High-Sr/Y granitic rocks,characterized by intermediate–high SiO2 contents(>56 wt.%),high La/Yb ratios(>20),and low Y(<18 ppm)and Yb(<1.9 ppm)contents,have drawn much attention from geologists in the past ...High-Sr/Y granitic rocks,characterized by intermediate–high SiO2 contents(>56 wt.%),high La/Yb ratios(>20),and low Y(<18 ppm)and Yb(<1.9 ppm)contents,have drawn much attention from geologists in the past decades.They were originally named adakites and defined as granitic magma in convergent plate margins formed by partial melting of subducted young(<25 Ma),hot,and hydrated oceanic slab.Recent studies suggest that these rocks can also be formed in a variety of non-subduction-related tectonic settings through different petrogenetic processes including partial melting of thickened lower continental crust,providing important constraints on crustal growth and evolution throughout the Earth’s history.The Tibetan plateau,generally considered as an archetype of collisional orogen,was formed by the successive accretion of different tectonic terranes to the southern margin of the Eurasia plates since the Early Paleozoic.Geologically,the Tibetan plateau comprises several W–E trending terranes(the Kunlun,North Qiangtang,South Qiangtang,and Lhasa terranes)and northernmost India(the Himalaya region).These terranes have undergone significant crustal thickening during the continental collision events.The crustal thickness of the Tibetan plateau is asymmetric,being thickest under the Lhasa terrane and thinning more gradually towards the north than to the south.From south to north,the crustal thickness increases from^50 km in the Himalaya region,to^70–80 km under the Lhasa terrane,and decreases to^65 km under the North and South Qiangtang terranes and^55 km beneath the Kunlun terrane.The present crustal thickness of the Tibetan plateau has generally been attributed to the Cenozoic collision between the Indian and Eurasian plates and the subsequent intracontinental convergences since^55 Ma.However,doubts were raised about this interpretion in the past few years.For example,recent studies indicated the crust of the Lhasa terrane had been significantly thickened during ca.140–130 Ma in response to the subduction of the Neo-Tethyan Ocean,although the Cretaceous marine strata across the Lhasa terrane demonstrates that it was thin enough to be within the reach of sea level during at least the early Late Cretacous.Whether the South Qiangtang terrane underwent crustal thickening before Cenozoic or not remains an unanswered question.In this study,new zircon U–Pb geochronologic and Hfisotopic,and whole-rock geochemical data of the Late Cretaceous high-Sr/Y granitoids from the eastern South Qiangtang terrane are presented.LA–ICP–MS zircon U–Pb dating produced ages(92.0±1.1 Ma,88.8±0.7 Ma,and 91.5±0.6 Ma)of formation for the Leiwuqi granitoids of the South Qiangtang terrane,Tibetan Plateau,indicating that they were formed during the Late Cretaceous.The granitoids have low A/CNK ratios(0.94–1.06;<1.10)and P2O5 contents(0.02–0.12 wt.%),geochemically similar to I-type granitoids.They are characterized by high Sr(350–938 ppm,most>400 ppm),low Y(5.94–9.67 ppm),with high Sr/Y(36–119,most>40)and(La/Yb)N ratios(67–394).They exhibit negative zirconεHf(t)values(–9.8 to–1.1)and low MgO(0.32–0.69 wt.%),Cr(2.07–12.17 ppm),and Ni(1.10–5.94 ppm)contents.These features suggest that the granitoids probably were derived from partial melting of thickened lower continental crust(>50 km).The crust of the South Qiangtang terrane(at least its eastern part)may have been significantly thickened before the Late Cretaceous.展开更多
The metamorphosed Early Paleoproterozoic granitoids in the Quanji massif, Northwest- ern China provide constraints for relationship between the Tarim Craton and North China Craton. Among granitoids batholiths, rocks o...The metamorphosed Early Paleoproterozoic granitoids in the Quanji massif, Northwest- ern China provide constraints for relationship between the Tarim Craton and North China Craton. Among granitoids batholiths, rocks of the Mohe quartz-diorite show typically adakitic geochemical characteristics, with medium K2O/Na2O ratios （0.56-1.17） and high Sr （519-619 ppm） low Y （9.37-20.40 ppm） and Yb （0.97-1.77 ppm） concentrations. The rocks have εNd（t） values between ＋2.4 and ＋4.4 and depleted mantle Nd model ages of 2.43-2.59 Ga. The magmatic zircons have positive εHf（t） values ranging from ＋0.40 to ＋7.60 and depleted mantle Hf model ages of 2.43-2.70 Ga, with major peaks at -2.54 and -2.65 Ga. The geochemical and Nd-Hf isotopic characteristics indicate that the Mohe quartz-dioritic rocks might be formed by partial melting of high-pressure metamorphosed juve- nile crustal rocks in post-orogenic extensional regime in the Early Paleoproterozoic. It suggests that important crustal growth occurred in the Quanji massif and the Tarim Craton at -2.4 and 2.5-2.7 Ga. The Quanji massif and Tarim Craton might share a similar crustal evolution history with the North China Craton in the Neoarchean.展开更多
