0016-7622/2014-84-2-197/$ 1.00 © GEOL. SOC. INDIA JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.84, August 2014, pp.197-208 Petrogenesis of Fractionated Basaltic Lava Flows of Poladpur-Mahabaleshwar Formation around Mahabaleshwar, Western Ghats, India BABITA R. CHOUDHARY and GAJANANRAO N. JADHAV Deapartment of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai - 400 076 Email: jadhav@iitb.ac.in Abstract: Geochemical investigations of Wai sub-group volcanic flows (in and around Mahabaleshwar) have been undertaken to determine the petrogenetic processes involved in the formation of volcanic flows. In comparison to the Ambenali Formation, Mahabaleshwar Formation flows were affected more by crustal materials, which left a signature consisting of enriched levels of K, Rb, Ba, Ti and P. Ratios of Nb/Zr and Ba/Y were sensitive to fractional crystallization; Mahabaleshwar formation flows showed the highest Nb/Zr ratios. Ba was noted as a boundary marker element between the Ambenali (47.3 to 63.9 ppm Ba) and Mahabaleshwar (83.1 to 180 ppm) formations. The general trend of incompatible element concentrations increasing from lower Poladpur to upper Mahabaleshwar flows with increasing Zr and the linear array on the plot are consistent with the fractionation of olivine and clinopyroxene. MgO ranged from 4.8 to 7.1 wt%, TiO 2 from 1.8 to 4.6 wt%, SiO 2 from 47 to 52 wt% and Al 2 O 3 from 12 to15.5 wt%. The Mg number (Mg#) was much lower, ranging from 36 to 50. The K 2 O/ P 2 O 5 ratio showed the role of assimilation in the basaltic flows. TiO 2 , Y, Zr, Nb and Mg# were used to determine fractional crystallization, whereas Ba, Rb, K 2 O and SiO 2 were used for monitoring the fractional crystallization effects of crustal contamination. The range of Zr/Y and TiO 2 > 1.8 wt% appears to have been generated by fractional crystallization starting from enriched mafic precursors. Keywords: Deccan basaltic flow; Volcanism; Magmatic evolution; Crustal contamination; Geochemistry, Maharashtra. already established observed spectrum of variation in analyzed samples from the study area. SAMPLING LOCATIONS AND GEOLOGICAL SETTING For this study representative samples were collected from the road sections of an area within the Wai sub-group. The lava flows have been sampled at different heights within the three formations of the Wai sub-group (Fig 1b) using the geological map (Subbarao and Hooper, 1988). During the excursion, a vertical profile of the different flows was prepared. The studied section, located in the SW part of Maharashtra state in the Satara district, lies 17° 55’ N and 73°40’ E. The three formations of the Wai sub-group have a combined thickness of 1200 m at Mahabaleshwar and numerous flow exposures have been characterized in the study area (e.g., Najafi et al. 1981; Mahoney et al. 1982; Cox and Hawkesworth, 984, 1985). The present field excursion has identified 46 flows. The Wai sub-group actually consists of five formations: Poladpur, Ambenali, INTRODUCTION The Deccan volcanic province (DVP) in western India represents one of the largest accumulations of continental lava flows on earth. The DVP hosts the Deccan traps, a thick sequence of nearly horizontal basalt flows, that erupted at the Cretaceous - Tertiary boundary. Extensive petrological and geochemical studies have produced a well-defined chemical stratigraphy for the DVP (Mahoney et al. 1982; Cox and Hawkesworth 1985; Devey and Lightfoot 1986; Beane et al. 1986; Hooper et al. 1988; Lightfoot et al. 1990; Melluso et al. 1995; Greenough et al. 1998; Mahoney et al. 2000; Melluso et al. 2002; Higgins and Chandrasekharam 2007). The focus of this work is the part of the DVP (Fig 1a) in and around Mahabaleshwar, in SW Maharashtra, India, which stratigraphically falls in the Wai sub-group (the uppermost sub-group of the DVP). The present study includes a field-petrography and geochemical investigation of the study area. The main aims of the paper are (i) to geochemically characterize the Wai subgroup, flow by flow (Poladpur - Mahabaleshwar) and (ii) to correlate it with the