GEOCHEMISTRY OF BLACK SHALES FROM THE NEOARCHAEAN SANDUR SUPERTERRANE, INDIA: FIRST CYCLE VOLCANOGENIC SEDIMENTARY ROCKS IN AN INTRAOCEANIC ARC-TRENCH COMPLEX

Abstract

The ~2.7 Ga Sandur Superterrane (SST), of the western Dharwar craton, is a collage of greenstone terranes having distinct lithotectonic associations; volcanic associations are prevalent. Fine-grained metasedimentary rocks, which are optimal for provenance studies, are sparse in greenstone terranes of this craton. However, extensive shale sequences are present in the eastern volcanic terrane (EVT) and the eastern felsic volcanic terrane (EFVT) of the SST. Within the EVT, the black shales are stratigraphically associated with black cherts, metabasalt and banded iron formation (BIF), and underlain by greywackes. Shales have compositions of tholeiitic basalt in terms of TiO2, Cr, Co, Ni, V, and Sc contents, and plot near the arc basalt endmember on the Th/Sc versus Sc mixing hyperbola. In contrast, Archean average upper continental crust of Taylor and McLennan [Taylor, S.R., McLennan, S.M., 1985. The Continental crust: Its Composition and Evolution. Blackwell, Oxford, 307p.; Taylor, S.R., McLennan, S.M., 1995. The geochemical evolution of the continental crust. Rev. Geophys. 33, 241-265], plots mid-hyperbola indicative of bimodal arc magma provenance. Accordingly, the Sandur shales likely had a catchment in an oceanic arc or back-arc dominated by tholeiitic basalts. Specifically, Nb/Th ratios 1.5-2.5 in shales are close to those of Archean arc basalts (1-4), so a plateau or ocean island basalt source, where Nb/Th >8, can be ruled out. Compositionally, cherts are shale highly diluted by silica, with positive Eu anomalies, and are interpreted to be hydrothermal sediments precipitated from reduced fluids during periods of limited siliciclastic input. In the shales, variable SiO2 and Fe2O3 contents, depletions of MnO, MgO, and Na2O, and positive to negative Eu anomalies, but gains of K relative to arc basalt compositions, are interpreted as due to hydrothermal alteration. Greywackes underlying the shales have two compositions. Type I is similar to the shales, whereas Type II has fractionated REE with negative Eu anomalies consistent with a cratonic granitoid catchment [Manikyamba, C., Naqvi, S.M., Moeen, S., Gnaneswar Rao, T., Balaram, V., Ramesh, S.V., Reddy, G.L.N., 1997a. Geochemical heterogeneities of metagreywackes from the Sandur schist belt: implications for active plate margin processes. Precambrian Res. 84, 117-138]. Collectively, the results are in keeping with an intraoceanic arc outboard of a continental margin. During transgression the trench has a low energy shale facies with dominant arc contribution, but for regression high energy greywackes are deposited from a cratonic provenance. © 2006 Elsevier Inc. All rights reserved.