Geochemical and Isotopic Constraints of Neoarchaean Fossil Plume for Evolution of Volcanic Rocks of Sandur Greenstone Belt, India

Metavolcanics of 20 km wide Sultanpura volcanic block of the Neoarchaean Sandur (greenstone) schist belt are divided into tholeiitic basalts, high Mg basalts, Al-depleted and Al-undepletcd komatiitic ultramafic schists. Metabasalts are metamorphosed to amphibolite facies, but still preserve their pillow structures. Ultramafic komatiitic rocks are transformed to actinolite-tremolite schists with no recognizable original textures or mineralogy. Mctabasalts and ultramafic komatiitic schists are interbedded with thin layers of sulphidic banded iron formations, argillaceous carbonate rocks and carbon phyllites that are interpreted as metamorphosed pelagic sediments of the deep ocean. No terrigenous sediments are found in Sultanpura block indicating that eruption of these submarine volcanic rocks took place in the deeper part of the ocean, away from the western and eastern shelf parts of the Sandur belt, where terrigenous sediments are abundant. Sultanpura block in its west and east is discordantly bounded by thrust, subduction complex and shelf facies sediments. These observations are interpreted to indicate that Sultanpura block is a telescoped proto-oceanic part between the two shelves and island are complexes. MgO of metavolcanic rocks varies from 6 to 30%, with a gap between 16 to 22%. Al2O3/TiO2 shows characteristic variation for tholeiites (10-15), high Mg basalts (13-21), Al-undepleted ultramafic komatiitic schists (9-23) and Al-depleted ultramafic komatiitic schists (11-20). CaO/Al2O3 of tholeiites and high Mg basalts is ~1, whereas for the ultramafic komatiitic schists, this ratio exhibits a range between 0.5 to 2, as a consequence of CaO mobility. REE, HFSE and 143Nd/144Nd data from Sultanpura volcanic rocks vary between CHUR (Chondrite Uniform Reservoir), primitive mantle and depleted mantle, but appear to be derived from primitive mantle and have been probably contaminated by continental crust. Although the abundance of REE varies from 2 to 12 chondrite, the patterns are smooth and flat with small negative or positive Eu anomalies as artifacts of alteration. Generally positive, but in few samples negative Nb anomalies are also found, with (Ce/Sm)N and (Gd/Yb)N being near chondritic. Ti/V, Ti/Zr, Zr/Y, Sc/Y, Nb/La, Nb/Th, Nb/U, MgO/TiO2, MgO/FeO and Al2O3/TiO2 also for many samples are near chondritic, εNd=+0.8649±0.00024 resembling CHUR. Th/Nb, Nb/U and some other ratios are near to those of Ontong Java and Gorgana plateaus (0.80 Ga) and the tholeiite-komatiitic sequence found in 2.7 Ga Southern volcanic zone, Abitibi belt of Canada and 2.1 Ga Birimian belt of West Africa. Collectively, these data indicate that a mantle plume, derived from an enriched mantle, possibly played an important role in the oceanic volcanic sequence of Sultanpura block. Some of the HFSE follow the olivine control line, whereas other elements following the olivine control line define a narrow array tube. Formation of such array tubes on the plots of some HFSE elements and their ratios, and the scatters of HFSE/REE ratios, probably suggest dynamic melting of the plume during ascent. Entrenchment, mixing of Archaean ocean ridge basalts (AORB), crustal contamination and subduction of such a plume-fed slab may have generated the compositional heterogeneities observed in the Sultanpura metavolcanic rocks.