The geochemistry of surface soils from the Garhwal region, NW Lesser Himalaya: an evidence for neotectonic activity in the area

摘要

The soil samples collected from the Garhwal region, Uttaranchal, NW Lesser Himalaya are characterised by fine to medium grained textures with brown and black as major colours. The soil samples have variable silica contents ranging from 55-72 wt% with moderate SiO_2/Al_2O_3, K_2O/Na_2O ratios but relatively high Fe_2O_3 + MgO (5-6 wt%) and moderate CaO contents. Relative to the post-Archean North American Shale Composite (NASC) and Post Archean Average Shales from Australia (PAAS), the Garhwal samples have lower large ion lithophile elements (LILE) and high field strength elements (HFSE) abundances and exhibit negative anomalies in a NASC-normalised spider diagram. The wide spectrum of SiO_2 contents, large variation in ratios like Cr/Zr (0.3-3.5), Sc/Th (0.6-2.43), Th/U(> 2), Th/Co and Zr/Y indicate that the weathering of mostly felsic rocks with minor inputs of mafic rocks might have played an important role in producing these soils. In the discrimination functions (F1-F2) diagram, the Garhwal samples exhibit scattered pattern and plot in the fields of Quartzose sedimentary provenance, intermediate igneous provenance and mafic igneous provenance thus substantiating the above inference. The relationship among alkali and alkaline earth elements, the chemical index of alteration (CIA) suggests that the source rocks have undergone moderate chemical weathering. The weathering conditions of any source region can also be depicted from Al_2O_3(A)-CaO + Na_2O(CN)-K_2O(K) diagram. The soil samples plot near idealized average shale composition and between shale and granodiorite in A-CN-K diagram pointing towards moderate weathering history of the source region. Despite the fact that the study area is more favourable for physical, chemical and biological weathering, striking similarities were observed between the composition of soils and source/bed rocks without much loss of elements. This inference has led to the conclusion that the area is undergoing periodic neotectonic activity.