Evolution of the Indian Shield: A New Approach

Abstract

Indian shield has five distinct cratonic blocks, namely Dharwar craton (DC), Bastar craton (BC), Singhbhum craton (SC), Bundelkhand craton (BkC) and Aravalli craton (AC), all comprising greenstone-gneiss in different abundance ratios. As in other Archean terrains, this ocean-continental rock association dichotomy is explained by extrusion of mafic lava and its transformation into greenstone along with its melting at depth to generate tonalitic magma that crystallized and deformed to form Archean tonalite, trondhjemite, granodiorite (TTG) gneisses, now seen interspersed with the metabasic components. Interior of the three Indian cratons viz. DC, BC and SC, contains nucleus of Archean gneisses (3.6–3.4 Ga), occurring either as separate crustal components or as inclusions in younger granitic intrusions. The remaining cratons of Bundelkhand and Aravalli are dominantly granitic gneisses, interpreted here as having a possible genetic relationship with the Singhbhum granite complex (SG I and SG II) as the product of partial melting of amphibolites of Older Metamorphic Group (OMG) and its possible basement called Older Metamorphic Tonalite Group (OMTG). This proposition is supported by their geochemical similarities; TTG rocks of the BkC and AC areas show fractionated LREE, weak negative Eu anomaly and gentle sloping HREE, similar to those found in Singhbhum granite complex (SG I and SG II) located to the south of the E-W trending Son-Narmada lineament. The earliest sedimentation in SC occurred at 3.5 Ga with deposition of OMG but 200 Ma later in DC with deposition of Sargur (3.3–3.1 Ga), and in uncertain period of deposition for the Sukma and Amgaon group of rocks in the BC. However, the first supracrustals over the Banded Gneissic Complex (BGC)-Berach granite, defining Aravalli craton, and also over the Bundelkhand craton, located to the north of the Son-Narmada lineament, were deposited after 2.5 Ga when all the Indian cratonic blocks of the Indian shield had stabilized with maximum growth. Because of unequal erosion, differing metamorphic grades and varying proportions of greenstone and TTG gneisses, the five cratons within the Indian shield cannot be correlated and thus negating the proposition that the Indian shield is formed by accretion of separate continental fragments. It is further argued that the joining of different Proterozoic fold belts of relatively short elongation and occurring singularly, or in parallel belts, at the cratonic margins such as that of AC, SC and BC of the Indian shield, is untenable because of their ensialic orogenesis involving little horizontal movement of the colliding crustal blocks.