浙西南八都杂岩早中生代泥质麻粒岩变质作用及构造意义

Metamorphism and Its Tectonic Implications of Early Mesozoic Pelitic Granulites from Badu Complex of Southwestern Zhejiang Province,South China

遂昌-大柘泥质麻粒岩出露于华夏地块东北部的浙西南八都杂岩中,该岩石保留了典型的减压反应结构.但其变质演化特点、变质作用时代及构造意义目前尚不明确.通过系统的岩相学、矿物化学和同位素年代学分析,结果表明遂昌-大柘泥质麻粒岩记录了4个阶段的变质矿物组合,其中早期进变质阶段M_1的矿物组合为石榴石+黑云母+石英;压力峰期变质阶段M_2的矿物组合为石榴石+铝绿泥石+金红石+蓝晶石+刚玉+黑云母+石英±十字石,该矿物组合可能预示着岩石曾经历了超高压变质作用过程;峰期变质阶段M_3的矿物组合为石榴石+黑云母+夕线石+石英±钾长石±斜长石±钛铁矿;峰后近等温降压M_(4-1)阶段的矿物组合为石榴石+黑云母+夕线石+堇青石+石英+钛铁矿±尖晶石±斜长石±钾长石;M_(4-2)阶段的矿物组合为石榴石+堇青石+夕线石+斜长石+黑云母+石英±钾长石.相平衡模拟结合传统地质温压计限定其峰期变质阶段的温压条件为T=780~810℃、P=8.0~9.2kbar;峰期后近等温降压的M_(4-1)阶段的温压条件为T=780~860℃和P=5.7~6.0kbar,M_(4-2)阶段的温压条件为T=~700℃和P=~4.4kbar,具有典型的顺时针近等温减压型P-T轨迹特征.LA-ICP-MS U-Pb定年结果表明其麻粒岩相变质作用时代为233.5~238.9Ma.变质作用历史说明浙西南地体可能卷入了古特提斯洋域内印支-华南-华北板块之间的俯冲-碰撞过程,并经历了早中生代的麻粒岩相变质作用后快速折返至地表.

Suichang-Dazhe pelitic granulites,showing distinct decompressional textures,crop out in Badu complex of the southwestern Zhejiang Province,the Cathaysia block.However,its metamorphic evolution characteristics,metamorphism ages and tectonic implications remain unclear.Results of our study of its petrology,mineral chemistry and LA-ICP-MS zircon U-Pb ages indicate that the rocks contain four distinct metamorphic assemblages,namely the early prograde（M1）,pressure peak（M2）,peak（M3）and the post-peak（M4）stages.The early prograde（M1）assemblage consists of garnet（core）＋biotite＋quartz.The pressure peak（M2）assemblage consists of garnet（mantle）＋sudoite＋rutile＋kyanite＋corundum＋biotite＋quartz±staurolite,the mineral assemblage may indicate that the rock has undergone the process of ultrahigh pressure metamorphism.The peak（M3）assemblage consists of garnet（rim-mantle）＋biotite＋sillimanite＋quartz±K-feldspar±plagioclase±ilmenite.Post-peak near-isothermal decompressional（M4）stage following the peak stage could be subdivided into two stages M4-1 and M4-2.The M4-1 stage assemblage consists of garnet（rim）＋cordierite＋biotite＋sillimanite＋quartz＋ilmenite±spinel±plagioclase±K-feldspar,and the M4-2 stage is represented by garnet（rim）＋cordierite＋sillimanite＋plagioclase＋biotite＋quartz±K-feldspar.Quantitative phase equilibria modeling in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2,in combination with traditional thermobarometry,was applied to obtain P-T conditions of 780-810℃ and 8.0-9.2 kbar for M3,780-860℃ and 5.7-6.0 kbar for M4-1,about 700℃ and about 4.4 kbar for M4-2.The combination of the mineral assemblages,mineral compositions,and metamorphic reaction history in the Suichang-Dazhe pelitic granulite defines a clockwise P-T path that involves near-isothermal decompression that followed the peak granulite-facies metamorphism.Analyses of LA-ICPMS zircon U-Pb dating indicate that metamorphic zircons recorded metamorphic age of the Suichang-Dazhe pelitic granulite is233.5-238.9 Ma.The clockwise P-T path reveals that the southwestern Zhejiang Province terrane involved in the subduction or collision followed by exhumation and cooling events and experienced the Early Mesozoic（233.5-238.9 Ma）granulite-facies metamorphism with the amalgamation of the Indochina-South China-North China block in the paleo-Tethyan domain.