藏南雅鲁藏布江结合带东段琼结杂岩早白垩世变辉绿岩地球化学特征及其地质意义

Early Cretaceous metadiabases from the Qiongjie ophiolitic mélanges in the eastern Yarlung Zangbo suture zone,southern Xizang

琼结杂岩(琼结蛇绿构造混杂岩)是1∶5万区域地质调查新划分的填图单元,大体相当于1∶25万地质图中朗县蛇绿混杂岩的一部分,位于雅鲁藏布江结合带东段,主要由基质、块体两大部分组成,其中块体中蛇绿岩残片主要由变辉长岩、变辉绿岩和变玄武岩组成。本文通过详细的野外工作,对洛林地区琼结杂岩中的变辉绿岩进行了LAICP-MS锆石U-Pb定年以及地球化学研究,获得的岩浆结晶年龄为111Ma。全岩的主量及微量元素地球化学特征显示,SiO_2含量为46. 06%～49. 27%,具有高Fe OT(平均为11. 71%)、高钛(Ti O_2平均为1. 30%)、高Mg#(Mg#平均为59. 39)等特点;轻、重稀土元素具较弱分馏,LREE/HREE比值为1. 83～2. 05,(La/Yb)N比值平均为1. 35,球粒陨石标准化稀土元素配分型式显示为平坦型,基本不具Eu异常(δEu=0. 81～1. 02),富集高场强元素(HFSE)(Nb、Ta、U、Th)、亏损大离子亲石元素(LILE)(Ba、Sr),呈现出正常洋中脊玄武岩(N-MORB)的地球化学特性。岩浆来自正常洋中脊玄武岩区域,为尖晶石二辉橄榄岩10%～20%左右部分熔融的产物;岩浆上升过程中未受到明显的地壳混染。结合区域地质资料,本文认为琼结杂岩变辉绿岩形成于典型洋中脊构造环境,代表典型的洋中脊残留,揭示出雅鲁藏布江缝合带东段蛇绿岩形成于晚三叠世中期—早白垩世晚期。

The Qiongjie Complexes(Qiongjie ophiolitic melanges)in the eastern Yarlung Zangbo suture zone,southern Xizang are delineated as a newly defined geological mapping unit of 1:50 000 regional geological survey,roughly corresponding to part of the Nangxian ophiolitic melanges in the 1:250 000 geological map.These ophiolitic melanges consist dominantly of matrix and blocks.The ophiolitic fragments in the blocks are composed of metagabbro,metadiabase and metabasalt.The present paper gives a detailed geochemical study of the metadiabases and reports a LA-ICP-MS zircon U-Pb age of ca.111 Ma for the crystalline age of magmatic rocks.The metadiabases have low SiO2 contents ranging from 46.06%to 49.27%,high FeO^T contents(an average of 11.71%),high TiO2 contents(an average of 1.30%),high Mg^#values(an average of 59.39),and slightly differentiated light/heavy rare earth elements(LREE/HREE=1.83-2.05,(La/Yb)N=1.35).In the chondrite-normalized REE diagram,the metadiabases in the Qiongjie ophiolitic melanges display the smooth chondrite-normalized REE distribution patterns without Eu anomalies(δEu=0.81-1.02),enrichment in high field strength elements Nb,Ta,U and Th,and depletion in large ion lithophile elements Ba and Sr.Geochemically,the metadiabases exhibit the geochemical signatures of normal mid-oceanic ridge basalts,and represent the products of the 10%-20%partial melting of spinel-iherzolite,without crustal contamination during the ascending of the magmas.Tectonically,the metadiabases in the study area are interpreted to be formed in the typical mid-oceanic ridge tectonic setting.It is concluded that the ophiolites in the eastern Yarlung Zangbo suture zone were initiated during the middle Late Triassic to the late Early Cretaceous.