Numerous Indosinian igneous rocks in the North Qaidam(NQ)provide crucial insights into the tectonic evolution of the Paleo-Tethys Ocean.This paper presents a comprehensive study of the petrography,mineralogy,geochemis...Numerous Indosinian igneous rocks in the North Qaidam(NQ)provide crucial insights into the tectonic evolution of the Paleo-Tethys Ocean.This paper presents a comprehensive study of the petrography,mineralogy,geochemistry,zircon U-Pb geochronology,and Hf isotope composition of dioritic rocks from the eastern NQ.Zircon U-Pb dating results indicate that the dioritic rocks were formed during the Middle Triassic(244-240 Ma).The rocks exhibit high-K calc-alkaline characteristics with variable SiO_(2)(55.25-65.39 wt%)and elevated K_(2)O+Na_(2)O(4.81-6.94 wt%)contents.They show enrichment in LILEs(Rb,Ba,K)and depletion in HFSEs(Nb,Ta,Ti),with slight negative Eu anomalies(Eu/Eu*=0.89-0.97).Zirconε_(Hf)(t)values(−20.93 to+5.60)and T_(DM2)ages(0.85-1.72 Ga)suggest mixed sources.Petrographic and mineralogical analysis reveals that the plagioclase phenocrysts exhibit disequilibrium textures(including reverse zoning),primarily composed of andesine and labradorite,with a small amount of oligoclase.The clinopyroxenes are all augites and have high crystallization temperatures(1111-1151℃).These features,particularly the reverse zoning of plagioclase,support a petrogenetic model involving mantle-derived magma underplating that induced melting of ancient lower crust,followed by mixing of underplated basaltic magma with crust-derived felsic magma.Our results indicate formation in a back-arc extensional setting during subduction of the Zongwulong Paleo-Tethys Ocean.展开更多
High-pressure(HP)or ultrahigh-pressure(UHP)rutile-quartz veins that form at mantle depths due to fluid-rock interaction can be used to trace the properties and behavior of natural fluids in subduction zones.To explore...High-pressure(HP)or ultrahigh-pressure(UHP)rutile-quartz veins that form at mantle depths due to fluid-rock interaction can be used to trace the properties and behavior of natural fluids in subduction zones.To explore the fluid flow and the associated element mobility during deep subduction and exhumation of the continental crust,we investigated the major and trace elements of Ti-rich minerals.Additionally,U–Pb dating,trace element contents,and Lu–Hf isotopic composition of zircon grains in the UHP eclogite and associated rutile-quartz veins were examined in the North Qaidam UHP metamorphic belt,Yuka terrane.The zircon grains in the rutile-quartz veins have unzoned or weak oscillatory zonings,and show low Th/U ratios,steep chondrite-normalized patterns of heavy rare earth elements(HREEs),and insignificant negative Eu anomalies,indicating their growth in metamorphic fluids.These zircon grains formed in 4313 Ma,which is consistent with the 4322 Ma age of the host eclogite.As for the zircons in the rutile-quartz veins,they showed steep HREE patterns on one hand,and were different from the zircons present in the host eclogite on the other.This demonstrates that their formation might have been related to the breakdown of the early stage of garnet,which corresponds to the abundance of fluids during the early exhumation stage.The core-rim profile analyses of rutile recorded a two-stage rutile growth across a large rutile grain;the rutile core has higher Nb,Ta,W,and Zr contents and lower Nb/Ta ratios than the rim,indicating that the rutile domains grew in different metamorphic fluids from the core towards the rim.The significant enrichment of high field strength elements(HFSEs)in the rutile core suggests that the peak fluids have high solubility and transportation capacity of these HFSEs.Furthermore,variations in the Nb vs.Cr trends in rutile indicate a connection of rutile to mafic protolith.The zircon grains from both the rutile-quartz veins and the host eclogite have similar Hf isotopic compositions,indicating that the vein-forming fluids are internally derived from the host eclogite.These fluids accumulated in the subduction channel and were triggered by local dehydration of the deeply subducted eclogite during the early exhumation conditions.展开更多
基金supported by China Geological Survey Project(DD20190069 and DD20221636)Science and Technology Plan Project in Shaanxi Province,China(2023-JC-ZD-14,2023-JC-YB-236,2024JC-YBQN-0249,and 2022JQ-286).
