Nd and Sr isotope systematics and geochemistry of plume related early Cretaceous alkaline-mafic-ultramafic igneous complex from Jasra, Shillong

Abstract – This paper describes an early Cretaceous Jasra alkaline-mafic-ultramafic igneous complex related to Kerguelen hotspot-mantle plume system of Indian Ocean. This complex, emplaced in the Shillong plateau, consists mainly of pyroxenite, gabbro and nepheline syenite and is closely associated to the Barapani-Tyrsad shear zone, Kopali faults, and Um Ngot lineaments. Pyroxenite and gabbro occur as separate plutons, whereas nepheline syenites occur either in the form of small dikes in pyroxenites or as differentiated bodies in the gabbros. A few mafic dikes, contemporaneous to gabbro, cut pyroxenite and granite bodies. Mineral compositions classify the pyroxenites into pyroxenite and alkali pyroxenite and the gabbros into essexite and olivine gabbro. Chemical and normative compositions of mafic-ultramafic rocks also show their alkaline nature. Nepheline syenite samples are miaskitic in character (agpaitic index < 1) suggesting involvement of CO2 related phase in their genesis. Chemical data do not support any simple genetic relationship between the different rock units of the complex. Field-data also support this conclusion and suggest that they have different genetic histories. Major element based discrimination function diagrams suggest ocean-island basalt (OIB) affinity of Jasra samples. OIB nature of these samples is further corroborated by their radiogenic isotopi