Interstitial silicic glasses in the Vikramgad-Murbad tholeiitic dyke swarm, Konkan Plain, western Deccan Traps: implications for silicic magma genesis in continental flood basalt provinces

The petrogenesis of silicic (> 63 wt.% SiO₂) magmas found in continental flood basalt provinces involves mechanisms ranging from fractional crystallisation to crustal anatexis. In the western Deccan Traps, India, a swarm of tholeiitic basalt and basaltic andesite dykes exposed in the Vikramgad-Murbad area likely fed the 2 km-thick, exclusively tholeiitic Western Ghats volcanic sequence. Thermobarometric calculations for the dykes indicate co-crystallisation of plagioclase (An_{81 − 55}) (1181–1083 ± 14 °C), clinopyroxene (Mg# 78 − 63) (1176–1115 ± 35 °C) and olivine (Fo_{81 − 55}) (1168–1135 ± 23 °C), at pressures of 3.5–0.1 (± 2.2) kbar (1σ errors). A universal groundmass feature of these dykes is interstitial rhyolitic glass, containing innumerable microlites of plagioclase (An_{42 − 21}), anorthoclase (An₆Ab_{69 − 66}Or₂₈₋₂₅), Fe-rich augite (Mg# 46 − 12), pigeonite (Mg# 32 − 15), Fe-olivine (Fo_15 − 4), Fe-Ti oxides, and apatite. Textures, whole-rock geochemistry, mineral and glass chemistry, thermobarometry, mass balance calculations, and rhyolite-MELTS modelling show that advanced (~ 76–94%) closed-system fractional crystallisation of anhydrous to hydrous (0.5 wt.% H₂O) tholeiitic dyke magmas at low pressures (~ 3–0.5 kbar) could generate the residual rhyolitic melts. Rapid cooling and solidification of the dykes at a shallow crustal depth quenched these melts to glass and thus trapped them within the crystal mush. This explains the notable lack of silicic extrusive or intrusive units in the Western Ghats volcanic sequence, despite the widespread production of silicic melts at depth. Such interstitial silicic glasses, common in tholeiitic flood basalts worldwide, are evidence for silicic magma genesis by fractional crystallisation, though without residual silicic melt segregation, intrusion and eruption.