Weathering of lower crustal rocks in the Kaveri river catchment, southern India: Implications to sediment geochemistry

In the upper catchment of the Kaveri river in the Sahyadri mountains of southern India, middle to lower crustal Archean granulite grade mafic and felsic rocks with similar structures and textures are exposed under the conditions of active tectonics, high rainfall and thick tropical vegetation. Occurrence of the two major rock types in close association under identical geological, geographical and biological conditions provides an uncommon situation for the study of weathering, elemental mobilization and sediment generation processes. Field observations, mineralogical and geochemical data including major, trace and rare earth elements (REE) of fresh rocks and variably weathered saprolite samples suggest that close association of mafic and felsic rocks accelerates the denudational processes by early weathering of mafic minerals in felsic rocks and mafic rocks in the terrain. Due to differential weathering of rocks, unweathered to less weathered felsic grains are likely transferred to the coarser fraction of fluvial sediments deposited on the floodplains of the river imposing an upper continental crust (UCC) geochemical signature. It is found that during chemical weathering, in addition to other factors, weatherability of host minerals of REE control the mobility of REE in the weathering profile. It is suggested from the observations on the weathering process and on the geochemistry of derivative sediments, that in a tectonically active system with a climate maximum, as in Sahyadris, an equilibrium could be dynamically maintained between weathering and erosional regimes. Also we infer from our study that there exist certain commonalities between surface denudational and mantle-magmatic geochemical differentiation processes. Similarity of these processes, therefore, may have implication to common UCC-like geochemistry of Post Archean sediments.