上扬子西缘晚二叠世宣威组凝灰岩:对峨眉山大火成岩省火山活动及古特提斯弧火山作用的约束

Late Permian Xuanwei Formation tuff from the western margin of theUpper Yangtze:constraints on volcanic activity and Paleotethyanarc volcanism in the Emeishan Large Igneous Province

扬子西缘二叠系—三叠系之交凝灰岩的研究对探讨峨眉山大火成岩省及古特提斯周缘岩浆弧火山活动的时限约束具有重要意义。本次工作在上扬子西缘永善地区晚二叠世宣威组中发现多层凝灰岩。对其底部和顶部凝灰岩进行LA-MC-ICP-MS锆石U-Pb测年和Hf同位素研究显示:底部凝灰岩加权平均年龄为258.9±0.7 Ma,锆石Th/Nb比值为2.53~13.05,ε_(Hf)(t)值为3.0~5.0,T_(DM2)为988~871 Ma;锆石微量元素、Hf同位素及二阶模式年龄显示其具有与峨眉山玄武岩相关的特征,为峨眉山大火成岩省后期产物。顶部凝灰岩加权平均年龄为254.9±0.6 Ma,锆石Th/Nb比值为36.50~311.85,ε_(Hf)(t)值为-7.4~-6.9,T_(DM2)为1573~1529 Ma;锆石微量元素、Hf同位素及二阶模式年龄特征表明其形成于宋马-哀牢山-金沙江构造带弧火山活动。研究表明:宣威组下部凝灰岩为峨眉山大火成岩省后期酸性火山活动的产物,研究区峨眉山火山活动至少持续到了258.9±0.7 Ma。古特提斯弧火山作用至少于254.9±0.6 Ma就已启动。

The tuff at the intersection of Permian-Triassic in the western margin of Yangtze is of great significance in constraining the chronology of volcanic activity in the Emeishan Large Igneous Province and the magmatic arc around the Paleotethys.In this work,multi-layer tuff was found in the Late Permian Xuanwei Formation in the Yongshan area,western margin of Upper Yangtze.LA-MC-ICP-MS zircon U-Pb dating and Hf isotope study of the tuff at the bottom and top show that the weighted average age of the tuff at the bottom is 258.9±0.7 Ma,the zircon Th/Nb ratio is 2.53~13.05,theε_(Hf)(t) value is 3.0~5.0,and the T_(DM2) value is 988~871 Ma.Zircon trace elements,Hf isotope and two-stage model ages(T_(DM2)) shows that its characteristics are related to the Emeishan basalt,and that it is a late product of the ELIP.LA-MC-ICP-MS zircon U-Pb dating and Hf isotope study of the tuff at the bottom and top show that the weighted average age of the tuff at the top is 254.9±0.6 Ma,the zircon Th/Nb ratio is 36.50~311.85,theε_(Hf)(t) value is-7.4~-6.9,and the T_(DM2) value is 1573~1529 Ma.Zircon trace elements,Hf isotope and two-stage model ages(T_(DM2)) indicate that it was formed during arc volcanic activity in the Songma-Ailaoshan-Jinshajiang suture zone.The results show that the tuff at the bottom of the Xuanwei Formation is a product of acid volcanic activity in the later stage of the Emeishan Igneous Province.The volcanic activity of the Emeishan in the study area lasted until 258.9±0.7 Ma.Paleotethyan arc volcanism started at 254.9±0.6 Ma or earlier.