环东冈瓦纳大陆周缘的古生代造山作用：东喜马拉雅构造结南迦巴瓦岩群的岩石学和年代学证据

Paleozoic circum-Gondwana orogens:Petrology and geochronology of the Namche Barwa Complex in the eastern Himalayan syntaxis,Tibet.

喜马拉雅造山带东端的南迦巴瓦岩群是高喜马拉雅结晶岩系的一部分,主要由麻粒岩相和角闪岩相变质的片麻岩、斜长角闪岩、片岩和钙硅酸盐岩组成。长英质片麻岩主要由斜长石、钾长石、石英、石榴石、黑云母和褐帘石组成。片麻岩中的锆石具有核.边结构,由一个大的继承岩浆核和一个窄的变质生长边组成。锆石岩浆核具同心韵律环带,其 REE配分模式以 HREE 富集和负 Eu 异常为特征,并具有高的 Th/U 比值。锆石 U-Pb 年代分析表明,这种继承岩浆锆石给出的加权平均年龄为490～500Ma。地球化学特征表明,这些片麻岩的原岩是花岗岩和花岗闪长岩,形成在俯冲带的岩浆弧构造环境。钙硅酸盐岩中的锆石具有高级变质岩中变质生长锆石的典型特征,即具有相对较低的 REE 含量,不明显的负 Eu异常和较低的 Th/U 比值。变质锆石所获得的 U-Pb 加权平均年龄为505Ma。本文和现有的研究结果表明,喜马拉雅造山带是一个复合造山带,它经历了古生代的原始造山作用,在新生代印度与欧亚板块的碰撞过程中发生了再造山作用。喜马拉雅的古生代造山带作用是原特提斯洋向冈瓦纳大陆北缘俯冲和亚洲微陆块(包括拉萨和羌塘地块)增生的结果,是在冈瓦纳大陆拼合之后其边缘发生的安底斯型造山作用,因此,它并不属于在冈瓦纳超大陆聚合过程中陆-陆碰撞形成的泛非造山带。

The Namche Barwa Complex located in the eastern Himalayan syntaxis is a part of the High Himalayan crystalline Complex, and consists of gneiss, amphibolite, schist and calc-silicate rocks which have been subjected to granulite-faeies metamorphism and amphibolite-faeies retrogression. The geochronological data shows that the magmatic and metamorphic rocks in the Namche Barwa Complex have the Paleozoic ages. The felsic gneisses consist of plagioclase, K-feldspar, quartz, garnet, and biotite with or without allanite, and show the typical textural and structural features of metamorphic rocks. However, zircons of the gneisses have concentric oscillatory zoning, HREE-enriched REE patterns with distinctly negative Eu anomalies, and high Th/U values, typical of zircon from granitic intrusives. These inherited magmatic zircons give U-Pb ages of 490 -500Ma. The ealc-silicate has a metamorphic mineral assemblage of scapolite, diopside, quartz and titanite. The zircons from a ealc-silicate have relatively low REE contents with slightly negative Eu anomalies and low Th/U ratios, and show no zoning, typical of zircon from high-grade metamorphic rocks. These metamorphic growth zircons give a weighted mean U-Pb age of 505Ma. The protoliths of these Paleozoic gneisses are granite and granodiorite, which were formed in the tectonic setting of magmatic arc. The present paper and available studies demonstrate further that the Himalaya is a composite orogenic belt, which was originally formed in the Paleozoic and then reworked in the Cenozoic by the collision between the India and Eurasian plates. It is considered that the Paleozoic orogeny was related to the Andean-type orogenic activity on the northern margin of the Indian continent, following the Gondwana assembly, and was driven by coupling across the plate margin either during on-going subduetion of the proto-Tethys ocean beneath to the India plate or through accretion of microcontinental ribbons, possibly represented by the Lhasa and Qiangtang blocks, but did not belonged to the Pan-Africa orogen which was formed by the continent-continent collision during the Gondwana assembly.