First Report of Tuff in Neoproterozoic Bhima Basin: Field Evidence, Petrography and Geochemical Studies

Abstract

Abstract We report here the presence of tuffs from the Neoproterozoic Bhima basin unreported hitherto. These are exposed near Hotpet, Hulkal, Kanchankavi, and Ukinal area along two different stratigraphic horizons (1) within carbonates of Shahabad Formation and (2) above shale of Hulkal Formation. Tuffs are of two types: 1. Ash fall tuff and 2. Lapilli tuff, occurring as beds, matrix of a conglomeratic unit and patchy discontinuous layers. Physically ash fall tuff is thinly bedded (2-6 cm) and light-weight, lapilli tuff is hard, compact and individual beds are not discernible in the field. Presence of petrological features like bipyramidal quartz, fiamme structures, embayed quartz grains, blue CL colours of quartz grains, volcanic layering and banding, altered glass, and stretched and elongated volcanic glasses, are clear indications of its volcanic affinity. The tuff is anomalously high in silica content ranging from 71.4-91.2wt% and have low alumina value ranging from 0.75-4.11 wt % compared with that of the normal felsic tuffs, rhyolites and high silica rhyolites. REE in the tuffs can be grouped into two types, 1. Ash fall tuff with enriched LREE and fractionated HREE, 2. Lapilli tuff with flat LREE and HREE pattern. Tuff shows high Hf, overall enrichment of large ion lithophile elements (LILE Rb, Th, K and Pb) and depletion in HFSEs (Nb, Ta, Ti, P). The tuff is probably generated from continental crust source and the depositions are episodic and have more than one eruptive pulses of volcanic activity.