Petrology and zircon U-Pb dating of the Neoarchean scapolite-garnet calc-silicate from the Namakkal Block of the Southern Granulite Terrain, India, and their geological implications

Abstract

本文对印度南部麻粒岩地体Namakkal陆块Tammampatti地区方柱石石榴子石钙硅酸盐岩进行了详细的岩石学、锆石U-Pb年代学和变质相平衡模拟研究，以研究其岩石成因和地质意义。岩相学观察识别出两阶段变质矿物组合：第一阶段为石榴子石+方柱石+斜长石+榍石+钛铁矿；第二阶段为石榴子石边部的绿帘石和方柱石边缘的方解石、斜长石和石英冠状体。CL图像分析显示锆石可分为两种，分别为高亮度和低亮度的变质锆石。LA-ICP-MS锆石U-Pb定年得到高亮度变质锆石²⁰⁷Pb/²⁰⁶Pb加权平均年龄为2562±17Ma，而低亮度变质锆石的²⁰⁷Pb/²⁰⁶Pb加权平均年龄稍年轻，为2495±15Ma。基于相平衡模拟计算了2个样品18ID-24和18ID-25的P-T视剖面图，确定它们峰期变质PT条件分别为4.3~7.1kbar、800~960℃和4.0~7.8kbar、750~854℃。高亮度变质锆石年龄2562±17Ma与Namakkal陆块紫苏花岗岩的原岩结晶年龄相当，其代表了紫苏花岗岩的原岩侵入导致的接触交代变质作用形成方柱石石榴子石钙硅酸盐岩的时代；低亮度变质锆石年龄2495±15Ma与该地区大约2530~2440Ma的高温-超高温变质作用时代相吻合，因此认为其代表区域性变质作用叠加的时代。根据全岩成分以及矿物组合，我们推测该岩石为中酸性岩浆岩（紫苏花岗岩原岩）与碳酸盐岩发生交代变质作用的产物。;Granulite terranes are considered as windows open to the lower crustal processes of the Earth. In this paper, we present detailed petrology, zircon U-Pb dating and phase equilibrium modeling of scapolite-garnet calc-silicate samples from the Tammampatti area of the Namakkal Block, Southern Granulite Terrain, India. On the basis of petrographic observation, two stages of metamorphic mineral assemblages were determined, which are garnet+scapolite+plagioclase+sphene+ilmenite (M1) and epidote corona+the symplectite of calcite, plagioclase and quartz. Cathodoluminescence (CL) imaging reveals that there are two types of metamorphic zircons: zircons with high luminescence and another is characterized by low luminescence. Zircon U-Pb dating using LA-ICP-MS yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2562±17Ma for the zircons with high luminescence, whereas those with low luminescence have a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2495±15Ma. The peak metamorphic conditions of two samples 18ID-24 and 18ID-25 are constrained to be 800~960℃/4.3~7.1kbar and 750~854℃/4.0~7.8kbar, respectively. The metamorphic age of 2562±17Ma is the similar as the crystallization age of the charnockite pluton within the Namakkal Block, which is interpreted to represent the timing of contact metasomatism caused by magma emplacement of the charnockite. The metamorphic age of 2495±15Ma is considered to be the overprinted time by the previously reported ca. 2530~2440Ma regional HT-UHT metamorphism. The calc-silicate should be resulted from metasomatism of intermediate-acid pluton (protolith of charnockite) and carbonatite.