Magmatic evolution of Girnar volcano-plutonic complex of Deccan Traps, India: Sr-Nd-Pb-Hf isotopic evidence of multiple sources

Close spatial association of plutonic and volcanic rocks in Large Igneous Provinces is very rare. Occurrence of various mafic rocks (syeno-diorite, gabbro, basalt and mafic dyke) and silicic rocks in the Girnar volcano-plutonic complex of Deccan Traps provides a unique opportunity to understand complex petrogenetic processes. Alkaline rocks (syeno-diorite and mafic dyke) display an enriched LREE pattern (La/Sm_N ~5.0 to 7.8), whereas the tholeiitic gabbro and basalt show a relatively flat LREE pattern (La/Sm_N ~0.8 to 1.8) with a positive Eu anomaly (1.1 to 1.3). Trace elements modelling of alkaline rock compositions are consistent with their origin by 5–9% partial melting of a spinel lherzolite source, with the melt experiencing 74–88% fractional crystallization. Tholeiitic gabbro and basalt could be generated from 20% and ~5% melting of a depleted MORB source, respectively. Distinct isotopic composition of silicic rocks (⁸⁷Sr/⁸⁶Sr_i = 0.7204–0.7275, ε_Nd(i) = −6.8 to −7.3, ²⁰⁶Pb/²⁰⁴Pb_i = 18.74–19.02, ²⁰⁷Pb/²⁰⁴Pb_i = 15.76–15.79, ²⁰⁸Pb/²⁰⁴Pb_i = 39.63–40.03, ε_Hf(i) = −6.0 to −7.3) indicate an origin from melting of a crustal source contaminated with (≤5%) mantle-derived mafic melts. The presence of shattered clinopyroxene crystals in silicic rocks and variation in Ti content of quartz are attributed to a sudden release of pressure during magma ascent. The Girnar Complex is emplaced in a reactivated fault lineament where mafic rocks (both alkaline and tholeiitic) were uplifted by several kilometers after its formation in a shallow magma chamber. Silicic rocks were emplaced along the concentric marginal fault of the Girnar Complex, synchronously or just after this uplift event.