东昆仑东段东昆中蛇绿岩年代学及构造属性研究

The ophiolites distributed in orogenic belt is always the important object being used to study the paleooceanic attribute and reconstruct the paleo continent-ocean tectonic framework. The ophiolites along the central fault of East Kunlun record the oceanic expansion events and the evolution process of continent-ocean during Proterozoic-early Palaeozoic era. Thus, we firstly dissect the ophiolite occurrence, scale and material composition through the detailed field survey, and present geochronological data by high-precision zircon/baddeleyite U-Pb isotope dating in order to establish preliminarily the accurate geochronological lattice of ophiolites. On the other hand, this project will confirm the geochemical feature and tectonic attribute of the ophiolites on basis of detailed research for the rock assemblages, and analyze the forming time, nature, scale of paleo-ocean basin represented by ophiolites through the comprehensive comparison with the.ophiolites outcropped in southern margin of East Kunlun around the same period. This research is crucial to reconstruct the paleo continent-ocean tectonic framework in middle part of East Kunlun and the entire Kunlun, and also has the important scientific significance to explore the evolution history, early orogenic process of the East Kunlun orogenic belt in Proto-Tethys tectonic domain and other fundamental problem of continental dynamics.

造山带中残存的蛇绿岩常常成为研究造山带古洋陆构造格架的重要对象与载体。东昆仑东段东昆中蛇绿岩带为一重要构造边界，记录了东昆仑地区元古代—早古生代重要的洋壳扩张事件及洋陆演化过程,是探讨原特提斯构造域古昆仑洋性质及洋陆构造格局的关键所在。本项目在前期蛇绿岩产出状态、物质组成初步研究基础上，拟利用高精度锆石U-Pb同位素测年技术系统研究该带多个蛇绿岩块的精确形成时代，试图初步建立东昆中地区统一的高精度蛇绿岩年代学格架。在蛇绿岩物质组成、相互接触关系细致研究基础上，通过系统的岩石/矿物地球化学数据综合确定蛇绿岩性质及构造属性，并在与大致同时期的东昆仑周缘蛇绿岩对比研究基础上，分析其所表征的东昆中古洋盆的时代和性质，恢复重建原特提斯构造域东昆中乃至整个东昆仑地区的古洋陆构造格架，对于探索东昆仑造山带形成演化及早期造山过程等大陆动力学基本问题具有重要意义。

本项目详细研究了东昆中断裂带出露的多个蛇绿岩的物质组成、相互接触关系及其锆石U-Pb同位素年龄，并通过系统的岩石/矿物地球化学数据确定了东昆中蛇绿岩性质及构造属性，恢复重建原特提斯构造域东昆中及其周缘地区的古洋陆构造格架。取得了如下重要进展：（1）对东昆中蛇绿岩中辉长岩/辉绿岩大量的锆石U-Pb年龄测试，限定了东昆中蛇绿岩的形成时代为早古生代，而非前人认为的元古代和石炭纪；（2）首次在东昆中蛇绿岩带的阿此特-清泉沟地区识别出一套弧前玄武岩（FAB）、玻安质辉长岩和难熔地幔橄榄岩组合，综合研究表明其形成于初始弧前扩张环境，形成时代510Ma，并限定东昆中洋初始俯冲年龄早于510Ma；（3）曲什昂蛇绿岩岩浆源区为介于EMORB和OIB之间富集地幔，形成于活动陆缘体系的初始弧后盆地（SSZ型蛇绿岩），类似于现今西太平洋地区冲绳海槽；（4）科科可特、清水泉地区蛇绿岩基性单元TiO2值较高，且εNd(t)值为负值或接近于0，地球化学特征具岩石圈地幔（SCLM）与软流圈地幔混合的富集地幔地球化学属性，形成于洋脊俯冲构造环境；（5）系统的地质、地球化学、构造地质学及Sr-Nd-Hf同位素地球化学研究，确定了东昆中蛇绿岩带是一个包含四种不同类型洋/弧壳残片的混杂带，并进一步认为东昆仑东段于早古生代早期具有类似于“东南太平洋-智利-南美Patagonia地区”洋壳俯冲与洋脊俯冲并存的复杂的洋陆构造格局。以上成果对于研究原特提斯构造域东昆中古洋陆构造格架具有重要意义。