文摘Numerous Indosinian igneous rocks in the North Qaidam(NQ)provide crucial insights into the tectonic evolution of the Paleo-Tethys Ocean.This paper presents a comprehensive study of the petrography,mineralogy,geochemistry,zircon U-Pb geochronology,and Hf isotope composition of dioritic rocks from the eastern NQ.Zircon U-Pb dating results indicate that the dioritic rocks were formed during the Middle Triassic(244-240 Ma).The rocks exhibit high-K calc-alkaline characteristics with variable SiO_(2)(55.25-65.39 wt%)and elevated K_(2)O+Na_(2)O(4.81-6.94 wt%)contents.They show enrichment in LILEs(Rb,Ba,K)and depletion in HFSEs(Nb,Ta,Ti),with slight negative Eu anomalies(Eu/Eu*=0.89-0.97).Zirconε_(Hf)(t)values(−20.93 to+5.60)and T_(DM2)ages(0.85-1.72 Ga)suggest mixed sources.Petrographic and mineralogical analysis reveals that the plagioclase phenocrysts exhibit disequilibrium textures(including reverse zoning),primarily composed of andesine and labradorite,with a small amount of oligoclase.The clinopyroxenes are all augites and have high crystallization temperatures(1111-1151℃).These features,particularly the reverse zoning of plagioclase,support a petrogenetic model involving mantle-derived magma underplating that induced melting of ancient lower crust,followed by mixing of underplated basaltic magma with crust-derived felsic magma.Our results indicate formation in a back-arc extensional setting during subduction of the Zongwulong Paleo-Tethys Ocean.
基金funded by the Fundamental Research Funds for National Universities, China University of Geosciences (Wuhan) and China Geological Survey (Grant Nos. 12120113032800, 21201011000150004, DD20190069)
文摘High-pressure(HP)or ultrahigh-pressure(UHP)rutile-quartz veins that form at mantle depths due to fluid-rock interaction can be used to trace the properties and behavior of natural fluids in subduction zones.To explore the fluid flow and the associated element mobility during deep subduction and exhumation of the continental crust,we investigated the major and trace elements of Ti-rich minerals.Additionally,U–Pb dating,trace element contents,and Lu–Hf isotopic composition of zircon grains in the UHP eclogite and associated rutile-quartz veins were examined in the North Qaidam UHP metamorphic belt,Yuka terrane.The zircon grains in the rutile-quartz veins have unzoned or weak oscillatory zonings,and show low Th/U ratios,steep chondrite-normalized patterns of heavy rare earth elements(HREEs),and insignificant negative Eu anomalies,indicating their growth in metamorphic fluids.These zircon grains formed in 4313 Ma,which is consistent with the 4322 Ma age of the host eclogite.As for the zircons in the rutile-quartz veins,they showed steep HREE patterns on one hand,and were different from the zircons present in the host eclogite on the other.This demonstrates that their formation might have been related to the breakdown of the early stage of garnet,which corresponds to the abundance of fluids during the early exhumation stage.The core-rim profile analyses of rutile recorded a two-stage rutile growth across a large rutile grain;the rutile core has higher Nb,Ta,W,and Zr contents and lower Nb/Ta ratios than the rim,indicating that the rutile domains grew in different metamorphic fluids from the core towards the rim.The significant enrichment of high field strength elements(HFSEs)in the rutile core suggests that the peak fluids have high solubility and transportation capacity of these HFSEs.Furthermore,variations in the Nb vs.Cr trends in rutile indicate a connection of rutile to mafic protolith.The zircon grains from both the rutile-quartz veins and the host eclogite have similar Hf isotopic compositions,indicating that the vein-forming fluids are internally derived from the host eclogite.These fluids accumulated in the subduction channel and were triggered by local dehydration of the deeply subducted eclogite during the early exhumation conditions.
基金supported by the China Geological Survey Project "An Integrated Study on Precambrian Geology and Paleo-zoic Tectono-Palaeogeography in Tibetan Plateau" (1212010610102)
