Early Cretaceous ultramafic-alkaline-carbonatite magmatism in the Shillong Plateau-Mikir Hills, northeastern India – a synthesis

Abstract A comprehensive mineralogical, geochemical and isotopic review of six ultramafic-alkaline-carbonatite magmatic intrusions the Shillong Plateau (Sung Valley, Jasra, Swangkre-Rongjeng, Mawpyut) Mikir Hills (Samchampi-Samteran Barpung) is presented here, using published data. These emplaced ca. 115–102 Ma ago, thus are significantly younger than tholeiitic flood basalts erupted in Rajmahal-Sylhet province (ca. 118–115 Ma). The intrusive lithologies vary from ultramafic (dunites, clinopyroxenites, melilitolites) to mafic (ijolites, gabbros sensu lato , shonkinites), felsic (syenites, nepheline syenites) carbonatites (mostly calcite-rich varieties). volcanic-subvolcanic facies (lamprophyres, phonolites) not abundant. range chemical compositions phases various assemblages notable; rocks result crystallization magmas variably evolved, independent liquid-lines-of descent, generally alkaline/strongly alkaline lineages sodic-to-potassic affinity. large variations Sr–Nd ratios silicate ( ) suggest a role shallow-level crustal contamination during their formation. Sung Valley Samchampi-Samteran have different isotope associated rocks, some affinity with Group I Rajmahal Traps an ultimate genesis carbonate-bearing lithospheric mantle.