Mineral chemistry of chrome-diopside bearing lamprophyre from Mesoproterozoic Settupalle igneous complex of Prakasam alkaline province: insights on the magma dynamics at shallow lithospheric mantle beneath the NE margin of the Cuddapah basin, Southern India

Abstract

The Eastern Dharwar Craton shows considerable variations in the activity of kimberlite, lamproite and lamprophyre and their related lithospheric thickness, thermal structure and magmatic source characteristics from the innermost core to the margin. These variations have been revealed through seismic studies and the analysis of xenoliths, and xenocrysts carried in alkaline magmas, particularly kimberlites. This study reports the occurrence of chrome-diopside phenocrysts from a lamprophyre of the Settupalle pluton, located on the northeastern margin of the Cuddapah basin, Prakasam alkaline province in South India. Studied lamprophyre are characterized by phenocrystic clinopyroxene (Cpx) and biotite, embedded in a groundmass of amphibole, biotite, opaques, calcite, feldspar, nepheline and sodalite, with the latter minerals forming ocellar structures. Clinopyroxene displays normal zoning, with a high Mg–Cr rich diopside core (up to 1.92 wt%) and a Na 2 O-, Al 2 O 3 , FeO and TiO 2 -enriched salite rim. These chemical variations, along with elemental substitutions from core to rim, indicate the significant role of fractionation in the evolution of lamprophyre magma. The lack of chemical disparity among the studied Cpx suggests closed system fractionation, under high oxygen fugacity conditions. Thermobarometry results indicate polybaric crystallization for the Cpx (1206–1269°C, 11.82–18.04 kbar) and biotite (825–856°C, 3.49–6.94 kbar). The gradient inferred from the pressure of the Cpx suggests that its crystallization occurred at a depth of 43–66 km, implying shallow lithospheric magma contribution for the genesis of the lamprophyre magma.