Subaqueous effusive and explosive phases of late Deccan volcanism: evidence from Mumbai Islands, India

Abstract

About 30 m of massive basaltic lava sheets and pillows belonging to the Deccan Traps are exposed at Borivali in the Sanjay Gandhi National Park, Mumbai (India). These lavas are conformably overlain by ~ 100 m of volcaniclastic deposits exposed in the Kanheri Caves. The pillows are plagioclase-clinopyroxene-phyric, very poorly vesicular, tholeiite basalts and commonly develop one to few central large cavities. Pillow rinds are holohyaline with swallow-tailed lanceolate or slender needle-shaped plagioclase and fan-spherulitic or dendritic textures in clinopyroxene that indicate rapid cooling under water. The volcaniclastic succession lying on top of the pillow lavas is chiefly composed of non-vesicular to scoriaceous/pumiceous, basaltic and andesitic clasts. The numerous sediment dykes within the volcaniclastic succession and the conformably underlying pillow lavas imply that the entire sequence was emplaced in a submarine environment. The fluidal texture and quenched margins in some of the volcanic clasts strongly suggest that at least part of the volcanic clasts were produced by underwater explosive eruptions. This study describes subaqueous erupted pillow lavas in context of continental flood basalts and provides a significant analogy to recent submarine-erupted intraplate oceanic basalts. The occurrence of pillow lavas and volcaniclastic deposits well above the present-day high tide line (160 m) indicates either high eustatic sea level during the Danaian age (61.8–62.9 Ma), or post-rifting regional uplift in the western Deccan Traps.