Metamorphism Of Migmatite And Amphibolite Lens From The Namakkal Block Of The Southern Granulite Terrain,India

Southern Granulite Terrain holds a central position in the Gondwana assembly and hence key to understand the process lead to the supercontinent formation.In this study,we present geochronology and phase equilibrium modelling in the Na₂O–Ca O–K₂O–Fe O–Mg O–Al₂O₃–Si O₂–H₂O–Ti O₂–O₂ system of melting restite of migmatite and amphibolite lens from the Namakkal area of Palghat-Cauvery Shear System(PCSS),Southern Granulite Terrain,in order to constrain their metamorphic P–T path and evaluate tectonic setting and geodynamic mechanism during late Neoproterozoic high-temperature(HT)metamorphism.Zircon U–Pb dating using laser ablation–inductively coupled plasma–mass spectrometry(LA-ICP-MS)yielded weighted mean²⁰⁶Pb/²³⁸U ages of 562.0±4.7 and 559.7±5.6 Ma for metamorphic zircon obtained from the melting restite and amphibolite lens respectively.Also,zircon U–Pb dating yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 560.2±3.6 for the metamorphic zircon within leucosome,zircon U–Pb dating for igneous zircon obtained from leucosome yielded a weighted mean ²⁰⁶Pb/²³⁸U ages of 559.5±9.4 Ma,representing their crystallization age.Hence the migmatization time is constrained to be ca.560 Ma.The pegmatite vein that crosscut the foliation of the migmatites has a crystallization age of ca.534.5±2.6 Ma,and igneous zircons within a nearby pegmatite intrusion define a weighted mean age of 556±3.8 Ma.The P-T conditions of garnet-bearing biotite–plagioclase gneiss(i.e.,melting restite of migmatite)has a peak mineral assemblage of garnet,biotite,plagioclase,K-feldspar,magnetite,quartz and melt,is found to be equilibrated at 800–870?C and 11.4–12.3 kbar(P_max),and 820–863?C and 8.5–10.3 kbar(T_max)as constrained by isopleths of X_An and X_Grs in the P-T pseudosection,suggesting granulite-facies metamorphism.The P-T pseudosection also constrains condition of metamorphism in the amphibolite lens to be 690–730?C at 5.3–6.8 kbar,which corresponds to re-equilibration during the retrograde metamorphism.The studied samples document a clockwise P–T path characterized by nearly isothermal decompression then a decrease in temperature and pressure,which may record the collision between the Northern Granulite Block(NGB)and the Madurai Block(MB)prior to ca.560 Ma,which induced partial melting.A later ultrahigh temperature(UHT)event at ca.530 Ma may have been related to post collisional extension.