Mesoproterozoic emplacement and enriched mantle derivation of the Racherla alkali syenite, Palaeo-Mesoproterozoic Cuddapah Basin, southern India: insights from in situ Sr-Nd isotopic analysis on apatite

Abstract

We report an in situ Sm–Nd isochron age of 1326±73 Ma, determined by LA-MC-ICP-MS (laser ablation-multiple collector-inductively coupled plasma-mass spectrometry), on crystalline apatite grains from the Racherla alkali syenite occurring in the Palaeo-Mesoproterozoic Cuddapah Basin, southern India. The obtained Mesoproterozoic age necessitates the Racherla syenite to be an intrusive into the sedimentary rocks of the Cuddapah Basin contrary to some previous suggestion that it represents an inlier of the basement pluton. Age of the dated syenite, within its error limits, is also coeval with that of the nearby Chelima lamproite (1354±52 Ma) and other syenite complexes (c. 1352 Ma) located elsewhere within the Eastern Ghats Mobile Belt, southern India, thereby implying a common geotectonic control in all of their generation. Incompatible trace element signatures (high La/Nb, Zr/Nb and La/Yb), ⁸⁷Sr/⁸⁶Sr (0.70432±10) and the lower Nd isotopic ratio (εNd(t) −8.9) of the apatite suggest derivation of the Racherla syenite parent magma from an enriched mantle source. Our findings provide compelling evidence for the existence of ancient (c. 2.6 Ga) metasomatized lithospheric mantle, at the Archaean–Palaeoproterozoic boundary, beneath the Cuddapah Basin. This enriched mantle appears to have had played a dominant role in the generation of co-spatial and coeval lamproites within and around the Cuddapah Basin and a plethora of alkaline and sub-alkaline syenites within the Eastern Ghats Mobile Belt. Implications of our study in the context of Indo-Antarctic collisional tectonics are also explored.