Metamorphic evolution of the UHT granulites from Mandikota, Eastern Ghats Mobile Belt, India: Constraints from phase equilibria modelling, fluid inclusions and monazite U-Th-total Pb geochronology

Abstract

The Eastern Ghats Mobile Belt (EGMB) had been subjected to extreme thermal metamorphic history at various sections of its terrane. One such terrane (Mandikota) located near the western part of the EGMB has been speculated based on asserted reaction textures, conventional geothermobarometry, geochronology, fluid inclusions, and pseudosection modelling. The present integrated study aims to understand the intricacies of the diversified and controversial tectono-metamorphic evolution of these polycyclic granulite terranes. The stable occurrence of sapphirine + quartz in contact is rare in its own and is a diagnostic feature of Ultra-High Temperature (UHT) metamorphism. Conventional geo-thermobarometeric tools applied, suggests an ordered series of reactions for the different stages of metamorphism experienced by the investigated granulites. Also, the P-T estimated by conventional methods lies in sound correlation with the peak P-T condition (in excess of 950 °C at nearly 9 kbar) attained in the constrained pseudosection model. The exhumation trajectory of the examined UHT terrane reflects P-T path responsible for the thrusting mechanism with isothermal decompression of 3 kbar followed by late-stage isobaric cooling. This study also presents the primary report for the systematic fluid inclusion study of the UHT granulites and its correlation with the evolved P-T trajectory. In addition, monazite dating using Electron Micro Probe Analysis (EPMA) is procured to assess the time frame in understanding the P-T-t evolution of the high-grade terrane. The geochronological study further revealed a new age (∼600 Ma) associated with the Visakhapatnam Domain which adheres to the Pan-African orogeny and marks the reworking phase of the widely reported the Grenvillian event in the EGMB province. As such, the results from the studied fluid inclusions displaying monophase inclusions of CO2-rich carbonic fluid, and its corroboration with the P-T pseudosection model interpreting the phase equilibrium conditions prevailing during the Neoproterozoic time (∼900 Ma).