Geochemistry and petrogenesis of mafic magmatic rocks of the Jharol Belt, India: geodynamic implication

Abstract

The belt consisting of deep-water meta-sediments of the Aravalli Supergroup hosts numerous mafic and ultramafic rocks occurring at two different structural levels. These are the basal Bagdunda volcanics and the Gopir magmatic rocks represented by Gopir dikes and flows associated with the ultramafic rocks along the Kaliguman lineament. The Bagdunda and Gopir mafic volcanic rocks are sub-alkaline, varying in composition from high Mg-tholeiite to basaltic komatiite. They are predominantly LREE depleted, but some flat REE patterns are also observed. Bulk geochemical data, especially the incompatible trace elements discount the possibility of crustal contamination. Gopir dikes are highly enriched in terms of their REE [(Ce/Sm) N and (Ce/Yb) N ] and other incompatible trace elements and closely resemble the basal Aravalli volcanics. Their ratio (rock)/ratio (PM) and normalized multi-element abundance patterns depict their overall enriched nature compared to N-MORB and primitive mantle. The Gopir and Bagdunda volcanics reflect trace element characteristics transitional between E-MORB and OIB. Gopir dikes show remarkable similarity with continental tholeiite and Continental Flood Basalts (CFBs) with negative Nb, P and Ti anomalies and low Nb/Ce ratio unlike the volcanics of both the suites. Petrogenetic modeling based on the compositionally corrected [Mg] and [Fe] abundances for Gopir dikes, Gopir and Bagdunda volcanics indicates (a) their derivation from non-pyrolitic sources (b) their derivation from sources that were variably enriched in [Fe/Mg] ratios with large variation in their [Fe] contents and (c) olivine was a major phase to fractionate, followed by a lesser amount of clinopyroxene in the case of Gopir volcanics, whereas in Bagdunda volcanics a combination of olivine and clinopyroxene fractionation is suggested. Based on the geochemical data, supported by field evidence, we propose a geodynamic model for the development of the Jharol Belt in which we suggest that the basement rock, i.e. the Banded Gneissic Complex (BGC) started rifting probably under influence of a mantle plume during the late Archaean–early Proterozoic period. During the opening of the basin, magma derived from asthenospheric mantle reached the surface contemporaneously with sedimentation. The first phase of volcanism is represented by Bagdunda volcanics. With continuous rifting, the crust became highly attenuated and facilitated asthenospheric upwelling, causing high degrees of melting (indicated by a large volume of mafic and ultramafic rocks) during the second phase of magmatism (Gopir volcanics) that occurred at later stages of Jharol sedimentation. Coeval melting of the sub-continental lithosphere under the adjoining BGC craton probably caused the emplacement of dikes along with the Gopir volcanics. At this stage of progressive rifting, an oceanic crust very similar to that in marginal basins developed in the Jharol Belt.