Charnockite Bedrock, Chennai Coast, Tamil Nadu: Micromorphology and Geochemical Signatures in Stages of the Weathering Processes

Abstract: Mineral weathering of bedrock in the context of landscape evolution provides useful insights in understanding the physico-chemical and climate processes. In this paper, we report mineral and petrographical features supported by geochemical results on the weathering of charnockite rocks exposed along the east coast, Chennai. The exposed rocks consist of medium to fine grained charnockite and are intensely weathered, fractured with weathering rind. Micromorphology of the unweathered charnockite core rock, weathering rind and soil show that the weathering processes are characterised by progressive physical and chemical attack on the labile minerals with the formation of neoformed clay minerals substituting the original rock fabric. The alteration of minerals has produced Fe oxides and clay minerals such as halloysite and kaolinite. Ferruginous oxides etch and impregnate into the plagioclase and othropyroxene microcracks and fractures during the advanced weathering stage. Microfabric alterations in microfractures and its variations occur in the weathering rind thereby, affect the chanrnockite weathering landscape. Geochemical data and chemical weathering indices (CIA, CIW, PIA and CWI) indicate insitu weathering due to stronger physical weathering compared to chemical alteration. UCC and chondrite normalized trace and REE data of the core rock, weathering and soil show similar spider spectra again pointing to insitu weathering with minor recycling. Negative Eu anomaly is due to the alteration of the alkali feldspars. Weathering of the core rock to soil grades towards the increase of quartz grains and Fe oxides in the weathering rind and soils. Weathering rind exhibits formation of clays such as kaolinite, halloysite and gibbsite formed under the humid, alternate dry and wet conditions.