Geochemical Stratigraphy of the Deccan Traps at Mahabaleshwar, Western Ghats, India, with Implications for Open System Magmatic Processes

Detailed stratigraphy based on whole-rock geochemistry is presented for a 1200 m sequence of basaltic lava flows in the Western Ghats escarpment near Mahabaleshwar. Five separate sections are used to define a regional dip of approximately 0–5° to the SW. From the base upwards the following formations are described: Bushe, Lower Poladpur, Upper Poladpur, Ambenali, and Mahabaleshwar. Inter-formation boundaries, with the exception of the Upper Poladpur-Ambenali, are sharp, and are particularly well defined by breaks in Sr-isotopic composition. Two of the formation bases are marked by abnormally mafic flows— the Kamshedi picrite horizon at the base of the Upper Poladpur, and the Kelghar mafic unit at the base of the Mahabaleshwar. Major element compositions are controlled throughout largely by the degree of gabbro fractionation. Intense crustal contamination further modifies compositions in the lower part of the sequence (Bushe-Upper Poladpur) and has strong effects on trace elements and Sr-isotopes. Contamination decreases up-sequence leading to the comparatively uniform Ambenali rocks. The Mahabaleshwar Formation represents a change towards magmatism generated in an enriched mantle with many characteristics similar to those of oceanic island basalts. The geochemical discussion deals mainly with two well-developed mixing lines, one between Ambenali magmas and granitic crust, the other between ambenali magmas and the products of the postulated enriched mantle source. The detailed stratigraphic sequences strongly support the RTF (replenished, tapped, fractionated) magma chamber model of O‘Hara &Mathews (1981) and the idea of periodical replenishment by picritic magmas (e. g. Huppert & Sparks, 1980b). This is believed to be the first demonstration of such processes operating on a large scale in a continental basalt province.