Paleosol at the Archean-Proterozoic contact in NW India revisited: Evidence for oxidizing conditions during paleo-weathering?

A number of fine-grained sericite bearing pelitic, schistose lithologies occur along the Archean ( Banded Gneiss Complex) - Proterozoic ( Aravalli Supergroup) contact ( APC) in the Udaipur valley in NW Indian craton. These Al-rich lithologies ( subsequently metamorphosed) have been described as 'paleosols', developed over a 3.3Ga old Archean gneissic basement and are overlain by Paleoproterozoic Aravalli quartzite. The paleosol was developed between 2.5 and 2.1, coincident with the globally recognized Great Oxidation Event ( GOE). In previous studies these paleosol sections were interpreted to have developed under reducing environment, however, the finding of a 'ferricrete' zone in the upper part of Tulsi Namla section ( east of Udaipur) during the present study ( in addition to earlier reported lithologies) has led to an alternative suggestion of oxygen- rich conditions during paleosol development. The Tulsi Namla paleosol section shows all the features characteristic of a complete paleosol section described from other Archean cratons. The paleosol includes sericite schist with kyanite as the prevalent Al-silicate in the lower part of pro. le while chloritoid and Fe-oxides typify the Fe- rich upper part. Alumina has remained immobile during the weathering process while Fe and Mn show a decrease in the lower part of the section and an abrupt rise in the upper part, in the ferricrete zone. The field and geochemical data indicate that the Tulsi Namla section is an in situ weathering pro. le and at least the upper part shows evidence of oxidizing conditions.