Neoarchean Greenstone Metamorphism in the Eastern Dharwar Craton, India: Constraints from Monazite U-Th-Pbtotal Ages and PT Pseudosection Calculations

Abstract

The Archean Hutti-Maski greenstone belt (HMGB) and the south Kolar greenstone belt (SKGB) are the two richest gold provinces in India. Here, we present results of texturally constrained U-Th-Pbtotal/CHIME dating of monazites and PT pseudosection analyses to understand the geochronological and metamorphic evolution of the two greenstone terrains. The ages of felsic volcanism are constrained at ca. 2669 ± 22 Ma in the HMGB and 2661 ± 32 Ma in the SKGB. The HMGB rocks underwent midamphibolite facies metamorphism at ca. 2564 ± 12 Ma. The inferred PT path indicates a contemporaneous PT increase of up to ∼6 kbar and ∼620°C followed by postpeak near-isothermal decompression for the HMGB rocks. The SKGB rocks underwent a lower amphibolite facies metamorphism at ca. 2546 ± 12 Ma and are characterized by a synchronous PT increase of up to ∼4.6 kbar and ∼600°C, followed by decompressional cooling. These results indicate a mixed arc/plume setting for the evolution of both the greenstone belts, as suggested by several earlier works. The HMGB rocks record a pervasive postpeak metamorphic K-rich fluid alteration event at ca. 2414 ± 18 Ma that is discernible by monazite growth proximal to K-feldspar veins and garnet breakdown domains. Similar, although less prevalent, monazite age record of ca. 2414 ± 26 Ma from the SKGB rocks is manifested by postmetamorphic shear-induced hydrothermal activity. Considering the worldwide scenario, the HMGB and SKGB are ∼100 Ma younger and witnessed a higher grade of metamorphism than the greenstone belts in the Abitibi province and the Yilgarn Craton.