Proterozoic Deep Marine Volcanic Eruptive Product (Obsidian Glass) in Chanda Limestone, Penganga Group, India: Its Implications for Basin Evolution and Metallogenesis

Abstract

Subaqueous effusive to low-explosive eruptive product composed of blocky and fluidal fragments of glass and sparsely crystalline glass of obsidian lava are present in fine-grained volcaniclastic debris flow conglomerate and turbidite, occurring in a deep marine sequence of thin- and even-bedded grey siliceous micritic limestone of the Chanda Limestone. The siliceous limestone conformably encloses stratiform manganese deposit. Obsidian glass fragments consist of coherent and amorphous glassy groundmass containing patches of devitrified fibrous glass, oriented microlites and microphenocrysts of quartz and K-feldspars. Euhedral crystals, resorbed-rounded and embayed crystals, spherical vesicles, microlites, and flow texture are distinctive volcanic features observed in these fragments chemically similar to obsidian lava. The blocky fragments represent the products of hydroclastic fragmentation whereas the fludial one may indicate formation under steam explosion. Gravitational instability of the resulting slurry of glass fragments and the ambient sea water-sediments at the site of eruption causes gravity flows that carries the fragments along with epiclasts away from vent to a distal site for final deposition. This study records maiden evidence of volcanism in the Chanda Limestone of the Neoproterozoic Penganga Group and infers rifting and partial melting of the sialic crust and silicic lava flow during deposition of limestone unusually rich in silica. The inferred tectono-sedimentary setting, facies association and mode of occurrence of the manganese deposit suggest a hydrothermal source for the metal.