西藏尼木地区变质火山岩的地球化学特征、锆石U-Pb年龄及其地质意义

The geochemistry and zircon U-Pb dating for the metamorphic volcanic rocks in Nimu area，Tibet

关键词:  尼木地区  地球化学  洋内弧和陆缘弧火山岩  锆石U-Pb年龄

Key words:  Nimu area  geochemistry  intra-oceanic arc and continental marginal arc  zircon U-Pb dating

摘要:

西藏尼木地区分布一套增生杂岩，其对冈底斯火山岩浆弧的演化具有重要意义。本文对该套增生杂岩中的变质火山岩的地质特征、矿物学、全岩地球化学和锆石U-Pb年代学等方面进行了综合研究。变质火山岩以斜长角闪岩、角闪斜长片麻岩为主，其中，角闪石主要为镁质角闪石，共生的斜长石主要为拉长石；变质火山岩经历了高温-中压变质作用；岩石富Al₂O₃和贫TiO₂，弱富集轻稀土元素（LREE），富集Rb、Sr、Ba等大离子亲石元素（LILE），亏损Nb、Ta、Ti等高场强元素（HFSE），其地球化学特征与火山弧玄武岩的地球化学特征相似，其形成的构造环境为洋内岛弧或活动大陆边缘弧；岩浆锆石U-Pb年龄值为151.4±1.6Ma和150.7±1.4Ma，表明岩石的形成时代为晚侏罗世。综合研究认为，增生杂岩中的变质火山岩是新特提斯洋在晚侏罗世北向俯冲的产物，在陆-陆碰撞之前卷入增生系统，该套变质火山岩不是以往所认为的变质结晶基底。

Abstract:

Located in the south of the Gangdise magmatic arc, Nimu area occurred a set of metamorphic volcanic rocks mainly composed of amphibolite and amphibole-bearing plagioclase gneiss. Previously, they are believed to be the late Paleozoic crystalline basement rocks of the Lhasa block. By detailed geological survey and studies on mineral associations, geochemistry, and zircon U-Pb dating, the authors believe these metamorphic volcanic rocks were the accretion complex related to the subduction of the New Tethys Ocean in the late Jurassic. Following their formation, these arc-type volcanic rocks then experienced the mid-pressure amphibolite facies metamorphism in the late Cretaceous and the high-temperature contact metamorphism in the Eocene. Mineral chemistry shows that hornblendes in the rocks are mainly magnesium hornblendes and the paragenetic feldspars are mainly labradorites. The geochemistry of the rocks shows that Al₂O₃ is in rich and TiO₂ is in poor, and LILEs such as Rb, Sr, Ba are in rich and HFSEs such as Nb, Ta, Ti are in poor, comparatively. The LA-ICP-MS U-Pb dating of zircons from the metamorphic volcanic rocks shows an age from 151.4±1.6Ma to 150.7±1.4Ma, which indicate that the studied rocks are originally the product of the northward subduction of the New Tethys Ocean in the late Jurassic.