中国东部在晚中生代时(晚侏罗世-早白垩世)有广泛的中酸性岩浆活动,按照花岗岩的地球化学特征,大致可以划分为东北、华北和华南3个岩区。本文研究表明,按照 Sr 和 Yb 的含量,大致可以将花岗岩分为5类,即:高 Sr 低 Yb 型(Sr >400μg/g...中国东部在晚中生代时(晚侏罗世-早白垩世)有广泛的中酸性岩浆活动,按照花岗岩的地球化学特征,大致可以划分为东北、华北和华南3个岩区。本文研究表明,按照 Sr 和 Yb 的含量,大致可以将花岗岩分为5类,即:高 Sr 低 Yb 型(Sr >400μg/g,Yb<2μg/g)、低 Sr 低 Yb(Sr<400μg/g,Yb<2μg/g)、低 Sr 高 Yb(Sr<400μg/g,Yh>2μg/g)、高 Sr 高 Yb 型(Sr>400μg/g,Yb>2μg/g)以及非常低 Sr 高 Yb 型(Sr<100μg/g,Yb=2-18μg/g)花岗岩。东北和华南以发育低 Sr 高 Yb 花岗岩为主,有少量高 Sr 低 Yb 和非常低 Sr 高 Yb 类型的花岗岩分布;而华北则以高 Sr 低 Yb 型花岗岩(埃达克岩)最发育,低 Sr 高Yb、低 Sr 低 Yb 型和非常低 Sr 高 Yb 型花岗岩有少量分布。本文着重探讨了华北和华南花岗岩的特征,认为华北和华南花岗岩地球化学的区别可能主要与花岗岩源区成分和深度有关,且主要受源区深度的控制。如果花岗岩熔融的源区残留相由榴辉岩组成(石榴石+辉石+金红石+/-角闪石),则花岗岩明显亏损 HREE、Nb、Ta 和 Ti,而富集 Sr 和 Al,无明显的负铕异常,属于高 Sr 低 Yb(埃达克岩)类型;如果源区深度浅,由斜长角闪岩或麻粒岩组成(斜长石+辉石+角闪石),则花岗岩相对贫Sr 富 Yb。作者认为,华北和华南花岗岩地球化学特征上的上述差异,表明在晚中生代时(晚侏罗世-早白垩世),华北和华南的地壳厚度不同:华北较厚,华南较薄;华北经历了下地壳拆沉而华南无;华北和华南的下地壳成分不,华北较基性的下地壳拆沉后,留下的地壳平均成分与华南比偏中性。展开更多
基金supported by the National Science Foundation of China(Grant No.41872240 and 91755103)Ministry of Science and Technology of China(Grant No.2016YFC0600304)the Chinese Geological Survey Project(Grant No.DD20190060 and DD20190370)
文摘High-Sr/Y granitic rocks,characterized by intermediate–high SiO2 contents(>56 wt.%),high La/Yb ratios(>20),and low Y(<18 ppm)and Yb(<1.9 ppm)contents,have drawn much attention from geologists in the past decades.They were originally named adakites and defined as granitic magma in convergent plate margins formed by partial melting of subducted young(<25 Ma),hot,and hydrated oceanic slab.Recent studies suggest that these rocks can also be formed in a variety of non-subduction-related tectonic settings through different petrogenetic processes including partial melting of thickened lower continental crust,providing important constraints on crustal growth and evolution throughout the Earth’s history.The Tibetan plateau,generally considered as an archetype of collisional orogen,was formed by the successive accretion of different tectonic terranes to the southern margin of the Eurasia plates since the Early Paleozoic.Geologically,the Tibetan plateau comprises several W–E trending terranes(the Kunlun,North Qiangtang,South Qiangtang,and Lhasa terranes)and northernmost India(the Himalaya region).These terranes have undergone significant crustal thickening during the continental collision events.The crustal thickness of the Tibetan plateau is asymmetric,being thickest under the Lhasa terrane and thinning more gradually towards the north than to the south.From south to north,the crustal thickness increases from^50 km in the Himalaya region,to^70–80 km under the Lhasa terrane,and decreases to^65 km under the North and South Qiangtang terranes and^55 km beneath the Kunlun terrane.The present crustal thickness of the Tibetan plateau has generally been attributed to the Cenozoic collision between the Indian and Eurasian plates and the subsequent intracontinental convergences since^55 Ma.However,doubts were raised about this interpretion in the past few years.For example,recent studies indicated the crust of the Lhasa terrane had been significantly thickened during ca.140–130 Ma in response to the subduction of the Neo-Tethyan Ocean,although the Cretaceous marine strata across the Lhasa terrane demonstrates that it was thin enough to be within the reach of sea level during at least the early Late Cretacous.Whether the South Qiangtang terrane underwent crustal thickening before Cenozoic or not remains an unanswered question.In this study,new zircon U–Pb geochronologic and Hfisotopic,and whole-rock geochemical data of the Late Cretaceous high-Sr/Y granitoids from the eastern South Qiangtang terrane are presented.LA–ICP–MS zircon U–Pb dating produced ages(92.0±1.1 Ma,88.8±0.7 Ma,and 91.5±0.6 Ma)of formation for the Leiwuqi granitoids of the South Qiangtang terrane,Tibetan Plateau,indicating that they were formed during the Late Cretaceous.The granitoids have low A/CNK ratios(0.94–1.06;<1.10)and P2O5 contents(0.02–0.12 wt.%),geochemically similar to I-type granitoids.They are characterized by high Sr(350–938 ppm,most>400 ppm),low Y(5.94–9.67 ppm),with high Sr/Y(36–119,most>40)and(La/Yb)N ratios(67–394).They exhibit negative zirconεHf(t)values(–9.8 to–1.1)and low MgO(0.32–0.69 wt.%),Cr(2.07–12.17 ppm),and Ni(1.10–5.94 ppm)contents.These features suggest that the granitoids probably were derived from partial melting of thickened lower continental crust(>50 km).The crust of the South Qiangtang terrane(at least its eastern part)may have been significantly thickened before the Late Cretaceous.
