Windowing petrogenesis of continental flood basalts through mineralogical investigations: a case study from the Eastern Deccan Volcanic Province

Abstract

Evaluation of the petrogenetic history of continental flood basalt using mineral–chemical data has been sparingly used for several flood basalt provinces of the world. For the Deccan flood basalts, such attempts are still rudimentary. The present contribution highlights several facets of petrogenesis of the Eastern Deccan Volcanic Province using details of constituent phases including glass. The present-study area around Khandwa (21°49′N, 76°21′E) indicates the presence of three distinct lava flows, which were identified on the basis of physical volcanological features based on a three-tier classification. Moreover, the occasional presence of chilled dykes and feeder dykes has been documented. The lava succession rests above (apparent) cumulate gabbro basement. The lava flows, in general, are characterized by dominant pyroxene (mostly augite, locally pigeonite) and plagioclase, opaque (magnetite and ilmenite) and glass, whereas olivine is rare. Both of these principal mineral phases (namely, plagioclase and pyroxene) occur both as phenocrysts and groundmass, while opaque phases occur only as groundmass. Pyroxene and plagioclase are generally zoned due to the changing milieu of crystallization conditions. Pyroxene thermometry data show the equilibration temperature range of 1050–1300 °C. Except for two samples of Flow II and one sample of chilled zone, the pyroxene thermometry data are broadly similar to that obtained from magnetite–ilmenite thermometry. As expected, deduced glass thermometric data reveal a relatively higher temperature. Crystallization history of parental magma was quantitatively modeled using Petrolog 3 software. The results indicate that the parental magma underwent both equilibrium and fractional crystallization (for example initially crystallized olivine reacted with ambient liquid to give rise to pyroxene). Phenocrystal and groundmass plagioclase and pyroxene show distinct patterns of compositional zoning corresponding to fluctuating physical parameters within the magma. Some of the glass [especially from the chilled dyke, feeder dyke, and Upper Colonnade Zone (UCZ) of flow II and flow III] represents ‘higher magnitude pressure ambience’ (fossilized glass) which is distinct from ‘surficial glass’ caused by equilibrium quenching. The glass and pyroxene compositions (using tectonic discrimination diagrams) suggest a non-orogenic continental setting of the parent magma. It is concluded that the studied Deccan volcanism shows a complex interplay of repeated pulses of magma ascent, fluctuation of crystallization condition, effervescence, and local magma mixing.