俯冲洋壳流体交代岩石圈地幔:西藏中部南羌塘地体钾质火山岩Li-Pb同位素的证据  

Lithospheric mantle metasomatized by oceanic crust-derived fluids:Li and Pb isotopic evidence from potassic volcanic rocks in the southern Qiangtang terrane,central Tibet

作　　者：李文强 许伟 田世洪 LI Wenqiang;XU Wei;TIAN Shihong(State Key Laboratory of Nuclear Resources and Environment,East China University of Technology,Nanchang 330013,China;School of Earth Sciences,East China University of Technology,Nanchang 330013,China)

机构地区：[1]东华理工大学核资源与环境国家重点实验室,江西南昌330013 [2]东华理工大学地球科学学院,江西南昌330013

出　　处：《地学前缘》2023年第4期376-388,共13页Earth Science Frontiers

基　　金：国家重点研发计划“战略性矿产资源开发利用”专项“我国西部伟晶岩型锂等稀有金属成矿规律与勘查技术(2021YFC2901900)”“北喜马拉雅锂等稀有金属找矿预测与勘查示范课题(2021YFC2901903)”;江西省“双千计划”创新领军人才长期项目(2020101003);东华理工大学核资源与环境国家重点实验室自主基金项目(2020Z08);东华理工大学高层次人才引进配套经费项目(1410000874);国家自然科学基金项目(42002095,41773014)。

摘　　要：Li稳定同位素在低温过程中会发生明显的分馏,已被广泛用于示踪壳幔相互作用。为识别岩石圈地幔中的壳源组分,本文对西藏中部南羌塘地体起源于岩石圈地幔部分熔融的晚始新世(约38 Ma)钾质火山岩(粗面英安岩和流纹岩)开展了Li和Pb同位素的测试。测试结果显示这些样品具有均一的Pb同位素组成(^(206)Pb/^(204)Pb=18.497 9~18.577 2,^(207)Pb/^(204)Pb=15.594 0~15.610 7,^(208)Pb/^(204)Pb=38.671 0~38.754 0),而Li同位素组成变化较大,其δ~7Li值变化于5.67‰到10.97‰之间(平均值为8.81‰,n=11),显著高于新鲜洋中脊玄武岩[δ~7Li=(3.4±1.4)‰]和全球俯冲沉积物[GLOSS-II,δ~7Li=(2.42±0.18)‰],但相似于蚀变洋壳(δ~7Li=6‰~14.5‰)。结合已发表的Sr-Nd同位素和Sr-Nd-Pb-Li同位素的三端员混合模拟,本文认为南羌塘地体下的岩石圈地幔受到过以俯冲洋壳流体为主要组成的壳源组分的交代,但不排除有俯冲碳酸盐岩熔体的贡献。Lithium(Li)isotopes show large isotopic fractionation during low-temperature processes and have been widely used to trace crust-mantle interactions.In order to identify the crust-derived components in the lithospheric mantle,this study reports new Li and Pb isotopic data for the Late Eocene(~38 Ma)lithospheric mantle-derived potassic volcanic rocks(trachydacites,rhyolites,etc.)from the southern Qiangtang terrane,central Tibet.The volcanic rocks have homogeneous Pb isotopic compositions(^(206)Pb/^(204)Pb=18.497918.5772;^(207)Pb/^(204)Pb=15.594015.6107;^(208)Pb/^(204)Pb=38.671038.7540)and variable Li isotopic compositions,withδ7Li values ranging from 5.67‰to 10.97‰(average 8.81‰,n=11),significantly higher compared to fresh mid-ocean ridge basalts(3.4‰±1.4‰)and global subducted sediments(2.42‰±0.18‰(GLOSS-II))but similar to that of altered oceanic crust(6‰14.5‰).Combined with published Sr-Nd isotopes and mixing models for Sr-Nd-Pb-Li isotopes,we suggest that mantle metasomatism occurred in the southern Qiangtang terrane by a crustal component dominated by subducted oceanic crust-derived fluids,but we do not preclude the possible contribution of subducted carbonated melts.

关 键 词：Li同位素 岩石圈地幔 钾质火山岩 洋壳流体 南羌塘地体 

分 类 号：P597.2[天文地球—地球化学] P588.1[天文地球—地质学] P542.5