Geochemistry of sericite deposits at the base of the Paleoproterozoic Aravalli Supergroup, Rajasthan, India: Evidence for metamorphosed and metasomatised Precambrian Paleosol

摘要

Fine grained sericite deposits occur at the interface between Archean Mewar Gneiss Complex and the Proterozoic Aravalli Supergroup independent of shearing. They show a gradational contact with the basement granites and gneisses and a sharp contact with the overlying quartz pebble conglomeratic quartzites. Rip-up clasts of these sericite schists are found in the overlying conglomerates. The sericite schists are rich in sericite towards the top and contain chlorite towards the base. The sericite in these schists was formed by metasomatic alteration of kyanite and not from the feldspars of the basement granitoids and gneisses. Uni-directional variations of SiO2 and Al2O3, high Al2O3 content (30%), positive correlation between Al2O3 and TiO2 , Ti/Al and Ti/Zr ratios, high pre-metasomatic chemical indices of alteration (90), and enrichment of heavy rare earth elements relative to the parent granites and gneisses a€” all these chemical characteristics combined with field evidence suggest that the sericite schists are formed from a paleosol protolith, which developed on Archean basement between 2.5 and a??2.1 Ga in the Precambrian of Rajasthan. The superimposed metasomatic alteration restricts the use of Fe2+/Ti and Fe3+/Ti ratios of these paleosols for interpretation of PO2 conditions in the atmosphere.