A Simple Petrologic Model for the Generation of Deccan Trap Magmas

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Abstract
The present study explores, on the basis of experimental phase equilibria of volatile-bearing mantle lherzolite, how tholeiitic, carbonatitic, and alkaline magmas all could have been produced from the Deccan plume. Deccan tholeiite parent magmas (olphyric, MgO = 7.5%) were produced by as much as 18 to 25% olivine fractionation from primary magmas containing about 14 to 15% MgO. Such olivine fractionation could have occurred at deeper levels (deep crust or mantle). The major oxide-oxide trends of tholeiites that define the liquid lines of descent (LLD) emanating from Deccan parental magmas with ∼7.5% MgO are a result of gabbroic fractionation (and mixing) at very low pressure (P ∼2kb) in crustal dikes and sills. Small increments of magma were extracted as the partially melted plume ascended adiabatically over a depth range of 100 to 60 km. Carbonatites and alkaline lavas all were generated by extremely low degrees of melting (F <0.01) from the cooler, more volatile-dominated apron of the plume head. Separation and extraction of these magmas may have been aided by the rift zones, such as SONATA. It also is possible that the volatile-rich parts of the plume disintegrated from the main plume and melted on the way up. Considerable uplift and faulting of the Indian landmass must have occurred in the Deccan region throughout the Cenozoic, as a result of a complex set of forces operating in this part of the Indian plate.