GEOCHEMICAL CHARACTERISTICS OF THE METASEDIMENTARY ROCKS OF THE DEOBAN AND MANDHALI FORMATIONS, LESSER HIMALAYAS

Abstract

Geochemical data of major oxides and trace elements for Mandhali Formation and Deoban Formation of inner Lesser Himalayas, was collected from several sources and studied for provenance, Depositional setting, paleoweathering conditions and tectonic setting by using several discrimination plots. The Mandhali shales have high SiO2, Al2O3, and K2O content while Deoban dolostones are high in CaO and MgO content. The Deoban dolostones are relatively poor in trace element content as compared to Mandhali shales. Positive correlation of K2O, Na2O and TiO2 with Al2O3 suggest that these elements are primarily controlled by dominant clay minerals. Tectonic discrimination diagrams for Mandhali shales revealed that these sediments were derived from passive margin setting. The binary plots of Ni vs Cr, Cr vs Zr and ternary diagram of V-Ni-Th*10 indicate that these shales were derived from felsic igneous source rocks. The Al2O3 vs SiO2 and MgO vs Na2O binary plots indicate that these shales are hydrogenous and were deposited under shallow marine conditions. Weathering indices values (CIA, CIW, PIA), CIA vs ICV binary plot and Al2O3-K2O Fe2O3 ternary plot suggest that granitic source rock underwent high degree of weathering and produced these compositionally mature sediments. Tectonic discrimination diagram for Mandhali shales depict passive continental margin as a tectonic setting of the source area. For Deoban Formation, CaO vs MgO bivariate plot indicate that dolomite is the host mineral for MgO present in the rocks, therefore lithology of this Formation is dolostone. These rocks are of sedimentary origin as indicated by MgO-CaO-Al2O3 plot. High V/(V + Ni) ratio suggest euxinic depositional conditions. Presence of redox sensitive trace elements like V and Co indicate reducing conditions. Low Na2O content indicate shallow marine depositional setting with sea water being the primary source of MgO for dolomitization.