Abstract:
The Singhbhum Craton of eastern India preserves distinct signatures of ultramafic-mafic-intermediate-felsic magmatism of diverse geodynamic affiliations spanning from Paleo-Mesoarchean to Proterozoic. Here we investigate the 2.25 Ga Malangtoli volcanic rocks that are predominantly clinopyroxene- and plagioclase-phyric, calc-alkaline in nature, display basalt-basaltic andesite compositions, and preserve geochemical signatures of subduction zone magmatism. Major, trace and rare earth element characteristics classify the Malangtoli volcanic rocks as arc basalts, boninites, high magnesian andesites (HMA) and Nb enriched basalts (NEB). The typical LILE enriched-HFSE depleted geochemical attributes of the arc basalts corroborate a subduction-related origin. The boninitic rocks have high Mg# (0.8), MgO (greater than 25 wt. percent), Ni and Cr contents, high Al sub(2)O sub(3)/TiO sub(2) (greater than 20), Zr/Hf and (La/Sm) sub(N) (greater than 1) ratios with low (Gd/Yb) sub(N) (less than 1) ratio, TiO sub(2), and Zr concentrations. The HMA samples are marked by moderate SiO sub(2) (greater than 54 wt. percent), MgO (greater than 6 wt. percent), Mg# (0.47) with elevated Cr, Co, Ni and Th contents, depleted (Nb/Th) sub(N), (Nb/La) sub(N), high (Th/La) sub(N) and La/Yb (less than 9) ratio, moderate depletion in HREE and Y with low Sr/Y. The NEBs have higher Nb contents (6.3-24 ppm), lower magnitude of negative Nb anomalies with high (Nb/Th) sub(pm) = 0.28-0.59 and (Nb/La) sub(pm) = 0.40-0.69 and Nb/U = 2.8-34.4 compared to normal arc basalts [Nb = less than 2 ppm; (Nb/Th)sub(pm) = 0.10-1.19; (Nb/La)sub(pm) 0.17-0.99 and Nb/U = 2.2-44 respectively] and HMA. Arc basalts and boninites are interpreted to be the products of juvenile subduction processes involving shallow level partial melting of mantle wedge under hydrous conditions triggered by slab-dehydrated fluid flux. The HMA resulted through partial melting of mantle wedge metasomatized by slab-dehydrated fluids and sediments during the intermediate stage of subduction. Slab-melting and mantle wedge hybridization processes at matured stages of subduction account for the generation of NEB. Thus, the arc basalt-boninite-HMA-NEB association from Malangtoli volcanic suite in Singhbhum Craton preserves the signature of a complete spectrum of Paleoproterozoic active convergent margin processes spanning from subduction initiation to arc maturation.
