U–Th–PbTotal monazite geochronology of the felsic volcanic rocks from the Hutti greenstone belt: Evidence of magmatic and hydrothermal events

Abstract

Petrochronology of monazite in volcanic rocks of the Hutti deposit in the Eastern Dharwar Craton (India) has been studied to understand the magmatic and hydrothermal events that have affected the area. The volcanic rocks occurring in the Hutti greenstone belt are bimodal in nature and contain both felsic and metabasalt. The felsic volcanic rocks in the area preserve accessory minerals like monazite, allanite, and apatite, whereas the same is absent in metabasalts. Petrographic observation of felsic volcanic rock shows corona growth of allanite and apatite (in addition to huttonite) clustering around monazite grains. This corona texture, termed allanite cluster, occurs parallel to the shear planes (C‐plane) of the felsic volcanic rock and characterizes late‐stage hydrothermal activity in the area. Texturally, monazite in the felsic volcanic rock occurs included within arsenopyrite (<1 μm), and in association with apatite, allanite, and occasionally with huttonite/thorite (~10–40 μm), in which monazite grains preserve geochemical heterogeneity in both intra‐ and inter‐grains. The U–Th–PbTotal dating of monazite associated with the allanite cluster shows two age peaks (both from intra‐ and inter‐grains) at 2658 ± 17 Ma (older) and 2534 ± 38 Ma (younger), in which the older age represents the magmatic/crystallization age of the studied felsic volcanic rock overprinted by a later hydrothermal event in the study area. Based on monazite geochemistry‐geochronology, the present study deduces two texturally constrained events related to the primary crystallization of monazite (~2.65 Ga) and subsequent alteration of the same during a later hydrothermal event (ca. 2.53 Ga) that has affected the felsic volcanic rock in the Hutti gold deposit.