Trace Element Variations in Deccan Basalts: Roles of Mantle Melting, Fractional Crystallization and Crustal Assimilation

The variation in incompatible trace element contents of lower basaltic dykes and flows from different parts of the Deccan province was assessed through modelling of partial melting of primitive mantle and fractional crystallization of the parental melts coupled with crustal assimilation (AFC). Variations in Ba/Zr, Rb/Y and Nb/Y can be attributed to different degrees of partial melting of mantle. Variations in Zr, rare earth elements and Rb/Nb indicate fractional crystallization and significant coupled crustal assimilation away from the Deccan plume center. The Zr variation near the plume center can be explained through only fractional crystallization without the involvement of crust. AFC modelling of available Sr-Nd isotope data requires very high and unreasonable amounts of crustal assimilant implying that the parental magmas may have acquired their isotopic characteristics before AFC occurred in crustal magma chambers. The occurrence of AFC away from the plume center in the Narmada-Tapti rift region may be related to longer and greater magma-wall rock interaction in shallow crustal magma chambers due to crustal extension-related enlargement of the magma chambers, recharge with fresh, hot magma and convective mixing.