西藏班公湖—怒江缝合带中段东巧地幔橄榄岩岩石成因及构造环境分析

东巧蛇绿岩位于班公湖—怒江缝合带中段,根据地理位置特征并以强玛镇为中心将东巧岩体划分为东西两个岩体。其中西岩体相对面积较大,由地幔橄榄岩、枕状玄武岩、辉长辉绿岩等组成;而东岩体面积较小,仅含地幔橄榄岩部分,各个不同单元之间呈断层接触关系。对东巧地幔橄榄岩开展岩石学、矿物学及地球化学研究发现:(1)东巧地幔橄榄岩以方辉橄榄岩为主,纯橄岩所占比例较小,约15%。豆荚状铬铁矿主要呈条带浸染状赋存在厚层且延伸较远的纯橄岩中。(2)东巧地幔橄榄岩中单斜辉石含量小于3%,矿物地球化学和全岩地球化学特征显示其来源于尖晶石相地幔源区的部分熔融,且部分熔融程度较高,估算在22%～28%,高于深海地幔橄榄岩的部分熔融程度(10%～22%)。(3)东巧地幔橄榄岩中的副矿物铬尖晶石Cr#值较高大于60,全岩具有U型球粒陨石标准化稀土元素配分模式,同时Rb、U、Zr和Sr相对富集,Hf和Nb相对亏损。全岩的地球化学特征指示了俯冲带之上的残余地幔与流体/熔体发生了反应,致使轻稀土元素以及部分微量元素选择性富集。综合东巧地幔橄榄岩的矿物化学组成成分以及全岩的地球化学特征,认为东巧地幔橄榄岩形成于大洋中脊的扩张环境中,后受到洋内俯冲作用的影响,导致俯冲带之上高度部分熔融的地幔橄榄岩与流体/熔体发生相互作用。

Genesis and tectonic setting of the Dongqiao peridotites in the central segment of the Bangong Co-Nujiang Suture Zone

Located in the central segment of the Bangong Co-Nujiang Suture Zone, the Dongqiao ophiolite is divided into east and west massifs. The research on the origin and tectonic setting of the ophiolite in the Bangong Co-Nujiang suture zone can provide key clues for revealing the tectonic evolution of the Neo-Tethys and the collision between the Indian plate and the Eurasian plate. The area of the west rock massif is relatively large, consisting of mantle peridotites, pillow-like basalts, gabbros, diabases, etc., while the area of the east massif is small and contains only part of mantle peridotites, with the faults being in contact between different units. Based on petrology, mineralogy and geochemistry of the peridotites in Dongqiao ophiolite, the authors have reached some conclusions:(1) The Dongqiao peridotites are dominated by harzburgites, with minor proportion of dunites (about 15%). The podiform chromitites mainly show banded and disseminated structures, and occur in a thick layer of far-extending dunites. (2) The clinopyroxene content of Dongqiao peridotites is less than 3%. The mineral and the whole-rock geochemistry show that Dongqiao peridotites were formed after 22%-28% degree of partial melting of the spinel-phase mantle source, higher than the partial melting of abyssal peridotites (10%-22%). (3) The Cr^# values of spinels in the Dongqiao peridotites are higher than 60. The chondritenormalized REE patterns of the Dongqiao peridotites display enrichments in LREE. In the primitive mantle-normalized spider diagrams, the Dongqiao peridotites exhibit slightly positive Rb, U, Zr, Sr anomalies and slightly negative Hf, Nb anomalies. These geochemical characteristics jointly indicate the selective enrichments of LREE and some trace elements according to interaction between the residual mantle and the fluid/melt above the subduction zone. Based on the mineral composition and the whole-rock geochemical characteristics of Dongqiao peridotites, the authors hold that the Dongqiao peridotites were formed in the dilated environment of the mid-ocean ridge, and then were influenced by intra-ocean subduction, resulting in highly partially melted peridotites interacting with the fluid/melt above the suprasubduction zone.