TRACKING KERGUELEN PLUME RELATED SOURCE FOR SAMCHAMPI-SAMTERAN ALKALINE COMPLEX, MIKIR HILLS, NORTHEASTERN INDIA: IMPLICATIONS FROM SR-ND ISOTOPE SYSTEMATICS

The Samchampi (26° 13´ N: 93° 18´E)-Samteran (26° 11´N: 93° 25´E) alkaline complex occurs as a plug-like intrusion within Precambrian gneisses in the Mikir hills of Northeastern India. The complex comprises a wide variety of lithologies, including syenites, ijolite-melteigite, alkali pyroxenite, and alkali gabbro. Carbonatite and nepheline syenite criss-cross the pluton as later dykelets.

The eruption of Bengal-Sylhet-Rajmahal flood basalts on the eastern and northeastern Indian shield (~117 Ma) is believed to mark the initiation of Kerguelen plume activity beneath the Indian plate. The Samchampi-Samteran complex represents one of the important differentiation centres of Bengal-Sylhet-Rajmahal basalts (BSRB) and manifests ultramafic-mafic-alkaline-carbonatite magmatism in the Mikir Hills.

The geochemical variation trends of the Samchampi-Samteran complex show differences in concentration between the high field strength elements, such as Y, Zr, Nb, P, and the rare earth elements between felsic and mafic units and preferential partitioning of these elements in the mafic-ultramafic members of the complex. The data favour liquid immiscibility as a dominant petrogenetic process for the formation of this complex. Distinct positive Nb and Ta anomalies and associated geochemical features clearly point towards lower degree partial melting of a metasomatized mantle in an intracontinental rift setting.

The overlapping ⁸⁷Sr/⁸⁶Sr_initial ratios of alkaline-silicate and carbonatite members of the Samchampi-Samteran complex suggest that the lithomembers are cogenetic, having been derived from a common parent magma, which is also supported by overlapping ¹⁴³Nd/¹⁴⁴Nd_initial ratios for these rock types. The ε_Sr - ε_Nd isotopic signatures (higher ε_Sr, lower ε_Nd) of rocks from the Samchampi-Samteran complex clearly point towards a Kerguelen plume-related enriched mantle (~EM II) source. Sr_initial and Nd_initial (105 Ma) isotopic compositions (⁸⁷Sr/⁸⁶Sr between 0.705562 and 0.724717 and ¹⁴³Nd/¹⁴⁴Nd between 0.511990 and 0.512449, corresponding to ε_Nd values of -10 to -1) are in line with those of Rajmahal tholeiites and Kerguelen plateau basalts. The overall geochemical character and isotopic compositions fingerprint a Kerguelen plume origin for the Samchampi-Samteran alkaline complex.