Significance of post-peak metamorphic reaction microstructures in the ultrahigh temperature Eastern Ghats Province, India

Abstract

Ultrahigh temperature (UHT) granulites in the Eastern Ghats Province (EGP) have a complex P–T–t history. We review the P–T histories of UHT metamorphism in the EGP and use that as a framework for investigating the P–T–t history of Mg–Al-rich granulites from Anakapalle, with the express purpose of trying to reconcile the down-pressure-dominated P–T path with other UHT localities in the EGP. Mafic granulite that is host to Mg–Al-rich metasedimentary granulites at Anakapalle has a protolith age of c. 1,580 Ma. Mg–Al-rich metasedimentary granulites within the mafic granulite at Anakapalle were metamorphosed at UHT conditions during tectonism at 960–875 Ma, meaning that the UHT metamorphism was not the result of contact metamorphism from emplacement of the host mafic rock. Reworking occurred during the Pan-African (c. 600–500 Ma) event, and is interpreted to have produced hydrous assemblages that overprint the post-peak high-T retrograde assemblages. In contrast to rocks elsewhere in the EGP that developed post-peak cordierite, the metasedimentary granulites at Anakapalle developed post-peak, generation ‘2’ reaction products that are cordierite-absent and nominally anhydrous. Therefore, rocks at Anakapalle offer the unique opportunity to quantify the pressure drop that occurred during so-called M2 that affected the EGP. We argue that M2 is either a continuation of M1 and that the overall P–T path shape is a complex counter-clockwise loop, or that M1 is an up-temperature counter-clockwise deviation superimposed on the M2 path. Therefore, rather than the rocks at Anakapalle having a metamorphic history that is apparently anomalous from the rest of the EGP, we interpret that other previously studied localities in the EGP record a different part of the same P–T path history as Anakapalle, but do not preserve a significant record of pressure decrease. This is due either to the inability of refractory rocks to extensively react to produce a rich mineralogical record of pressure decrease, or because the earlier high-P part of the rocks history was erased by the M1 loop. Irrespective of the specific scenario, models for the tectonic evolution of the EGP must take the substantial pressure decrease during M2 into account, as it is probable the P–T record at Anakapalle is a reflection of tectonics affecting the entire province.