LAUSR

Giant plagioclase basalts, (GPBs) that contain, plagioclase mega-crystals either alone or in parallel groupings or clusters (> 2cm and some with up to 10 cm long; Figure 1), were first reported from the Deccan Volcanic Province ( DVP) by Sowani and Peshwa (1964) from the Purandar Fort area in Maharashtra and later described by Karmarkar et al. (1971) from a number of localities (20 nos.) from the Western Ghats. Detailed field mapping and chemo-stratigraphic studies in the Western Ghats by the Geological Survey of India and the Indo-US programme on the DVP between Department of Earth Sciences, IIT Powai, Mumbai and the Department of Geology, Washington State University, Pullman, Washington, USA, besides other Groups Although volumetrically very minor, the GPBs or their petrological equivalents, found as flows, dykes and sills, have also been reported, often with different terminology from a number of regions of the world ranging in age from the Archean to Recent. These include the Scottish Hebridian Tertiary Volcanic Province (Big feldspar basalts), Columbia River Basalts, USA, Emeishan continental flood basalt province, China, the Keweenavan flood basalts of the Lake Superior region (‘Daisy stone’ lava flows with radiating balls of euhedral plagioclase) and the plagioclase, megacryst–bearing basalts in the Archean terrains of Canada, West Greenland, and Australia, emplaced within greenstone belts or high-grade gneiss terrains besides those found in MORB systems, Ocean Is., and oceanic plateau settings (c.f. Sheth, 2016, p. 917 and references therein). Recent studies on plagioclase ultraphyric basalts (PUBs) from Iceland, have enabled a better understanding of the formation of such basalts (Neave et al., 2014). The seven GPB flows, that have been used as ‘marker horizons’ in the Western Ghats are the most evolved (4-6% MgO; Mg# 52.8 – 39) and show considerable variation in their major and trace element abundances with An contents ranging from An59 An64. In spite of such diversities among them, it has been shown by Hooper et al. (1988) that each one of the GPBs from the Kalsubai subgroup, has a distinct chemistry as manifested in the TiO2 vs. Zr plot and Sr-isotope characters (Hooper et al., 1988) and recommends such geochemical routes to discern any challenges that may occur during field traverses while mapping the GPBs across ravines and Ghats that cannot be walked out so as to ‘distinguish unequivocally between them’ (c.f. Hooper et al., 1988, p.135). Thus for the DVP as a whole, variations in GPBs can be present since such a diversity depends on the parental magma and its evolution from the mantle source region and en route to the magma chamber from where they have paused, further evolved and erupted. Field, petrological, mineralogical and geochemical studies on GPBs thus have a direct bearing on a variety of magma chamber processes, such as assimilation crystal fractionation (ACF), assimilation and pseudo-equilibrium fractionation, differentiation (both crystal settling and floatation), crystal growth-duration and magma mixing from RTF (refilled, trapped and fractionated) (RTF) magma-plumbing systems (O’Hra and Mathews, 1981; Cox, 1988). Extrapolations of growth rates of plagioclase mega-crysts on eruption rates and the volume of magma erupted during DVC have also been postulated (Borges et al., 2014 and references therein). The GPBs differ from the porphyritic basalt flows, commonly found in numerous places in the DVP, especially in areas where a differentiated suite of basalts are found (e.g. Fig.1. Giant plagioclase basalt (GPB) showing the large plagioclase phenocryts either alone or in clusters from the Kalsubai Subgroup, top of Bhimashankar Formation, Ane Ghat.