Geophysical and geochemical alteration of rocks in granitic profiles during intense weathering in southern Purulia district, West Bengal, India

Abstract

Purulia district of West Bengal is geologically dominated mostly by Proterozic hard granite gneiss rocks including soft phyllite and mica schist of Singhbhum group as part of Chotanagpur plateau. Present study aims to understand the nature of geophysical and geochemical weathering processes for characterization of granitic crust as well as the geochemical ways of this alteration. During the field study 14 rocks and soils samples were collected from three sections of each weathering profile in three different blocks namely, Manbazar-I, Manbazar-II and Banduan. The samples were collected along the roads cuts, natural and other man-made exposures and restricted mostly within the exposed layers of the respective profiles. The samples were analyses in sieve for particle size distribution and in X-ray diffraction for mineralogical alteration. The thin section of selected samples were analysed under polarized optical microscope for understanding the nature of physical and chemical changes in parent rocks. From sieve analysis, the cumulative particle size distribution show that the size of weathered materials gradually reduces from saprolite to overlying soils in every profile. Mineralogical analysis by XRD shows that feldspar, muscovite, quartz and biotite are the primary minerals which are intensely weathered and have undergone some geochemical processes except quartz, to form some secondary clay minerals like montmorillonite, kaolinite and illite in the overlying soil. Optical microscopic analysis reveals that transformation of primary minerals to secondary clay minerals significantly reduced the rocks strength which leads the rocks disintegrate into smaller particles. Finally, the results show that there is an abundance of montmorillonite and altered primary minerals with gravel materials in the profiles are liable for further weathering to develop a mature soil.