Neoarchean microblock amalgamation in southern India: evidence from the Nallamalai Suture Zone

Abstract

A collage of crustal blocks ranging in age from Mesoarchean to Neoarchean accreted to the southern margin of the Dharwar Craton in Peninsular India preserve distinct evolutionary history, but share a common metamorphic record during Archean-Proterozoic transition. Here we investigate the tectonic boundary between two of these microblocks, the Shevaroy Block to the west and the Madras Block to east, termed as the Nallamalai Suture Zone (NLSZ). We present integrated field, petrological, geochemical and zircon U-Pb and Lu-Hf data from a suite of meta-igneous and metasedimentary units along the NLSZ and its flanks. Zircon grains from the meta-monzogranite, hornblende-biotite gneiss, amphibolite, granodiorite, diorite, charnockite, BIF (banded iron formation) and BMQ from this area show magmatic emplacement ages clustering around 2.50 to 2.56 Ga, except for the meta-monzogranite showing ages up to 3.2 Ga, correlating with long-lived convergent margin magmatism through multiple slab melting episodes. All the rocks show tightly constrained early Paleoproterozoic (ca. 2.46 to 2.48 Ga) metamorphic ages, including the metamorphic zircon in the BIFs, marking the timing of collision of the two continental blocks with consumption of the intervening oceanic lithosphere. The Epsilon Hf(t) values of magmatic zircon grains from the different rock types range from -4.1 to +5.7, and together with T sub(DM) sup(C) Hf model ages of 2672-3247 Ma, a dominantly juvenile crust growth is indicated, which initiated around 3.3 Ga and continued to 2.7 Ga, followed by crustal reworking in a continental arc towards the end of Neoarchean. The geochemical data suggest that tholeiitic to calc-alkaline parental melts for the mafic-intermediate-felsic suite were generated by low degree partial melting of a peridotitic mantle wedge metasomatized by subduction-derived fluids and sediments and continuous slab melting ensued by intracrustal fractional crystallization of melts with reworking of older continental crust. The BIF and BMQ samples display positive Eu anomalies, negative to negligible Ce anomalies, and superchondritic Y/Ho ratios suggesting their formation in an oceanic realm proximal to an active continental margin setting. The magmatic and tectonic attributes are consistent with eastward oceanic subduction and ocean closure along the NLSZ which is defined here as the trace of a suture welding the Shevaroy and Madras Blocks. We envisage that multiple subduction and amalgamation of several microblocks occurred in the Dharwar Craton and its southern domains, amalgamating several microblocks during the Archean - Paleoproterozoic transition.