Evidence of sub-continental lithosphericmantle sources and open-system crystallization processes from in-situ U–Pb ages and Nd–Sr–Hf isotope geochemistry of the Cretaceous ultramafic-alkaline-(carbonatite) intrusions from the Shillong Plateau, north-eastern India

Abstract

Newin-situU–Pb ages and Sr–Nd–Hf isotopic data on mineral phases of the Sung Valley and Jasra ultramafic-alkaline-(carbonatite) intrusions (Shillong Plateau, India) shed new light on the petrogenetic processes of volca-nism in north-eastern India during the Cretaceous. Perovskites of Sung Valley dunite, ijolite and uncompahgriteyieldedU–Pbagesof109.1 ± 1.6, 104.0 ± 1.3 and 101.7 ± 3.6Ma,respectively. A U–Pbage of 106.8± 1.5 Mawasobtained on zircons of a Sung Valley nepheline syenite. Perovskite of a Jasra clinopyroxenite yielded an age of101.6 ± 1.2 Ma, different from the U–Pb age of 106.8 ± 0.8 Ma on zircon of Jasra syenites. The variation in Sr-Nd-Hf isotopic compositions [initial87Sr/86Sr = 0.70472 to 0.71080;εNd i=−10.85 to +0.86;εHf i=−7.43to +1.52] matches the bulk-rock isotopic composition of the different rock units of Sung Valley and Jasra. Calciteand apatite in the carbonatites, the perovskite in a dunite, and the bulk-rock carbonatites of Sung Valley intrusionhave the lowest initial87Sr/86Sr andεNd, taken to be the best proxies of the mantle source composition, which isdominated by components derived from the lithospheric mantle. The alkaline intrusions of north-eastern Indiaare significantly younger than the Sylhet tholeiitic magmatism. The silicate rocks of both intrusions have isotopiccomposition trending to that of the underlying Shillong crust, indicating the effects of fractional crystallizationand low-pressure crustal contamination during the emplacement of the various intrusive magma pulses.