Petrogenesis of mafic alkaline dikes from the ～2.18 Ga Mahbubnagar Large Igneous Province, Eastern Dharwar Craton, India: Geochemical evidence for uncontaminated intracontinental mantle derived magmatism

摘要

In this paper we present detailed field observations, petrology, and geochemical data, and discuss the petrogenet-ic aspects of the unusual high Mg-mafic dikes of alkaline to subalkaline nature from the widely known Paleo-proterozoic Mahbubnagar Large Igneous Province. These dikes traverse the granitoids of the peninsular Gneissic Complex and the 2.7 Ga metabasalts in the adjacent Gadwal greenstone terrane. The study area is geologically embedded between the eastern margin of the Gadwal greenstone terrane and the northwest margin of the Pro-terozoic Cuddapah basia in the eastern Dharwar craton of peninsular India. The dikes are fine to medium grained, massive, mesocratic to melanocratic in appearance, and range in dimension from a few hundred metres to over ten kilometres in strike length extending in the northwest to the southeast direction. These dikes are neither deformed nor metamorphosed, and they exhibit well-preserved igneous textures. They are predominantly composed of clinopyroxene (augite) and plagiodase (oligoclase-andesine) with subordinate biotite, amphibole, chlorite, and accessory titanite and ilmenite. They exhibit uniform geochemical compositions with ocean island basalt-like characteristics. Their compositions preclude any significant alteration, element mobility, or contamination by the upper continental crust during their ascent and emplacement Therefore, their major and incompatible trace element characteristics reflect a primary source feature. Numerical calculations suggest a melt segregation temperature of ～1390 °C at 2.2 GPa corresponding to an estimated depth of ～71 km Geochemical modelling indicates low-percentage partial melting (～2%-6%) of a peridotitic mantle source in the garnet ± spinel stability field. The regional tectonic framework and field observations corroborated by detailed geochemical studies favour an intracontinental setting for the emplacement of these high-Mg mafic dikes of alkaline affinity. Further, we propose that the Gadwal alkaline dikes evolved from a common parental magma as a consequence of the partial melting of a metasomatised subcontinental lithospheric mantle by adiabatic upwelling of an astheno-spheric plume source coupled with lithospheric thinning and extension.