Primary to re-equilibrated fluids and geochemical signatures for the evolution of the Nagthat siliciclastics in the Tons valley, Lesser Himalaya, India

Abstract

Medium to coarse-grained Neo-Proterozoic Nagthat siliciclastic rocks form a part of the Krol Formation in the Lesser Himalayan geotectonic zone. Fluid inclusion and geochemical studies have been carried out on the Nagthat siliciclastics from the Tons valley to determine their provenance during the Proterozoic and their recrystallisation during maximum burial to uplift. Fluid inclusion studies have been carried out on detrital, recrystallised quartz grains and quartz overgrowths. Major and trace element analyses of the siliciclastics, the relationships of SiO 2 with various trace elements, and the association of various trace elements with mineral species suggest a granitic source for these siliciclastics. Primary Q1 aqueous brine inclusions and Q3 H 2O–CO 2 fluid with 0.9 gm/cm 3 CO 2 density in detrital quartz grains characterised the protolith of the sandstone as granite or metamorphic rocks. H 2O–NaCl fluids participated in the cementation history, temperatures of quartz overgrowth from 198 to 232 °C show the effect of maximum burial. The re-equilibration of the primary fluid due to elevated internal pressure > confining pressure is evident from features like ‘C’ shaped cavities, stretching of the inclusions, their migration, decrepitation clusters, etc. During recrystallisation these inclusions were equilibrated at 187 ° and 235 °C in a restricted fluid of aqueous, moderately saline composition. The observed inclusion morphology is attributed to a decrease in external pressure related to isothermal decompression uplift.