Nd and Sr isotope systematics and geochemistry of a plume-related Early Cretaceous alkaline-mafic-ultramafic igneous complex from Jasra, Shillong plateau, northeastern India

Abstract

This article describes an Early Cretaceous Jasra alkaline-mafic-ultramafic igneous complex related to the Kerguelen hotspot-mantle plume system of the Indian Ocean. This complex, emplaced in the Shillong plateau, consists mainly of pyroxenite, gabbro, and nepheline syenite and is closely associated with 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 with gabbro, cut pyroxenite and granite bodies. Mineral compositions classify the pyroxenites into pyroxenite and alkali pyroxenite and the gabbros into essexite and olivine gabbro. The chemical and normative compositions of mafic-ultramafic rocks also show their alkaline nature. Nepheline syenite samples are miaskitic in character (agpaitic index < 1),suggesting the involvement of a 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 these rock units have different genetic histories. Major-element-based discrimination function diagrams suggest the ocean island basalt (OIB) affinity of the Jasra samples. The OIB nature of these samples is further corroborated by their radiogenic isotopic compositions. Sr and Nd isotopic compositions (⁸⁷Sr/⁸⁶ Srinitial between 0.706523 and 0.708891 and ¹⁴³Nd/¹⁴⁴ Nd_initial between 0.512258 and 0.512464) suggest that these rocks were derived from a mixing of mantle components such as HIMU (mantle with a high U/Pb ratio) and EM (enriched mantle) components, which show an isotopic composition similar to FOZO (focal zone) composition. Lherzolite mantle was metasomatized into an alkaline wehrlite by CO₂, released by low-degree melting of a carbonated mantle peridotite. Melting of such a metasomatized mantle source may produce ultrabasic alkaline silicate magma, from which the different rock units of the Jasra Complex were crystallized. The geological, geochemical, geochronological, and isotopic data also suggest a spatial and temporal association with the Kerguelen plume activity.