Phase Chemistry and Formational Conditions of Mesoproterozoic Gabbro and Diabase Dykes from Prakasam Alkaline Province, SE India: Insights into Crystallisation of Fractionated-Oxidised Calc-Alkaline Magmas of Lower-Mid Crustal Magma Chambers

Abstract

The texture and composition of rock-forming minerals help in understanding magma chamber dynamics because of their ability to preserve crystallisation conditions and latestage magmatic processes. This study investigates the mineral compositions of co-spatial gabbro and diabase dykes in the Prakasam Alkaline Province (PAP) to reveal their petrogenetic aspects. The studied rocks exhibit a similar mineral assemblage with distinct disequilibrium textures, and their corresponding chemistry indicates the fractionated nature of their parent magma. Crystallisation of clinopyroxene, amphibole, and biotite in the host gabbro and dykes occurred at 1141–682°C at 12.85–0.6 kbar and 1107–753°C at 10.2–0.6 kbar, respectively. The estimates of oxygen fugacity, crystallisation pressures, and temperatures indicate an increase in the oxidation state and P<inf>H2O</inf> of the magma during the final stages of crystallisation. Disequilibrium textures (skeletal, resorbed, and sieve) and a broad range of crystallisation pressures for individual minerals imply Polybaric/ Polythermal Open system Fractionation Reactions (POFR). Non-isochemical reactions between anhydrous phases and interstitial residual fluids led to the formation of amphibole and biotite coronas, emphasising the impact of hydration on early crystallised minerals at shallow depths. From a tectonomagmatic perspective, the synthesis of mineral chemistry supports the presence of a calc-alkaline magmatic source believed to originate from the Subcontinental Lithospheric Mantle (SCLM). The compositional coherence of minerals from both the host gabbro and dykes suggests a genetic relationship, although they originate from different batches. This study highlights the SCLM-related calc-alkaline mafic magmatism in the PAP, which is associated with Mesoproterozoic rifting. © 2025 Geological Society of India, Bengaluru, India.