62–11.8 ppm) and MgO(mostly from 0.86 wt.% to 1.57 wt.%) contents and weak Eu anomaly. It has initial 87Sr/86 Sr ratios of 0.7086–0.7091, εNd(t) values of.6.2 to.5.9 and zircon εHf(t) values mostly of.10.1 to.7.6. Based on the geochemical characteristics and simple modelling, it is suggested that the most likely generation mechanism of the Lingxi granodiorite is partial melting of a thickened Proterozoic lower continental crust at a pressure ≥12 kbar(or crust thickness ≥40km), leaving a garnet-bearing amphibolite residue. Combining our results and previous studies of the tectonic evolution of the Cathaysia Block, we propose that the crust was thickened to over 40 km by a compressive event occurring during the late Early Cretaceous, which is supported by the observation that there is an angular unconformity between the Upper Cretaceous Series and the early Lower Cretaceous or the Jurassic rocks. After this event, the Cathaysia Block experienced a lithospheric extension and thinning probably driven by the high-angle paleo-Pacific subduction. With the attenuation of lithosphere, the lower crust was heated to partial melting by upwelling asthenospheric materials, resulting in generation of the Lingxi granodiorite and other coeval granitoids in the Cathaysia Block. This study provides new information on the crustal evolution of the Cathaysia Block during the Early Cretaceous.
