Composition and weathering conditions of Paleoproterozoic upper crust of Bundelkhand craton, Central India: Records from geochemistry of clastic sediments of 1.9 Ga Gwalior Group

Abstract

The 1.9 Ga Gwalior Group represents one of the major Paleoproterozoic depositional basin of Bundelkhand craton and is comprised of ∼1 km thick unmetamorphosed and undeformed sedimentary succession, with basal clastic sequence of sandstone-shale association (Par Formation) and an upper chemogenic BIF rich sequence (Morar Formation). The geochemical examinations of the basal clastic rocks are the subject of present study. The compositional maturity and extreme quartz enrichment in sandstones of Gwalior Group suggest slow upliftment and high in situ weathering of basement terrains in a stable platformal setting. These clastic sediments are characterized by strong depletion of mobile constituents like Ca, Sr and Na, high Plagioclase Index of Alteration (PIA) values and strong positive correlation between Al 2O 3 and TiO 2, indicating severe weathering of upper crust. Positive correlation between FeO (T) and TiO 2 suggests Fe retention in paleoweathering profiles and is attributed to oxygenated atmosphere. Post depositional potash metasomatism is indicated in A-CN-K systematics of these sediments and K-enrichment from near zero to as high as 38% is inferred, with most of the shale samples show ∼30% extraneous K 2O additions. Depleted transition element (Ni, Cr, Co, Sc) contents, elevated large ion lithophile element (LILE) [K, Rb, Ba, Th, light rare earth elements (LREE)] contents, fractionated rare earth element (REE) patterns, strong negative Eu anomalies in these sediments point toward K-rich intracrustal granitic source component. Steeper REE patterns (Avg. La/Yb N ∼12.57) compared to basement granites and conspicuous depletion of Y over other LILEs in AUC normalized spidergram also suggest a heavy rare earth element (HREE) depleted TTG source component. Minor contribution from tholeiitic mafic rock is evidenced from Th/Sc–Sc, La/Sc–Sc/Th systematics and positive correlation of transition elements with FeO (T) and TiO 2. Mixing calculations on the REE data suggest that upper continental crust of Bundelkhand craton was constituted of 40% granite, 40% TTG and 20% mafic rocks. This is in contrast with the present day composition of Bundelkhand Granite Gneiss Complex (BGGC) that contains ∼90% high K granite. The data suggest unroofing of granite batholiths by means of erosion and upliftment of supracrustal TTG cover.