Recognition, characterization and implications of high-grade silicic ignimbrite facies from the Paleoproterozoic Bijli rhyolites, Dongargarh Supergroup, central India

A controversy regarding the distinction between the highly welded lava-like ignimbrites sometimes showing strongly rheomorphic characters, and the ext ensive silicic lava flow has been overwhelming in the recent literature. Ho wever, a rethinking, after Walker (1983), has brought into light the concep t of 'grade' referring to the degree and extent of welding between the pyro clasts. Various parameters and characteristics were suggested for strengthe ning the idea of densely welded ignimbrites, which differentiate them from lava. Here, a comprehensive study on early Proterozoic acid magmatic rocks forming lower part of the Dongargarh Supergroup, central India, has been ma de to suggest extensive occurrence of high-grade welded rheomorphic tuffs. The possibility of their being welded ignimbrite rather than lava flow has been explored in the light of facies analysis as well as detailed microscop ic evidences. Despite having overall monolithologic look various units bear distinction on account of their nature of welding, enrichment of phenocrys ts and degree of stretching. The presence of vitroclastic texture, melt inc lusions and radial fracturing of phenocrysts suggests pyroclastic nature of these deposits. Based on these characters four facies - A, B, C and D from bottom to the top respectively, have been identified from field studies ar ound Salekasa. Facies-A and B represent clast-supported/matrix-supported we lded pyroclastic flow deposits. Facies-C represents extremely welded thinly laminated rheomorphic tuffs while lava-like tuffs with an autobreccia cara pace is represented by facies D. A complete gradation of facies A/B to D th rough C exists. High to extremely high-grade nature of welding in these dep osits suggests a low column-height subaerial plinian to fissure eruption of a very high temperature silicic magma in a continental setting.