Petrogenesis of the Mesoproterozoic Lamproites from the Krishna Valley, Eastern Dharwar Craton, Southern India

摘要

Petrology and geochemistry of eleven Mesoproterozoic lamproites from the Krishna valley, Eastern Dharwar craton, are presented. The Krishna lamproites show wide variation in their petrography and modal mineralogy and constitute excellent examples of rare phenomenon of heteromorphism. Ti-phlogopite, clinopyroxene, richterite, Fe-rich sanidine, sphene, F-rich apatite, ilmenite, and rutile are the major minerals present in varying proportions. Their mineral compositions overlap with those present in world-wide Phanerozoic lamproites and casts doubt on recent suggestions regarding the absence of archetypal lamproites and kimberlites from the Indian cratons. Incompatible trace element ratios (i) rule out the Krishna lamproites to be products of direct plume- (asthenospheric) derived mantle as well as subduction-related components and (ii) demonstrate their derivation from sources similar to those of the co-spatial Cuddapah basin lamproites and Wajrakarur- and Narayanpet-kimberlites from the Eastern Dharwar craton. The source enrichment of the Krishna lamproites is related to the period of global-scale mantle upwelling at ～1.9 Ga that included the emplacement of Large igneous provinces of similar age and magmatism in the Dharwar and Bastar cratons (India), the Superior Craton (Canada) and the Kaapvaal craton (southern Africa). We find that the observed petrological and geochemical characteristics of the Krishna lamproites can be best explained by the vein-plus-wall-rock model involving phlogopite + amphibole + rutile + clinopyroxene + apatite + titanite occuring as metasomatic veins in a depleted lithospheric mantle, within the garnet stability field. Our study illustrates the significant role of the lithospheric thickness and its attendant metasomatism in influencing petrogenesis of the co-spatial Mesoproterozoic lamproites and kimberlites from the Eastern Dharwar craton.