Episodes 2020; 43(1): 88-108
Published online March 1, 2020
Copyright © International Union of Geological Sciences.
Fareeduddin* and D. M. Banerjee **#
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The continued existence of an Eo-Paleoarchean proto-continent through Proterozoic times in the Aravalli Mountain Range is now widely accepted. The protocontinent’s growth culminated with the outpouring of ~2.5 Ga old silicic magma by the end of Neoarchean, which produced polyphase granites-gneisses, and migmatites of the ‘Banded Gneissic Complex.’ The polycyclic Proterozoic orogenic processes involved crustal thickening and thermal perturbations, which reconstituted its pristine character. The term Banded Gneissic Complex (BGC) as a valid lithodemic unit has survived while nomenclature like Mangalwar Gneiss and Mewar Gneiss failed to satisfy the international stratigraphic code. BGC with relics of ancient supracrustals and komatiitic bodies display a wide range of relationship with the cover rocks. Its subdivision into BGC-I and BGC-II has gained acceptance in contemporary literature.
The Aravalli Mountain Range with Proterozoic successions of the Aravalli and Delhi systems (Aravalli-Delhi Mobile Belt: ADMB), encapsulates several independent geo-tectonic domains sharing between them stages of deformation and metamorphism.
The Paleoproterozoic age of the Aravalli Supergroup has been firmed up with new radiometric dates. The Aravalli Supergroup has been re-defined as (i) East Aravalli Terrane (EAT) of Nathdwara–Bhilwara-Hindoli and (ii) South-Central Aravalli Terrane (SCAT) of Nathdwara and Lunavada. No identifiable stratigraphic hiatus between the two terranes is discernable. The EAT is represented by medium to high-grade metamorphics with migmatised paragneisses and is affected by longitudinal shears making it challenging to establish a valid internal stratigraphic order. In contrast, the SCAT exposes un-metamorphosed to mildly metamorphosed sediments preserving well developed primary depositional structures, facilitating construction of a viable, although somewhat debatable stratigraphic order. Reassessment of stratigraphic position of the basal quartzite, basic volcanics and, paleosol units with the BGC helped to understand the stratigraphy of the Aravalli sedimentary pile in a new perspective. Inappropriate application of Lomagund-Jatuli heavy isotope event to assign an age to the Aravalli carbonate rocks is discussed. The possibility of two separate episodes of carbonate sedimentation events needed emphasis. Discussion included the use of the robust UPb dates in fixing the age of the Aravalli basin opening. The attention to the fallacy of using the age of Darwal Granite in assigning age to the Aravalli sedimentaries was drawn. Analyses of available structural and metamorphic data of the Aravalli Craton concerning the mobile belt was analysed.
The Delhi Supergroup is distributed either in a series of fault-bound basins in the northeast part of the Aravalli Mountain Range resting directly on the basement gneisses (North Delhi Fold Belt: NDFB) or directly overriding the Aravalli Supergroup of rocks in central and southwestern parts (South Delhi Fold Belt: SDFB). The stratigraphy of the NDFB is well constrained, but physical connection between the two belts is conjectural. The SDFB represents a collage of three independent linear fold belts, each with unique depositional and deformational attributes. The terrane between the extreme limits of NDFB and SDFB is occupied by undifferentiated granitic rocks. Both NDFB and the SDFB are made up of meta-arenites, carbonates, and pelites along with sporadic granites and gneisses. South of the main SDFB, an independent and possibly younger Sirohi basin, represents the final orogenic event in the cratonization of NW Indian shield. Geochronological data points to ca 1.72 Ga and ca 1.0 Ga ages for the collisional orogenies accompanied by polyphase deformation and metamorphisms.
The plate tectonic models for the generation of the Aravalli and Delhi basins postulate the existence of a cratonic segment towards northwestern parts of the Aravalli Craton. The recent report of Eoarchean and Neoarchean zircon in the Delhi hinterland in the NDFB indicates the existence of an elusive Marwar Craton.