Geochemical Characterisation of the Neoproterozoic Nagthat Siliciclastics, NW Kumaun Lesser Himalaya: Implications for Source Rock Assessment

The shallow marine Nagthat siliciclastics of the Jaunsar Group show extensive development of medium to thickly bedded arenites and wackes, and represent the terminal Proterozoic siliciclastic episodes of the Lesser Himalaya. Chemical variation between arenites and wackes of Nagthat siliciclastics is related to the elemental distribution within the different mineralogical phases. Inverse correlation of most of the major oxides and trace elements (Ni, Cr, Co and Nb) with SiO₂, indicate quartz dilution effect. The majority of arenites show the common presence of plagioclase, K-feldspar and clay minerals, but lack ferromagnesium minerals and therefore indicate minimal decomposition of felspar and greater decomposition of ferromagnesian minerals. This phenomenon is well documented by the positive correlation of Fe₂O₂^(t) Ni, CO, K₂O, Rb, Sr, Th, Nb and Y with Al₂O₃ hereby suggesting that elemental distributions are mainly controlled by clay minerals. A moderate correlation exists between Al₂O₃ and La and Yb suggesting that REEs are similarly controlled in part by clays or micas. The plots of Nagthat siliciclastics in A-CN-K diagram, along with their enriched K₂O contents suggest variable degree of post-depositional K-metasomatism. Presence of higher amounts of monocrystalline quartz than the polycrystalline quartz, characteristic major and trace element contents, and enriched LREE pattern with negative Eu anomaly indicate a source of granite or granitic gneiss. High Eu/Eu ratio, low Ni concentration and low La/Co, Th/Co and Cr/Th ratios corroborate the acidic source. Granite or granitic gneiss source signature, along with indicative palaeocurrent directions, suggest that the detritus for Nagthat siliciclastics may have been derived from the southerly direction where Banded Gneissic Complex (BGC) of Aravalli-Bundelkhand craton is widely developed.