藏南仲巴地区始新世花岗质岩石成因及其对拉萨地壳加厚的启示

The Gangdese batholith located in the southern part of Lhasa terrane, and distributed along the Yarlung-Zangbo Suture Zone, which is the great source of the Neo-Tethyan subduction and the India-Asian collision information. However, the results of the present study were coming from the eastern and central parts, while little is known about the western part of the Gangdese batholith. We focus on the Zhongba granitoids from the western part, and obtained a Zircon U-Pb age of 38 Ma during the pre-research, which means the Zhongba granitoids was formed during the origin magmatic quiescent period with little magmatic activity. The research of this Eocene granitoids could improve the spatial and temporal distribution of the Gangdese batholith. We will study the field character, geochemistry, whole rock Sr-Nd-Pb and zircon Hf isotopic data of the Zhongba granitoids, and discuss magma source, petrogenesis and tectonic evolution. All the data, combining with the current results from other batholiths in Gangdese belt, will be used to discuss the possibility of the mix of Indian plate in the source and provide new constraints on the crust thicken of the Lhasa terrane.

冈底斯岩基位于拉萨地体南缘，沿雅鲁藏布江缝合带北侧广泛分布，蕴含了新特提斯洋俯冲、印度-亚洲大陆碰撞的重要信息。但是，目前对冈底斯岩基的研究多集中在东段和中段。本项目选取研究程度薄弱的西段仲巴地区花岗质岩体为研究对象。在预研究中，获得其形成年龄为38 Ma，处于冈底斯岩浆活动的宁静期，对其进行深入研究有助于完善冈底斯岩基的时空格架。通过对仲巴花岗质岩体的岩石学、地球化学、全岩Sr-Nd-Pb同位素和锆石Hf同位素研究，探讨其岩浆源区、成岩模式和动力学机制。并将其与冈底斯岩基最新研究进展相结合，分析俯冲的印度大陆地壳物质对其源区的影响，以及对拉萨地壳加厚的约束。

仲巴花岗质岩体位于冈底斯岩基西段，本文对其进行了地球化学、年代学、全岩Sr-Nd-Pb和锆石Hf同位素研究。锆石U-Pb数据显示其成岩时代为始新世（38Ma）。仲巴样品富集LREE、LILE和Pb，亏损HFSE，其(87Sr/86Sr)i = 0.71080 ~0.71639，εNd(t)= −8.7 ~ −6.5，(206Pb/204Pb)t = 18.622 ~ 18.909，(207Pb/204Pb)t = 15.738~15.755，(208Pb/204Pb)t = 39.304~39.459，εHf(t)=-7.7~-2.6。仲巴花岗岩可能为，软流圈岩浆在上涌过程中，使俯冲的印度大陆地壳物质发生部分熔融形成熔体，并与之混合形成。结合前人研究成果，仲巴、冈仁波齐和泽东花岗岩记录了拉萨地体南缘，发生在42-38Ma的一期不同于同碰撞-后碰撞时期的岩浆活动。仲巴花岗岩是新特提斯洋板片断离引起的岩浆作用，记录了印度大陆地壳物质通过岩浆作用的方式对拉萨地体地壳加厚的影响。