雅鲁藏布江缝合带西段东波MORB型均质辉长岩的大洋核杂岩成因

为解决雅鲁藏布江缝合带西段南带中数个大型超镁铁岩体的成因问题，对南带西段约400 km2的东波蛇绿岩开展区域地质填图，研究蛇绿岩岩石组合和构造性质及西北缘均质辉长岩年代学和成因.研究表明，东波蛇绿岩以地幔橄榄岩、薄层洋壳和周缘出露大面积晚侏罗世-早白垩世残余海山为特征，地幔橄榄岩中发育大量拆离、韧性剪切和正断层及糜棱岩和糜棱岩化蛇纹岩和蛇绿角砾岩；均质辉长岩的锆石普遍受到流体交代，锆石U-Pb年龄为129.0±1.8 Ma，地球化学具有低Si、K、P、Fe和Ti，高Ca和Mg，N-MORB型的稀土配分特征及明显的Th、Nb、Sr和Pb负异常.认为均质辉长岩形成于慢速-超慢速大洋扩张阶段，在大洋核杂岩沿拆离断层侵位过程中形成. 

Dongbo MORB-Type Isotropic Gabbro Emplaced as an Oceanic Core Complex in Western Yarlung Zangbo Suture Zone,Tibet

The tectonic setting and genesis of the ophiolites in the southern belt (SB) of the western Yarlung Zangbo ophiolitic belt are still controversial.They occur much larger peridotite massifs in contrast with those in the northern belt that are made discontinuously of lensoidal ophiolitic bodies in serpentinite matrix mélanges. The Dongbo ophiolite in the SB has been investigated and mapped, especially a 1 002.06 m dominant peridotite core has been drilled in the northwestern margin in 2015. Geochemical and geochronological (U-Pb zircon age) data from isotropic gabbros are presented in this paper. Dongbo ophiolite consists dominantly of harzburgite, minor dunite and mafic intrusions, associated with thin oceanic crust.Dismembered Late Juarassic to Early Cretaceous volcanic-sedimentary sequences of seamounts overlie the peridotites in the margins.Detachment and ductile shear faults, mylonite and mylonitic serpentinite and ophiolitic breccia are found in the mantle peridotites. In-situ LA-ICP-MS analysis of zircon grains from isotropic gabbros yields 129.0±1.8 Ma.The geochemical data of these gabbros are characterized by low Si, K, P, Fe, Ti and high Ca, Mg, showing N-MORB-like chondrite-normalized REE patterns and remarkable Th, Nb, Sr and Pb negative anomalies in N-MORB normalized spider diagrams. It is proposed that part of Dongbo peridotite and isotropic gabbro formed in a slow-spreading mid-ocean ridge which exhumed along detachment faults as an oceanic core complex.