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
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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华南遵义剖面二叠系-三叠系界线粘土岩中锆石的年龄、微量元素和Hf同位素组成：对火山作用性质和构造环境的启示
DOI：
10.1007/s12583-015-0589-9
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发表时间：
2015-12
影响因子：
3.3
通讯作者：
O'Reilly Suzanne Y.
中科院分区：
文献类型：
--
作者：
Gao Qiuling;Chen Zhong-Qiang;Zhang Ning;Griffin William L.;Xia Wenchen;Wang Guoqing;Jiang Tengfei;Xia Xuefei;O'Reilly Suzanne Y.
越来越多的证据表明，二叠系-三叠系界线(PTB)附近的火山活动可能是引发二叠系-三叠系(P-Tr)大灭绝的关键因素。因此，中国南部PTB附近的火山灰床可能携带了这一事件的信息。利用LA-ICPMS和LA-MC-ICPMS分析了贵州遵义PTB岩层中出露的三个火山灰层的U-Pb年龄、微量元素和Hf同位素组成。锆石的成因主要为岩浆成因(241Ma279 Ma)，但也有两个继承/包裹型锆石(939 Ma和2325 Ma)。这些岩浆锆石的年龄表明，它们分别发生在中−晚二叠世界线(~261.5 Ma，MLPB)、乌家坪-长兴期界线(~254.5 Ma，WCB)和PTB(~250.5 Ma)附近的三期岩浆活动。MLPB和WCB附近的前两幕岩浆作用可能归因于岩浆的继承或再沉积。所有的岩浆锆石都有相似的微量元素和Hf同位素组成。它们具有Y、Hf、Th和U的含量，Nb/Ta比值是典型的硅质钙碱性岩浆中的锆石。这些锆石还显示了富含Hf的同位素组成，εHf(T)值为-11.4~+0.2值，表明三次岩浆事件涉及大陆地壳的熔融。ZY13层(~250.5 Ma)的Hf同位素组成较高(εHf(T)=-11.4−-4.8)，反映了古地壳物质对岩浆的更多输入。岩浆锆石的Hf同位素和微量元素组成的综合表明，这三幕岩浆作用可能是由于古特提斯洋的闭合而发生在中国西南或附近的会聚大陆边缘。
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 Middle−Late Permian boundary (~261.5 Ma, MLPB), the Wuchiapingian-Changhsingian boundary (~254.5Ma, 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 ε Hf (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 (ε Hf (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.
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