Sr-rich apatite and Nb-rutile from the Chelima lamproite dykes, Cuddapah Basin, southern India and their petrological significance

Abstract

The Chelima dykes intrude the Nallamalai Group of sedimentary rocks in the Cuddapah Basin, southern India, and constitute some of the world’s oldest (1.38 Ga) known lamproites. Electron probe micro-analysis reveals that the Chelima apatite is Sr-rich (SrO: 2.01–7.72 wt%) and LREE-poor (ΣLREE =<1 wt%), whereas the Chelima rutile is Nb-bearing (Nb2O5: 0.58–1.91 wt%) and Fe-poor (FeO: <0.79 wt%). Even though apatite and rutile are present, albeit in much lesser modal proportions, in the co-spatial and broadly coeval (1.25–1.56 Ga) Krishna lamproites at the NE margin of the Cuddapah Basin, these minerals are relatively much impoverished in their respective Sr (SrO: up to 0.11 wt%) and Nb (Nb2O5: up to 0.13 wt%) contents. Recent geochemical (isotopic) studies showed the existence of an ancient (2–2.5 Ga) and anomalously enriched (metasomatized) sub-continental lithospheric mantle beneath the Cuddapah Basin and its environs. The contrasting distribution of two highly incompatible trace elements (viz. Sr and Nb) in apatite and rutile from the Chelima and Krishna lamproites reflects either the difference in the nature of their metasomatized mantle source regions or varying melt fraction of their magmas, or both. The present study shows distinct petrogenesis of the co-spatial Cuddapah (Chelima) and Krishna lamproites and highlights the utility of trace element chemistry of accessory phases in unravelling the nature and evolution of the underlying subcontinental lithospheric mantle.