Origin of the primitive, strongly SiO2-undersaturated alkalic rocks from the Deccan Traps by low-degree mantle melting and high-pressure fractional crystallization

Strongly SiO2-undersaturated alkalic rocks (Mg# > 50, SiO2 ≤ 45 wt%, Na2O + K2O ≥ 3 wt%) occur in three early-stage (Sarnu-Dandali, Mundwara, Bhuj) and one late-stage (Murud-Janjira) rift-associated volcanic complexes the Cretaceous-Paleogene Deccan Traps flood basalt province of India. Thermobarometry based on clinopyroxene-liquid equilibrium suggests that they mostly crystallized beneath Moho at ~ 15 kbar/1270 °C to 11–12 kbar/1115–1156 pressures temperatures. Primary magma compositions with lherzolite were estimated through reverse fractionation calculations by incrementally adding phases olivine:clinopyroxene:spinel:phlogopite = 12:68:20:15 proportions low temperatures followed olivine:clinopyroxene:spinel 12:68:20 higher A comparison primary magmas experimentally generated melts shows their are consistent an origin from garnet sources < 1 wt% H2O CO2. Hornblendite, pyroxenite (except for some Bhuj rocks) carbonated eclogite unlikely rocks. The Sarnu-Dandali Murud-Janjira lamprophyres probably originated 5% melting 1.3 times Ti-enriched lherzolitic compared primitive mantle. basanites calculated assimilation-fractional crystallization assimilating lower crustal mantle xenoliths found younger lamprophyre dikes same area indicate contamination Indian lithosphere was during ascent. evolved mixing phonotephritic melts, a Ti-poor (0.7 times) source. temperature base 1325 1285 Murud-Janjira.