Weathering of gneissic rocks in the upper reaches of Cauvery river, south India: implications to neotectonics of the region

Abstract

In the upper reaches of the Cauvery catchment area in southern India, the Archean Peninsular Gneisses exhibit a greater degree of weathering in the field relative to the massive charnockites and granites. Although the gneisses have been physically broken down along foliation planes, fractures and compositional discontinuities, secondary mineral (such as chlorite, smectite, kaolinite and Fe-oxyhydroxide) formation is insignificant. Consequently, there is little chemical change even up to the stage of saprolite development. Only in the regolith there is some loss of Ca, Na, Sr, Ba, Mg and SiO₂. Other elements such as Fe, Mn, Al, Cr, Ni and REE all show enrichment relative to TiO₂ in areas which have developed profiles by in situ weathering. The chondrite normalized REE patterns, however, show little change excepting a slight flattening of the patterns because of greater HREE mobility during weathering. Clay minerals also have flattened REE patterns because of relative HREE enrichment. The depletion and the enrichment of different elements appear to be related to mild leaching of primary mafic minerals by meteoric water rather than by mineral breakdown. This and the extent of variation in Chemical Index of Alteration values suggest that that the gneisses have suffered only incipient chemical weathering for the extent of physical weathering undergone by them. It is likely that this contrasting weathering extents of these >2500 Ma rocks is due to their exposure to surface geological processes only recently because of ongoing periodic uplift and physical erosion of this region.