基金supported by the National Natural Science Foundation of China(Nos.40972042 and 41172069)Hong Kong RGC(No.HKU704312P)the Fundamental Research Funds for the Central Universities of of Ministry of Education,China
文摘The metamorphosed Early Paleoproterozoic granitoids in the Quanji massif, Northwest- ern China provide constraints for relationship between the Tarim Craton and North China Craton. Among granitoids batholiths, rocks of the Mohe quartz-diorite show typically adakitic geochemical characteristics, with medium K2O/Na2O ratios （0.56-1.17） and high Sr （519-619 ppm） low Y （9.37-20.40 ppm） and Yb （0.97-1.77 ppm） concentrations. The rocks have εNd（t） values between ＋2.4 and ＋4.4 and depleted mantle Nd model ages of 2.43-2.59 Ga. The magmatic zircons have positive εHf（t） values ranging from ＋0.40 to ＋7.60 and depleted mantle Hf model ages of 2.43-2.70 Ga, with major peaks at -2.54 and -2.65 Ga. The geochemical and Nd-Hf isotopic characteristics indicate that the Mohe quartz-dioritic rocks might be formed by partial melting of high-pressure metamorphosed juve- nile crustal rocks in post-orogenic extensional regime in the Early Paleoproterozoic. It suggests that important crustal growth occurred in the Quanji massif and the Tarim Craton at -2.4 and 2.5-2.7 Ga. The Quanji massif and Tarim Craton might share a similar crustal evolution history with the North China Craton in the Neoarchean.
文摘中国东部在晚中生代时(晚侏罗世-早白垩世)有广泛的中酸性岩浆活动,按照花岗岩的地球化学特征,大致可以划分为东北、华北和华南3个岩区。本文研究表明,按照 Sr 和 Yb 的含量,大致可以将花岗岩分为5类,即:高 Sr 低 Yb 型(Sr >400μg/g,Yb<2μg/g)、低 Sr 低 Yb(Sr<400μg/g,Yb<2μg/g)、低 Sr 高 Yb(Sr<400μg/g,Yh>2μg/g)、高 Sr 高 Yb 型(Sr>400μg/g,Yb>2μg/g)以及非常低 Sr 高 Yb 型(Sr<100μg/g,Yb=2-18μg/g)花岗岩。东北和华南以发育低 Sr 高 Yb 花岗岩为主,有少量高 Sr 低 Yb 和非常低 Sr 高 Yb 类型的花岗岩分布;而华北则以高 Sr 低 Yb 型花岗岩(埃达克岩)最发育,低 Sr 高Yb、低 Sr 低 Yb 型和非常低 Sr 高 Yb 型花岗岩有少量分布。本文着重探讨了华北和华南花岗岩的特征,认为华北和华南花岗岩地球化学的区别可能主要与花岗岩源区成分和深度有关,且主要受源区深度的控制。如果花岗岩熔融的源区残留相由榴辉岩组成(石榴石+辉石+金红石+/-角闪石),则花岗岩明显亏损 HREE、Nb、Ta 和 Ti,而富集 Sr 和 Al,无明显的负铕异常,属于高 Sr 低 Yb(埃达克岩)类型;如果源区深度浅,由斜长角闪岩或麻粒岩组成(斜长石+辉石+角闪石),则花岗岩相对贫Sr 富 Yb。作者认为,华北和华南花岗岩地球化学特征上的上述差异,表明在晚中生代时(晚侏罗世-早白垩世),华北和华南的地壳厚度不同:华北较厚,华南较薄;华北经历了下地壳拆沉而华南无;华北和华南的下地壳成分不,华北较基性的下地壳拆沉后,留下的地壳平均成分与华南比偏中性。
