Mesoarchaean clockwise metamorphic P-T path from the Western Dharwar Craton

Abstract

In this study, we utilize high-alumina pelitic schist samples from the Holenarsipur Greenstone Belt, a component of the ca. 3.30–3.15 Ga Sargur Supergroup in the Western Dharwar Craton, South India to establish a Mesoarchaean metamorphic history in the terrane. On the basis of integrated textural, mineral compositional, metamorphic reaction history and P-T pseudosection modelling studies, garnet-monazite trace element thermometry and coupled monazite electron probe micro-analyzer and sensitive high-resolution ion microprobe U-Pb dating results, we constrain for the first time a Mesoarchaean metamorphism along a hairpin clockwise metamorphic P-T path in the Dharwar Craton. The path reveals a two-stage prograde segment with an initial phase of heating accompanying burial (dP/dT gradient ~ 30 bar/°C) and a later phase of steep burial (∆P ~ 1.1 kbar, dP/dT gradient ~110 bar/°C), peak metamorphism at 7.5 kbar, 640 °C and a post-peak, steep decompression stage (P~6.3 kbar at 635 °C). Chemically zoned monazite grains shielded within garnet or occurring in low-strain matrix domains were formed in two stages during the Mesoarchaean metamorphism: (a) Monazite cores with relatively lower Y concentrations, higher Th/U ratios, a strong negative Eu anomaly; their REE partitioning relationships with the garnet inner rim reveal their crystallization was synchronous with the growth of garnet. (b) Monazite rims with higher Y and uraniferous compositions and lower Th/U ratios which, combined with Y and HREE enrichments in the outer rim of garnet, indicate their formation during the post-peak decompression stage associated with coupled decompositions of garnet and zircon. We correlate U-Pb monazite core and rim ages at 3144 ± 7 Ma and 3105 ± 10 Ma respectively with the timings of peak and retrograde metamorphism. Our findings, when evaluated with the available geological, isotopic, geochronological data of Meso- to Palaeoarchaean extant and extinct felsic crust as well as thermo-mechanical models, predict the ca.3.14–3.11 Ga metamorphism in the Holenarsipur Greenstone Belt as part of a widespread tectono-thermal event in the Western Dharwar Craton. The event marks late stages of sagduction tectonics in the craton as it evolves from a predominantly vertical to a horizontal shortening mode.