Origin of volcanic ash beds across the Permian-Triassic boundary, Daxiakou, South China: Petrology and U-Pb age, trace elements and Hf-isotope composition of zircon

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• 作者：Qiuling Gao ; Ning Zhang ; Wenchen Xia ; Qinglai Feng ; Zhong-Qiang Chen ; Jianping Zheng ; William L. Griffin ; Suzanne Y. O'Reilly ; Norman J. Pearson ; Guoqing Wang ; Sha Wu ; Wenli Zhong ; Xiufeng Sun
• 关键词：Ash beds ; Permian&ndash ; Triassic mass extinction ; Zircon trace elements ; Zircon Hf isotopes ; Silicic calc-alkaline volcanism ; Convergent continental margin volcanism
• 刊名：Chemical Geology
• 出版年：18 December, 2013
• 年：2013
• 卷：360-361
• 期：Complete
• 页码：41-53
• 全文大小：2693 K

文摘

Abundant volcanic ash layers, largely altered to clay, characterize the Permian-Triassic boundary beds of the Daxiakou section, South China. Volcanism may have had an important role in the Permian-Triassic mass extinction, so it is important to understand the origin of these ash beds. The lithology of ash bed 260, as constrained by modes of crystal fragments (66.7% plagioclase, 7.2% alkali feldspar, and 26.1% quartz), suggests that the ash represents dacitic volcanism. The U-Pb ages, trace elements and Hf-isotope compositions of zircons from ten ash beds have been analyzed using LA-ICPMS and LA-MC-ICPMS. The zircons can be divided into two groups: magmatic zircons (~ 97%, 227-279 Ma) and inherited zircons (~ 3%, 678-2424 Ma). Magmatic zircons have Y, Hf, Th and U contents and Nb/Ta ratios typical of zircons from silicic calc-alkaline volcanism. 蔚_Hf(t) values of magmatic zircons vary from 鈭?#xA0;10.9 to + 5.3, implying magmatic mixing between juvenile crustal material (probably Neoproterozoic in age) and ancient crustal material (probably Archean). Beds 252-259-b have relatively depleted Hf-isotope compositions with 蔚_Hf(t) values of 鈭?#xA0;10.9-+ 5.3 and averages of 鈭?#xA0;5.0-鈭?#xA0;2.5, while other beds have 蔚_Hf(t) values of 鈭?#xA0;10.8-+ 4.4 (mostly 鈭?#xA0;10.8-鈭?#xA0;3.5) and averages of 鈭?#xA0;8.9-鈭?#xA0;6.8, implying more input of juvenile crustal material in the volcanism of Beds 252-259-b. Integration of the Hf-isotope and trace-element compositions of magmatic zircons suggests that the volcanism producing the ashes took place along the convergent continent margins during the formation of the Pangea supercontinent. This intense silicic volcanism may have played an important role in causing the Permian-Triassic mass extinction in South China.