冈底斯岩浆弧东段沉积岩的晚白垩世变质作用及其构造意义

Late Cretaceous metamorphism of sedimentary rocks in the eastern Gangdese magmatic arc and its tectonic significance

位于青藏高原南部的冈底斯岩浆弧形成于中生代新特提斯洋俯冲过程中,并在印度与欧亚大陆碰撞过程中叠加了新生代岩浆作用和变质作用。冈底斯岩浆弧东段出露的中、新生代变质岩是研究其深部组成与形成演化的理想窗口。本文对冈底斯东段米林田兴村地区的变沉积岩,即石榴夕线黑云片岩、含石榴钙硅酸盐岩、黑云斜长片麻岩和大理岩进行了岩石学和锆石U-Pb年代学研究。研究结果表明,石榴夕线黑云片岩由夕线石、黑云母、石榴石、斜长石、钾长石、石英和钛铁矿组成,经历了中压麻粒岩相变质作用,变质条件为810~820℃和6.4~7.8kbar。锆石年代学研究表明,石榴夕线黑云片岩、含石榴钙硅酸盐岩和大理岩经历了87~83Ma的变质作用。本文和现有研究表明,冈底斯弧东段林芝和米林地区的高压麻粒岩相变质岩分布区代表该岩浆弧的下地壳,而包括本文研究点在内的中压麻粒岩相到角闪岩相变质岩分布区为其中地壳组成部分。本文认为晚白垩世大体积幔源岩浆的注入和随后的新特提斯大洋岩石圈平俯冲,导致了冈底斯岩浆弧地壳的生长、加厚和底冲,使上地壳的沉积岩和岩浆岩运移到中-下地壳,并经历了高温变质与部分熔融,形成了分布在上地壳的花岗岩。这表明岩浆弧的新生地壳在晚白垩世新特提斯洋俯冲过程中发生了再造。以长英质岩石为主的表壳岩进入深地壳很可能是岩浆弧中-下地壳由基性转变成中性成分的重要方式。

The Gangdese magmatic arc is located in the southern part of the Lhasa terrane,southern Tibet,and formed during the Mesozoic subduction of the Neo-Tethyan oceanic lithosphere,and overprinted by the Cenozoic magmatism and metamorphism during the collision between Indian and Asian continents.Middle-to high-grade metamorphic rocks exposed in the eastern Gangdese magmatic arc represent the middle and lower crusts of the arc,which are ideal area for studying the deep component and evolution of the continental magma arc.In this paper,we conducted a petrological and geochronological study for the metasedimentary rocks from the Tianxing area,Milin County,the eastern Gangdese arc.The studied rocks include garnet-sillimanite-biotite schist,garnet-bearing calc-silicate rock,biotite-plagioclase gneiss and marble.The garnet-sillimanite-biotite schist consists of garnet,sillimanite,biotite,plagioclase,K-feldspar,quartz and ilmenite,and experienced medium-pressure granulite-facies metamorphism under P-T conditions of 6.4~7.8kbar and 810~820℃.Zircons U-Pb dating from garnet-sillimanite-biotite schist,calc-silicate rock and marble yielded metamorphic age of 87~83Ma.This study and the existing results show that the high-pressure granulite-facies metamorphic belt in the Linzhi-Milin area,the eastern Gangdese magmatic arc,represents the lower crustal component of the arc,while the medium pressure granulite-facies and amphibolite-facies metamorphic belt represents the middle-lower crust of the arc.We suggest that the accretion and loading of voluminous mantle-derived magma and subsequent flat subduction of Neo-Tethyan oceanic lithosphere during the Late Cretaceous resulted in the crustal growth,thickening and underthrusting of the Gangdese magmatic arc,which transported the intrusions and hosting sedimentary xenoliths in the upper crust into the middle and lower crust.The thickened middle and lower rocks underwent high temperature metamorphism and partial melting to generate the granites in the upper crust,indicating that the juvenile arc crust underwent reworking during the Late Cretaceous subduction.The downward transportation of the upper crustal felsic magmatic rocks and sedimentary rocks is probably an important mechanism in transforming the mafic lower arc crust to andesitic one.