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

In the upper reaches of the Cauvery catchment area in southern India, the A rchean Peninsular Gneisses exhibit a greater degree of weathering in the fi eld relative to the massive charnockites and granites. Although the gneisse s have been physically broken down along foliation planes, fractures and co mpositional discontinuities, secondary mineral (such as chlorite, smectite, kaolinite and Fe-oxyhydroxide) formation is insignificant. Consequently, t here is little chemical change even up to the stage of saprolite developmen t. 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 relat ive to TiO2 in areas which have developed profiles by in situ weathering. T he chondrite normalized REE patterns, however, show little change excepting a slight flattening of the patterns because of greater HREE mobility durin g weathering. Clay minerals also have flattened REE patterns because of rel ative HREE enrichment. The depletion and the enrichment of different elemen ts appear to be related to mild leaching of primary mafic minerals by meteo ric water rather than by mineral breakdown. This and the extent of variatio n in Chemical Index of Alteration values suggest that that the gneisses hav e suffered only incipient chemical weathering for the extent of physical we athering undergone by them. It is likely that this contrasting weathering e xtents of these > 2500 Ma rocks is due to their exposure to surface geologi cal processes only recently because of ongoing periodic uplift and physical erosion of this region. (C) 2000 Elsevier Science B.V. All rights reserved .