摘要:
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,SiO2含量为46.06%~49.27%,具有高FeOT(平均为11.71%)、高钛(TiO2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。
Abstract:
The Qiongjie Complexes (Qiongjie ophiolitic mélanges) 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 mélanges in the 1:250 000 geological map. These ophiolitic mélanges 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 FeOT 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 chondritenormalized REE diagram, the metadiabases in the Qiongjie ophiolitic mélanges display the smooth chondritenormalized 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.
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