Geochemical characteristics of Mesoproterozoic clastic sedimentary rocks from the Chakrata Formation, Lesser Himalaya: implications for crustal evolution and weathering history in the Himalaya

Abstract

To evaluate the changes in the chemistry of upper continental crust during Precambrian time in the Himalaya, the oldest and best-preserved Mesoproterozoic clastic sedimentary rocks (pelites/shales and quartzites) from the Chakrata Formation, NW Lesser Himalaya, Uttar Pradesh have been analysed for major elements and a number of trace elements, including rare earth elements (REEs). When compared to post-Archean shales (for e.g. North American Shale Composite), the Chakrata pelites have high SiO 2, Al 2O 3, K 2O, Rb, Ba contents and low CaO, Fe 2O 3 t , TiO 2, Sr, Sc, Ni and Cr concentrations. The REE abundances are high (up to 296 ppm) in pelites and reveal enriched LREE and depleted HREE patterns (La N /Yb N =7.9–11.1). Strong negative Eu anomalies are evident in the Chakrata pelites (Eu/Eu ∗=0.46–0.58). The quartzites of the Chakrata Formation have high silica contents and low trace element abundances when compared to Phanerozoic sandstones. The chemical index of alteration values of these rocks ranges from 70 to 76, indicating intense weathering conditions in the source areas. Most pelites of the Chakrata Fm plot near the A–K line in the A–CN–K diagram which also suggests intense chemical weathering conditions. Major and trace element abundances and elemental ratios critical of provenance (e.g. high SiO 2/Al 2O 3, K 2O/Na 2O, La/Sc, Th/Sc, La/Yb and low Eu/Eu ∗ and Cr/Zr) suggest that the source rocks of these pelites were remarkably felsic in nature. The paleocurrent analyses reveal that the Chakrata pelites may have been derived from the granitoid rocks from the southerly direction (i.e. from Peninsular India). The geochemical similarities favour granitoid rocks from the Aravalli and Bundelkhand regions as the main source rocks for Chakrata sedimentary rocks.