Late Archaean Mantle Fertility : Constraints from Metavolcanics of the Sandur Schist Belt, India

Abstract

Late Archaean (2.7 Ga) mantle fertility and the processes of oceanic crust and greenstone formation have been inferred through a detailed geochemical study of the metavolcanics of the Sandur Belt. This belt is made up of two distinct lithotectonic assemblages. (1) The autochthonous sequence consists of Yeshwanthanagar Volcanic Block (YVB), Deogiri Block (DB), Western Volcanic Block (WVB), Central Volcanic Block (CVB) and Eastern Volcanic Block (EVB). (2) The allochthonous assemblages are divided into North Central Acid Volcanic Block (NCAVB), Sultanpura Volcanic Block (SVB) and Eastern Acid Volcanic block (EAVB). Autochthonous assemblage was formed as a pericontinental insitu sequence on a shelf to which the allochthonous blocks have been successively accreted along layer parallel faults. Lithological, structural, metamorphic and geochemical discontinuities are found across the different blocks. Volcanic components of these blocks comprise of ultramafic komatiites and/or cumulates, basaltic komatiites, high Mg-basalts, high Fe-tholeiites, tholeiitic dacites, andesites and rhyolites, metamorphosed upto amphibolite facies, but sometime preserving relictigneous mineralogy. REE abundances of most of the metabasalts are 10-40 times of those of chondrite and have generally unfractionated patterns with (La/Yb) N ∼1-5. Fractionated REE patterns are found in the felsic amphibolites of CVB (Hospet zone) and the intermediate-acid volcanic members present only in NCAVB and EAVB. Abundances and ratios of several incompatible diagnostic elements of comparable partioning coefficients such as Nb/Th, Hf/Sm, Zr/Hf, Zr/Yb, Sc/Yb, Nb/Yb, Nb/Th, Th/Ta, Th/Tb and others show large scale variation within and between different blocks. These ratios for the high Mg-komatiitic basalts are nearer to those of primitive mantle. The marked differences in geochemistry between the metabasalts of different blocks indicate that the entire volcanic sequence may not have been derived from the same mantle. Their compositions in general are not comparable to any of the basalts from modern tectonic settings. However a mix of 80 and 20% enriched and normal MORB is very close to their overall composition, which is defined as Archaean Oceanic Ridge Basalt (AORB). Geochemical data suggest that Sandur basalts were generated from a fertile and heterogenous mantle. This fertile mantle either was tapped at the Archaean ridges directly or enriched magma was fed to the ridges from nearby hotspots. Partial subduction of the AORB possibly gave rise to the mafic-felsic volcanics of the NCAVB and EAVB; the two subduction complexes having interbedded turbidites and other sediments derived from within the basin. FeO/MgO, Ce/Nb, Nb/U, Nb/Th and Nb/La ratios of samples having MgO >11% indicate that hot spot tectonics might also have played a role for the evolution of this belt, but the present level of information is equivocal. However, derivation of the Sandur metabasalts from a relatively fertile mantle in the form of oceanic ridge basalts seems to be a strong possibility as around 2.7-2.8 Ga ago only small fraction of the continental crust was extracted from the mantle. Subsequent compressional processes appears to have telescoped the two continental volcanic margins.