ABSTRACT
Chemical weathering occurs when the earth’s materials are exposed to variable climatic conditions associated with the hydrosphere, atmosphere and biosphere. Under the influence of such processes, the primary constituents of the rock are mobilized and reorganized to establish equilibrium with newly imposed physico-chemical conditions. Though there are many studies available on elemental mobility and weathering, these processes in an orogenic belt are not well understood. An attempt has been made to study the weathered crust profile of the volcano-sedimentary sequence of Bhowali-Bhimtal and Berinag regions in the Kumaun Lesser Himalaya, which is one of the well-known young orogenic belts of the world. Geologically, these are dominantly constituted by alternating strata of volcanic and sedimentary units with variable thickness at different locations. Samples have been collected from selected weathered crust profiles of the region, which include weathered as well as fresh forms of the rock. Field and laboratory measurements of magnetic susceptibility data are heterogeneous in nature, which suggests a differential rate of mobility of ferromagnesium minerals. Mineralogical studies reveal various evidence of alterations, where plagioclase has been mostly found as albitic, and the original clinopyroxene composition has been mostly converted into amphiboles and chlorites. Empirical estimation based on eigenvectors (XPT, YPT, Xm, Ym) derived from major elements of clinopyroxenes classifies them as diopside augite and their evolution in quartz-tholeiitic (0.5 < Ti/Mg < 0.8) basaltic magma in a temperature range of 1050–1100°C at pressure <2 kbar. The values of chemical index of alteration (CIA) derived from major elements of weathered crust profiles range from 48 to 80, which distinctly show a compositional shift towards Al2O3 apex in an A-CN-K diagram. On a molar A-CNK-FM triangular plot, the samples show enrichment of chlorite smectite composition. Trace element chemistry suggests that large-ion lithophile elements(LILE) are somewhat affected, while high-field strength elements (HFSE), Ba and Nd remain intact during secondary processes. The comparative study of fresh and weathered counterparts of samples suggests that rare earth elements (REE) were not much affected during secondary processes, although the range of variation in altered samples is higher. Paleoclimate estimation based on lithological and lithochemical parameters suggests a humid climate with enhanced greenhouse effect.
