The Chogat-Chamardi subvolcanic complex, Saurashtra, northwestern Deccan Traps: Geology, petrochemistry, and petrogenetic evolution

Abstract

The Saurashtra region in the northwestern part of the Deccan continental flood basalt province (India) is notable for its many volcano-plutonic complexes, compositional diversity, and the abundance of rhyolite and granophyre. In this first detailed study of the Chogat-Chamardi subvolcanic complex in eastern Saurashtra, we report microgranite and granophyre plutons, gabbro plutons cut by basaltic andesite dykes, as well as rhyolite plugs. Bulk-rock geochemical (including Nd-Sr isotopic) data suggest a strong contribution from ancient, Rb-rich basement crust to the silicic magmas. The mafic rocks are also crustally contaminated but less than the silicic rocks, in conformity with observations elsewhere (e.g., the Skye and Skaergaard granophyres). The Chogat-Chamardi silicic rocks have initial (87)Sr/(86)Sr (for t = 65 Ma) as high as 0.72914, and initial epsilon(Nd) values as low as -13.9. Trends defined by the Chogat-Chamardi and other Deccan rhyolitic suites on plots of Sr concentration vs. Sr isotopic ratio are modeled with concurrent assimilation and fractional crystallization (AFC) processes involving a basaltic magma and granitic basement rocks. Considering both Nd and Sr isotopic variations, the Chogat-Chamardi silicic rocks notably require crustal end members very different from those inferred in most previous isotopic studies of Deccan rocks. (C) 2011 Elsevier Ltd. All rights reserved.