Mantle sources and crustal input as recorded in High-Mg Deccan Trap basalts of Gujarat (India)

Near-primitive picritic basalts in the northwestern Deccan Traps have MgO > 10 wt.% and consist of two groups (low-Ti and high-Ti) with markedly different incompatible element and Nd-Sr-Pb isotope characteristics. Many elemental characteristics of the low-Ti picritic basalts are similar to those of transitional or normal ocean ridge basalts. However, values of ratios like Ba/Nb (13-30) and Ce/Pb (4-11), and isotopic ratios e.g., epsilon(Nd)(t) +0.3 to -6.3 (Pb-207/Pb-204), 15.63-15.75 at (Pb-206/Pb-204 18.19-18.84, delta O-18(olivine) as high as +6.2 parts per thousand) are far-removed from ocean-ridge-type values, indicating a significant contribution from continental crust. The crustal signature could represent crustal contamination of ascending magmas; alternatively, it could represent a minor component within the Indian lithospheric mantle of anciently subducted sedimentary material or fluids derived from subducted material. In contrast, the high-Ti picritic basalts are chemically and isotopically rather similar to recent shield lavas of the Reunion hotspot (e.g., epsilon(Nd)(t) +2 to +4) and to volcanic rocks along the postulated pre-Deccan track of this hotspot in Pakistan. Neither type of picritic basalt is parental to the voluminous flows comprising the bulk of the Deccan Traps. However, many of the Deccan primary magmas could have been derived from mixtures of a high-Ti-type, Reunion-like source component and a component more similar to, or even more incompatible-element-depleted than, average ocean-ridge mantle.