Petrogenesis of a Mesoproterozoic shoshonitic lamprophyre dyke from the Wajrakarur kimberlite field, eastern Dharwar craton, southern India: Geochemical and Sr-Nd isotopic evidence for a modified sub-continental lithospheric mantle source

Abstract

Mineralogy and geochemistry of the Udirpikonda lamprophyre, located within the Mesoproterozoic diamondiferous Wajrakarur kimberlite field (WKF), towards the western margin of the Paleo-Mesoproterozoic Cuddapah basin are presented. The lamprophyre is characterised by a panidiomorphic-porphyritic texture imparted by clinopyroxene, olivine and biotite set in a groundmass of feldspar and spinel. Olivine occurs as the microphenocrysts with a composition range of Fo87–78. Clinopyroxenes display reverse as well as oscillatory optical zoning and are diopsidic in nature with a variation in the composition from core (Wo47 En28 Fs20Ac5) to rim (Wo46En41Fs11Ac3). Biotite (Mg#<0.6) is the only mica present and spinels are titano-magnetites showing ulvospinel- magnetite solid solution. Plagioclase is the dominant feldspar with a variable compositional range of An41–8Ab82–56Or33–3. Based on the mineralogy, the lamprophyre can be classified to be of calc-alkaline variety but its geochemistry display mixed signals of both alkaline and calc-alkaline lamprophyres. K2O/Na2O ranges from 1.49 to 2.79, making it distinctly potassic and highlights its shoshonitic character. Moderate Mg# (60–65), Ni (110–200ppm) and Cr (110–260ppm) contents in the bulk-rock indicate substantial fractional crystallization of olivine and clinopyroxene. Fractionated chondrite normalized REE patterns (average (La/Yb)N=37.56) indicates involvement of an enriched mantle source from within the garnet stability field whereas slightly negative Ta-Nb-Ti and Hf anomalies displayed on the primitive mantle normalized multi-element spider gram highlight involvement of a subducted component in the mantle source. Given the spatial disposition of the studied lamprophyre, the age of the emplacement is considered to be coeval with WKF kimberlites (~1.1Ga) and the initial 143Nd/144Nd (0.510065–0.510192) and 87Sr/86Sr (0.705333–0.706223) are strikingly similar to those observed for the Smoky Butte lamproites, Montana, USA. Fluid-related subduction enrichment of the mantle source is apparent from the enriched ratios of La/Nb, Ba/Nb and (Hf/Sm)N, (Ta/La)N<1. Petrogenetic modelling reveals melt generation from 1 to 2% partial melting of an enriched mantle source that subsequently underwent fractional crystallization. Our study provides geochemical and isotopic evidence for a sub-continental lithospheric mantle (SCLM) modified by subduction and asthenospheric upwelling in the Eastern Dharwar Craton. The partial melting of a resulting heterogeneous Eastern Dharwar Craton SCLM to generate Udiripikonda lamprophyre and Wajrakarur kimberlites has been attributed to the Mesoproterozoic regional lithospheric extension event.