Metamorphism of pelites in the eastern Gangdese magmatic arc and its tectonic implications

Abstract

冈底斯岩浆弧东端林芝地区出露的中-高级变质岩来自岩浆弧的中、下地壳，是研究岩浆弧深部组成与形成演化的窗口。本文对林芝布久地区西部产出的泥质片岩进行了岩石学和锆石U-Pb年代学研究。研究表明，含夕线石石榴石云母片岩的峰期矿物组合为石榴石+黑云母+斜长石+白云母+夕线石+石英+金红石，经历了角闪岩相变质作用和部分熔融，峰期变质温度和压力条件为~7.4kbar和~715℃。片岩的进变质和部分熔融作用很可能开始于~70Ma，退变质和熔体结晶作用发生在61~48Ma。本文和现有研究成果表明，冈底斯岩浆弧东端变质岩的变质条件存在明显的空间变化，角闪岩相和麻粒岩相变质带变质岩分别代表岩浆弧的中地壳和下地壳组成。同时，岩浆弧的下地壳主要由变质的基性和长英质岩浆岩组成，含少量变质沉积岩，而中地壳主要由变质的花岗质岩浆岩和变质沉积岩组成。我们认为在晚中生代-早新生代，印度大陆与亚洲大陆的碰撞和俯冲的新特提斯洋岩石圈的断离，引起了岩浆弧地壳构造缩短加厚和幔源岩浆增生，进而导致上地壳的沉积岩被埋藏到中、下地壳，并经历长期持续的高温变质和部分熔融作用。本研究不仅揭示出冈底斯岩浆弧经历了晚中生代-早新生代的变质作用，也为岩浆弧的地壳组成与空间变化提供了重要信息。;The Nyingchi area, eastern Gangdese magmatic arc, exposed middle-to high-grade metamorphic rocks, representing the middle and lower crustal components of the arc, and therefore, it provides a window into investigating the deep components, formation, and evolution of the arc. In this paper, we conduct a study of petrology and zircon U-Pb geochronology of pelitic schist from the west of the Bujiu, the Nyingchi area. The results show that the sillimanite-bearing garnet mica schist has the peak assemblage of garnet+biotite+plagioclase+muscovite+sillimanite+quartz+rutile. Phase equilibrium modeling and geothermobarometry reveal that the schists experienced amphibolite-facies metamorphism and partial melting under peak metamorphic conditions of ~7.4kbar and ~715℃. The prograde process and associated partial melting of these schists probably initiated at ~70Ma, and the retrograde process and melt crystallization occurred at 61~48Ma. Combined available results, we conclude that the metamorphic conditions of metamorphic rocks from the eastern Gangdese arc have obvious spatial variation, and amphibolite-facies and granulite-facies metamorphic belt correspond to the component of arc middle crust and lower crust, respectively. Moreover, the Gangdese arc lower crust is composed mostly of meta-mafic and felsic igneous rocks, including minor meta-sedimentary rocks, and meta-granitic igneous rocks and meta-sedimentary rocks are the main components of the middle crust. We suggest that the continental collision of Indo and Asia and the roll-back and breakoff of the subducted Neo-Tethyan slab during Late Mesozoic-Early Cenozoic, resulted in the shortening and thickening of the Gangdese arc crust and the accretion of voluminous mantle-derived magma, which in turn induced the sedimentary rocks from upper crust transported into the middle and lower crust and experienced long-lasting high-temperature metamorphism and partial melting. The present study does not only reveal that the eastern Gangdese arc experienced Late Mesozoic-Early Cenozoic metamorphism, but also provides a key constraint on the component and spatial variation of the arc crust.