Petrogenesis of the Girnar Complex in the Deccan Traps Province, India

Abstract

AbstractThe c. 66 Ma Girnar Complex of the Deccan Traps consists of various mafic to silicic rocks and provides a unique setting for understanding the dynamics of magma chamber processes. The intrusive alkaline rocks in Girnar are diorites/monzodiorites and contain syenite/nepheline syenite veins. Several (alkaline) mafic dykes cut the alkaline rocks. These rocks are surrounded by intrusive tholeiitic gabbro and extrusive basaltic flows and a silicic (SiO2 > 65 wt%) ring dyke at the outer margin of the complex. The tholeiitic gabbro and basaltic lavas have relatively flat light rare earth element (REE) patterns (La/SmN ~ 0.8 to 1.8) with positive Eu anomalies of 1.1–1.3. The tholeiitic basalts formed by 3–5% partial melting of an incompatible element depleted mid-ocean ridge basalts (MORB)-like source. Accumulating c. 20–30% of (olivine, orthopyroxene, clinopyroxene, and plagioclase) crystals from the tholeiitic melt generated the tholeiitic gabbro. The alkaline rocks display enriched light REE patterns (La/SmN ~ 5.0 to 7.8) consistent with (~9%) partial melting of a spinel lherzolite source, different from the MORB-like source for tholeiitic rocks. The subsequent 58–72% fractional crystallization of olivine, clinopyroxene, plagioclase, orthoclase, spinel, and apatite mineral assemblage formed the alkaline rocks. Reverse and oscillatory zoning in plagioclase (An30 to An50) phenocrysts of the silicic rocks suggest that convective movement of crystals in a magma chamber led to initial crystallization of high-An plagioclase in the hotter zone of the magma chamber, followed by addition of low-An plagioclase in cooler regions of the magma chamber. Pressure estimates from shattered (xenocryst or antecryst) clinopyroxene crystals in silicic rocks give a minimum value of 450 MPa. Ti-in-quartz geothermometry gives a crystallization temperature between 690°C and 1080°C. The distinct isotopic composition of the silicic rocks (87Sr/86Sri = 0.7204–0.7275, εNd(i) = −6.8 to −7.3, 206Pb/204Pbi = 18.74–19.02, 207Pb/204Pbi = 15.76–15.79, 208Pb/204Pbi = 39.63–40.03, εHf(i) = −6.0 to −7.3) further indicates that the silicic rocks assimilated 7 to 9% granitic basement rocks. Overall, therefore, the Girnar Complex started to form with the emplacement of mafic rocks (both alkaline and tholeiitic) in the central part along a reactivated fault lineament and silicic rocks along the concentric marginal fault. Petrography, mineral chemistry, and whole-rock geochemical and Sr-Nd-Pb-Hf isotope ratios of the Girnar Complex rocks indicate that despite the close spatial association of tholeiitic gabbro and basalt, alkaline rocks, and silicic rocks, these rocks originated from multiple sources and evolved in a complex magma drainage and storage network in a continental setting of the Deccan Large Igneous Province (LIP).