Early Cretaceous Lamprophyre Dykes from Nongchram Fault Zone, Meghalaya, Northeastern India

A swarm of alkaline and mafic dykes occurs in the Archaean Gneissic Complex of western Meghalaya, Northeastern India, spatially associated with a N-S trending deep-seated fault - Nongchram Fault. Lamprophyre constitutes the dominant member among the alkaline dyke rocks; other petrological variants include tinguaite, ijolite, trachyte and orthoclasite. Rare carbonatite is also present. Lamprophyre contains country rock fragments, as well as mantle derived ultramafic nodules, aggregated megacrysts and lapilli like structures. A feature characteristic of most lamprophyre dykes of the area is the presence of ocelli, commonly zoned, with carbonate-rich (rarely analcime) inner zone and an outer zone rich in alkali felspars, mica, pyroxenes and opaque minerals. Petrographically, the lamprophyres of the area are classified as camptonite, but they have some affinity towards ultramafic and carbonatitic lamprophyres. Field and petrographic studies suggest that the lamprophyre and carbonatite of Nongchram Fault Zone (NFZ) are genetically linked to a common parental magma, which differentiated by the process of liquid immiscibility to carbonate and silicate fractions, leading to the emplacement of discrete bodies. The best candidate for the parental magma appears to be mantle-derived alkali basalt magma. Lamprophyre and other sodic alkaline rocks and carbonatite in NFZ indicate rift-controlled magmatism triggered by a mantle plume. The spatial and temporal distribution of lamprophyre dykes (107 Ma) of the region with ultramafic-mafic-alkaline-carbonatite complexes of Assam-Meghalaya plateau and Kerguelen-related Sylhet-Rajmahal- Bengal Basin basalts (105-118 Ma) suggest their genetic link with early Kerguelen hotspot magmatism.