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

摘要

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-Pb-total/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 similar to 6 kbar and similar to 620 degrees 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 similar to 4.6 kbar and similar to 600 degrees 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 similar to 100 Ma younger and witnessed a higher grade of metamorphism than the greenstone belts in the Abitibi province and the Yilgarn Craton.