The Southeastern Saurashtra dyke swarm, Deccan Traps: Magmatic evolution of a tholeiitic basalt–basaltic andesite–andesite–rhyolite suite

The Deccan Traps continental flood basalt (CFB) province of India contains several dyke swarms, which are dominated by tholeiitic basalts and basaltic andesites. The Southeastern Saurashtra dyke swarm, containing mainly these rock types, also contains an andesite and several rhyolites. Based on petrographic, mineral chemical, and whole-rock major and trace element and Sr-Nd isotopic data, we discuss the magmatic evolution and pressure-temperature conditions of crystallisation of these dyke magmas. The tholeiitic basalts and basaltic andesites have low to moderate TiO2 contents. The andesite and the rhyolites have low CaO, MgO, Fe2O3t, TiO2 and P2O5, and high K2O, Rb, Ba and light rare earth element contents. Thermobarometric calculations for equilibrium mineral–whole-rock pairs indicate plagioclase crystallisation at 1200–1170 °C, overlapping with clinopyroxene crystallisation at 1181–1143 °C, and a pressure range of 0.1–3.6 kbar indicating crystallisation during magma ascent or storage at shallow crustal levels. Major and trace element modelling is consistent with the rhyolites being produced by advanced fractional crystallisation of basaltic parental magmas. However, a broad trend of increasingly radiogenic Sr and nonradiogenic Nd isotopic ratios, fromthe mafic rocks through the andesite to the rhyolites, suggests a combined assimilation and fractional crystallisation (AFC) process between basaltic magma and ancient granitic crust. The rhyolites contain the largest crustal contributions and display the most “enriched” Sr-Nd isotopic characteristics yet recorded from the Deccan Traps (εNdt = −20.6, (87Sr/86Sr)t = 0.74855). The widespread occurrence of mafic enclaves in the rhyolite dykes suggests that processes such as magma mixing may also be responsible for some of the andesitic rocks known in Saurashtra.