An experimental petrologic and geochemical study of Deccan trap basalts : part I, Deccan traps from Tamia, central India ; part II, crystallization relationships of Deccan basalts at 6.35 kbars

New major, minor, and trace element data for the Chakhla- Delakari sill and three nearby lava flows in the proximal area of Tamia are presented. Based on abundances of trace elements, it is inferred that the sill was the feeder of the two uppermost lava flows which tapped magmas from a LREE-, Rb-enriched source. The oldest (lowermost) flow shows greater affinity with the western Deccan lavas. The association of the feeder sill with a large network of parallel trending sills and dikes, and the elongate Satpura Dome, is taken to infer that a major locus of eruption of the Deccan Trap lavas existed in the eastern part of the province. A model is presented for the evolution of the magma source of the sill and the two uppermost flows in a chamber beneath the Satpura Dome in the easternmost Deccan Traps. Results of equilibrium crystallization experiments at 6.35 kbars conducted on two compositions which represent probable near primary liquids are presented. One of these starting compositions has 9.5% MgO and the other 13% MgO. The sequence of crystallization in both starting liquids at 6.35 kbars is as follows: olivine; olivine + Ca- clinopyroxene; ol + Ca-clinopyroxene + Ca-poor clinopyroxene; olivine + Ca-clinopyroxene + Ca-poor clinopyroxene + plagioclase. Based on the present experimental data, it is inferred that typical Deccan magmas (MgO 5-9%) are produced by early fractionation of olivine, pyroxene (Ca-augite and pigeonite), and late fractionation of plagioclase. Deccan basalts with MgO 9-10% appear to have crystallized from melts lower in Ti02 than the starting materials of this study which fractionated olivine and plagioclase. Basalts with MgO2source which has fractionated olivine and plagioclase will produce of the bulk of Deccan basalts.