Chemical weathering of metamorphic rocks from low elevations in the southern Himalaya

Early chemical weathering involving the acid hydrolysis of banded gneisses under warm temperate and subtropical conditions in the Likhu catchment of the Middle Hills of Nepal has resulted in the formation, in situ, of residual soils (2–3-m thickness) grading downwards into saprolites that extend to thicknesses exceeding 6 m. Chemical analyses of bulk samples from three detailed profiles at elevations between 800 and 1000 m above sea level, reveals a uniformity of chemical change throughout the weathering sequence within each profile in terms of loss of Ca and Na. However, there is also a progressive change between profiles with the more strongly weathered profiles showing almost total loss of Ca and Na. The uniformity of change to depth within profiles is facilitated by the free-draining sandy and silty textures of the weathering residues, together with the frequent flushing by monsoon rains. Substantially greater retention of K and Mg has occurred in all profiles; only subtle decreases in both MgO and K2O are found to occur from the base of the residual soils upwards to the top of the soil. These changes in bulk chemistry are consistent with the clay mineralogy and the stream water chemistry analyses. It is suggested that the K and Mg are preferentially retained in some of the clay minerals (illite, chlorite, palygorskite and hydrobiotite), whereas the Na and Ca are readily leached to the stream waters. The general pattern of chemical erosion is consistent with that observed in other warm and humid, tectonically active mountain terrains, although the widespread formation of kaolinite is less commonly reported in such environments.