喜马拉雅造山带东构造结南迦巴瓦岩群地质年代学和前寒武纪构造演化

Geochronology and Precambrian tectonic evolution of the Namche Barwa complex from the eastern Himalayan syntaxis,

位于喜马拉雅造山带东构造结,印度—雅鲁藏布江缝合带以南的南迦巴瓦岩群经历了高压变质作用和强烈的部分熔融与混合岩化作用。本文选择广泛分布的长英质片麻岩进行了岩石学和年代学研究。除个别岩石保存了由石榴石+蓝晶石十三元长石+石英组成的高压泥质麻粒岩相变质矿物组合以外,大多数片麻岩具有角闪岩相变质矿物组合,它们的原岩包括闪长岩和花岗闪长岩,并具有岩浆弧花岗岩的化学成分特征。片麻岩中的锆石普遍具有核-边结构。SARIMP 和 LA-ICP-MS 原位分析表明,锆石的边缘给出了古生代至新生代的多期变质和岩浆事件年龄(500～10Ma),而锆石的核部给出了前寒武纪年龄,但主要集中在～2500Ma,～1800Ma,～1600Ma 和～1000Ma。所分析的锆石区域具有明显的岩浆结晶环带和高的 Th/U 比值,表明它们所指示的是多期岩浆活动事件年代。这些年代峰值与整个高喜马拉雅结晶杂岩及印度陆块所获得的前寒武纪构造热事件年龄及分布特征基本上可以对比。因此,我们认为南迦巴瓦岩群及高喜马拉雅结晶杂岩的原岩是由新太古代至新元古代形成的多期岩浆岩组成,并作为印度陆块的一部分经历了 Columbia、Rodinia 和 Gondwana 超大陆的形成与裂解过程,以及喜马拉雅期的区域变质与岩浆作用再造。

The Namche Barwa complex located in the eastern syntaxis of the Himalayan orogenic belt and south of the India-Yarlung Tsangpo suture zone has been subjected to the high-pressure regional metamorphism and intensely migmatization. The felsic gneisses distributing widely in this area were chosen to study its lithology and isotopic geochronology. Most of the gneisses have the metamorphic mineral assemblages of amphibolite facies except minor pelitic gneisses with a high-pressure granulite facies mineral assemblage of garnet ＋ kyanite ＋ ternary feldspar ＋ quartz. Most of their protoliths are diorite and granodiorite characterized by the chemical composition of igneous rocks formed in a magmatic arc environment. The zircons in gneisses commonly have a core-rim texture. SHRIMP and LA-ICP-MS U-Pb zircon in-situ analyses indicate that the zircon rims give multi-stage metamorphic and magmatic event ages ranging from 500Ma to 10Ma, while the zircon cores yield at least four populations of Precambrian ages： -2500Ma, - 1800Ma, - 1600Ma and - 1000Ma. The zircon grains analyzed show obvious magmatic crystallization zoning and have relatively high Th/U ratios, typical of magmatic origin, and thus these four populations of ages are interpreted as ages of muhiphase Precambrian tectonicmagmatic events. Moreover, these ages are comparable to those of Precambrian tectonic-magmatic events of the Greater Himalayan Crystalline complex and the Indian craton. Therefore, we conclude that the protoliths of the Namche Barwa complex and the Greater Himalayan Crystalline rock may have been derived from the intrusive rocks formed from the late Archean to the late Proterozoic, recorded the formation and breakup of the Columbia, Rodinia and Gondwana supercontinent, and undergone the regional metamorphism and magmatism during the Himalayan orogeny.