冈底斯岩浆弧东端石榴斜长角闪岩的晚白垩世岩浆与变质作用记录及构造意义

Late Cretaceous magmatism and metamorphism records and its tectonic significance of garnet amphibolite from the eastern end of the Gangdese magmatic arc

冈底斯岩浆弧位于青藏高原拉萨地体南部,形成在新特提斯洋向北俯冲的安第斯型造山和印度与欧亚大陆碰撞的喜马拉雅型造山过程中,是研究青藏高原中-新生代构造演化的理想地区。本文对冈底斯岩浆弧东端石榴斜长角闪岩进行了岩石学、相平衡模拟以及锆石U-Pb年代学研究。石榴斜长角闪岩主要由石榴石、角闪石、斜长石、绿帘石、白云母和石英组成,含少量金红石和钛铁矿,具有基性岩浆岩的化学成分。相平衡模拟结果表明,石榴斜长角闪岩经历了高压麻粒岩相变质作用和部分熔融,峰期变质作用的温度和压力条件为～820℃和～1.67GPa,形成深度相当于55km。石榴斜长角闪岩中的锆石具有继承的岩浆核和变质边。锆石岩浆核具有条带状环带,相对高的HREE含量(平均值为1150×10^(-6))和高的Th/U比值(0.51～0.95),给出了90.4±1.7Ma(MSWD=0.32)的原岩结晶年龄。锆石的变质边不具环带,具有较低的HREE含量(平均值为58.5×10^(-6))和低的Th/U比值(0.29～0.40),给出了86.7±5.7Ma(MSWD=0.15)的变质年龄。本文研究结果表明,冈底斯岩浆弧东端晚白垩世辉长岩经历了近侵入同期的高压高温变质作用和部分熔融,大体积幔源岩浆的底垫和增生导致岩浆弧发生了显著的新生地壳生长和加厚。

The Gangdese magmatic arc,located at the south Tibet,was formed during the evolution of the Andean-type orogeny derived from the northward subduction of the Neo-Tethyan oceanic lithosphere and the Himalayan-type orogeny related to India-Asia collision,and therefore representing a natural experimental laboratory for studying the Mesozoic to Cenozoic tectonic evolution of the Tibetan Plateau.In this paper,we report the petrology,phase equilibria modeling and zircon U-Pb geochronology of the garnet amphibolite from the eastern Gangdese magmatic arc.The studied rock is composed of garnet,amphibole,plagioclase,epidote,muscovite and quartz,with minor rutile and ilmenite,showing chemical compositions of typical mafic rocks.Phase equilibria modeling indicates that the garnet amphibolite experienced high-pressure granulite facies metamorphism and significantly partial melting under peak metamorphic conditions of^820℃and^1.67GPa,corresponding to the formation depth of^55km.The zircons from the garnet amphibolite have inherited magmatic core and metamorphic rim.The magmatic core of zircon has a banded zoning,relatively high HREE content(with a mean value is 1150×10-6)and Th/U ratio(0.51~0.95),and yields a protolithic crystallization age of90.4±1.7Ma(MSWD=0.32).The zircon rim shows no zoning,has relatively low HREE content(with a mean value is 58.5×10-6)and low Th/U ratio(0.29~0.40),and yields a metamorphic age of 86.7±5.7Ma(MSWD=0.15).The present study shows that the Late Cretaceous gabbros in the eastern Gangdese arc experienced nearly syn-intrusive high-grade metamorphism and associated partial melting,and that the underplating and accretion of voluminous mantle-derived magma resulted in significantly growth and thickening of juvenile crust.