Mafic/Ultramafic xenoliths from Saurashtra peninsula of Gujarat; northwestern Deccan Trap, India

The crustal growth forming processes at the crust-mantle interface or within the crust due to magma underplating is important for the formation and emplacement of continental flood basalt and large igneous provinces. Mafic/ultramafic xenoliths from lower crust or upper mantle provide clue to characterize the underplated material and magmatic processes. Earlier study of ultramafic xenoliths suggested magma underplating and crustal growth in Kuchchh, Gujarat, northwestern Deccan Trap (NWDT). Absence of such xenoliths in Saurashtra peninsula (SP) of NWDT however could not supplement this. Here, we report the mafic/ultramafic xenoliths entrained in high MgO basaltic lava flows of NWDT of SP in Rajkot district of Gujarat, India. The xenoliths are medium to coarse grained, meso - to melanocratic, elongated to angular pyroxenite (Type-I), two pyroxenes gabbro (Type-II) and anorthosite (Type-III) showing sharp contact with host basalt flows. Type-I xenoliths dominated by clinopyroxene (cpx) (Wo49-45 En49-38) with olivine (ol) (Fo84-78), exhibit cumulate texture, Type-II composed of cpx (core-Wo49-48 En42-41), orthopyroxene (opx) (core- En77-76 Fs23-22) and plagioclase (plag) (Ab35-28 An71-64) and Type-III, composed dominantly of plag (Ab67-29 An68-28) with minor opx (En78-76 Fs20-18) and a grain of hercynite (Al2O3=59%) in close association with plag. The basaltic lavas are porphyritic containing ol (Fo88-75), cpx (Wo50-48 En39-37), plag (Ab43-26 An74-54) and opaques. Whole rock geochemical data of xenolith entrained lava flows indicates high MgO (10-11 wt%) with high Ni (421-430 ppm) and Cr (795-1076 ppm). The equilibration temperature calculated from cpx - opx (adjacent grain of cpx and opx, pair-A; inclusion of cpx within opx, pair-B) for Type-II xenolith indicates 778°C and 789°C (pair-A) and 821°C and 832°C (pair-B) at 5 kbar and 10 kbar pressure respectively. Present study suggests that the possibility of magma underplating at crust-mantle interface or presence of layered igneous sill/dyke complex at sub-crustal level. This is corroborated by seismic and other geophysical studies suggesting the magmatic underplating and presence of high velocity layer (Vp 7.1 km/s) with high Poisson's ratio at lower crustal level or crust-mantle interface beneath the NWDT of SP, Gujarat.