柴北缘早古生代中低压麻粒岩相变质作用--以乌兰地区泥质麻粒岩为例

The Early Paleozoic Middle-Low Pressure Granulite-Facies Metamorphism:The Evidence from Pelitic Granulite in Wulan Area

柴北缘乌兰地区出露有大量早古生代中低压泥质麻粒岩。据岩石学和矿物化学研究,将乌兰地区早古生代泥质麻粒岩变质作用划分为两期。其中峰期变质阶段矿物组合为Grt1+Bt1+Sil+Kfs+Pl+Qtz+melt,退变质阶段矿物组合为Bt2+Ms+Grt2+H2O±Pl±Qtz。通过对泥质麻粒岩样品进行锆石LA-ICP-MS U-Pb定年,获得泥质麻粒岩峰期变质时代为(482±3)Ma。为厘清乌兰地区早古生代泥质麻粒岩变质演化历史,揭示麻粒岩变质机制,通过相平衡模拟技术和黑云母Ti温度计,获得乌兰地区早古生代泥质麻粒岩峰期和退变质温压条件分别为:710℃~730℃/5~6.2 kbar和560℃~650℃/5~6.2 kbar。由于岩浆的冷却使其退变质阶段经历等压降温变质作用。

The Early Paleozoic middle-low pressure pelitic granulite was exposed in the Wulan area in the northern Qaidam.Petrological and geochronological studies reveal that the Early Paleozoic pelitic granulite in Wulan area underwent two stages metamorphism.The peak mineral assemblage of pelitic granulite is garnet+biotite+sillimanite+K-feldspar+plagioclase+quartz+melt.The retrograde mineral assemblage consists of biotite+muscovite+garnet+H2O±plagioclase±quartz.The LA-ICP-MS U-Pb dating of zircons indicate that the granulite-facies metamorphic age is 482±3 Ma.In order to reveal the metamorphic evolutionary history and metamorphic mechanism of the Early Paleozoic pelitic granulite in Wulan area,we conclude the pelitic granulite metamorphic P-T conditions by phase equilibria and the result of Tiin-biotite thermometer.And the P-T conditions of the peak granulite-facies metamorphism and retrograde metamorphism are 710℃-730℃/5-6.2 kbar and 560℃-650℃/5-6.2 kbar,respectively.Duo to the cooling of magma,the pelitic granulite underwent isobaric cooling metamorphism during retrograde metamorphism.