Ages, Trace Elements and Hf-Isotopic Compositions of Zircons from Claystones around the Permian-Triassic Boundary in the Zunyi Section, South China: Implications for Nature and Tectonic Setting of the Volcanism  被引量:9

Ages, Trace Elements and Hf-Isotopic Compositions of Zircons from Claystones around the Permian-Triassic Boundary in the Zunyi Section, South China: Implications for Nature and Tectonic Setting of the Volcanism

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作  者:Qiuling Gao Zhong-Qiang Chen Ning Zhang William L.Griffin Wenchen Xia Guoqing Wang Tengfei Jiang Xuefei Xia Suzanne Y.O'Reilly 

机构地区:[1]Exploration & Development Research Institute,Zhongyuan Oilfield Company [2]School of Earth Sciences, China University of Geosciences [3]Australian Research Council Centre of Excellence for Core to Crust Fluid Systems and GEMOC National Key Centre,Department of Earth and Planetary Science, Macquarie University [4]State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences [5]Shandong Lunan Institute of Geo-Engineering Exploration [6]Jianghan Oilfield Company

出  处:《Journal of Earth Science》2015年第6期872-882,共11页地球科学学刊(英文版)

基  金:supported by an aid grant from Chengdu Center, China Geological Survey (No. 12120113049100-1);the National Natural Science Foundations (Nos. 40572068, 40839903 and 41272044);the "111" Program (No. B08030);an aid grant (No. GBL11206) from the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan), China

摘  要:A growing body of evidence shows that volcanism near the Permian-Triassic boundary(PTB) may be crucial in triggering the Permian–Triassic(P–Tr) mass extinction. Thus, the ash beds near the PTB in South China may carry information on this event. Three volcanic ash layers, altered to clay, outcropped in the PTB beds in Zunyi Section, Guizhou Province, Southwest China. The U-Pb ages, trace elements, and Hf-isotope compositions of zircon grains from these three ash beds were analyzed using LA-ICPMS and LA-MC-ICPMS. The zircons are mainly magmatic in origin(241-279 Ma) except for two inherited/xenocrystic zircons(939 and 2 325 Ma). The ages of these magmatic zircons indicate three episodes of magmatism which occurred around the MiddleLate Permian boundary(-261.5 Ma, MLPB), the Wuchiapingian-Changhsingian boundary(-254.5 Ma, WCB), and the PTB(-250.5 Ma), respectively. The first two episodes of magmatism near the MLPB and WCB may be attributed to magmatic inheritance or re-deposition. All magmatic zircons share similar trace-element and Hf-isotope compositions. They have Y, Hf, Th and U contents and Nb/Ta ratios are typical of zircons from silicic calc-alkaline magmas. These zircons also exhibit enriched Hf-isotope compositions with _(εHf)(t) values of-11.4 to +0.2, which suggests that the three magmatic episodes involved melting of the continental crust. The more enriched Hf-isotope composition (_(εHf)(t)=-11.4--4.8) of Bed ZY13(-250.5 Ma) implies more input of ancient crustal material in the magma. Integration of the Hf-isotope and trace-element compositions of magmatic zircons suggest that these three episodes of magmatism may take place along the convergent continent margin in or near southwestern South China as a result of the closure of the Palaeo-Tethys Ocean.A growing body of evidence shows that volcanism near the Permian-Triassic boundary(PTB) may be crucial in triggering the Permian–Triassic(P–Tr) mass extinction. Thus, the ash beds near the PTB in South China may carry information on this event. Three volcanic ash layers, altered to clay, outcropped in the PTB beds in Zunyi Section, Guizhou Province, Southwest China. The U-Pb ages, trace elements, and Hf-isotope compositions of zircon grains from these three ash beds were analyzed using LA-ICPMS and LA-MC-ICPMS. The zircons are mainly magmatic in origin(241-279 Ma) except for two inherited/xenocrystic zircons(939 and 2 325 Ma). The ages of these magmatic zircons indicate three episodes of magmatism which occurred around the MiddleLate Permian boundary(-261.5 Ma, MLPB), the Wuchiapingian-Changhsingian boundary(-254.5 Ma, WCB), and the PTB(-250.5 Ma), respectively. The first two episodes of magmatism near the MLPB and WCB may be attributed to magmatic inheritance or re-deposition. All magmatic zircons share similar trace-element and Hf-isotope compositions. They have Y, Hf, Th and U contents and Nb/Ta ratios are typical of zircons from silicic calc-alkaline magmas. These zircons also exhibit enriched Hf-isotope compositions with _(εHf)(t) values of-11.4 to +0.2, which suggests that the three magmatic episodes involved melting of the continental crust. The more enriched Hf-isotope composition (_(εHf)(t)=-11.4--4.8) of Bed ZY13(-250.5 Ma) implies more input of ancient crustal material in the magma. Integration of the Hf-isotope and trace-element compositions of magmatic zircons suggest that these three episodes of magmatism may take place along the convergent continent margin in or near southwestern South China as a result of the closure of the Palaeo-Tethys Ocean.

关 键 词:Permian-Triassic boundary zircon trace elements Hf isotope silicic volcanism convergent continental margin South China. 

分 类 号:P597[天文地球—地球化学] P534[天文地球—地质学]

 

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