Petrogenesis of fractionated basaltic lava flows of Poladpur-Mahabaleshwar formation around Mahabaleshwar, Western Ghats, India

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%, TiO2 from 1.8 to 4.6 wt%, SiO2 from 47 to 52 wt% and Al2O3 from 12 to15.5 wt%. The Mg number (Mg#) was much lower, ranging from 36 to 50. The K2O/ P2O5 ratio showed the role of assimilation in the basaltic flows. TiO2, Y, Zr, Nb and Mg# were used to determine fractional crystallization, whereas Ba, Rb, K2O and SiO2 were used for monitoring the fractional crystallization effects of crustal contamination. The range of Zr/Y and TiO2 > 1.8 wt% appears to have been generated by fractional crystallization starting from enriched mafic precursors.