Geochemistry and petrogenesis of sodic and potassic mafic alkaline rocks in the Deccan Volcanic Province, Mumbai area (India)

Major element, trace element, Sr- and Nd-isotopes and mineral chemical data are reported for alkaline rocks (lamprophyres, tephrites, melanephelinites, nephelinites and nepheline syenites) cross-cutting the Deccan Trap lava flows south (Murud-Janjira area) and north of Mumbai (Bassein). These rocks range from sodic to potassic and have a large span in MgO (12???2???wt%). The lamprophyres have high content of incompatible elements (e.g., TiO2???>???3.8 wt%, Nb???>???130???ppm, Zr???>???380???ppm, Ba???>???1200???ppm), and relatively high initial (at 65???Ma) 143Nd/144Nd (0.5128) and low 87Sr/86Sr (0.7038???0.7042). They are likely to be small-degree melts (2???3%) of volatile- and incompatible element-enriched mantle sources, similar to other alkaline rocks in the northern Deccan, though slightly more potassium-rich. The nepheline-rich rocks have highly porphyritic textures (up to 57% phenocrysts of diopside???±???olivine), and anomalously low contents of incompatible elements (e.g., TiO2???<???1.3???wt%, Nb???<???24???ppm, Zr???<???100???ppm) indicating that they could not represent liquid compositions. Moreover, their very low initial 143Nd/144Nd ratios (0.5116???0.5120), at 87Sr/86Sr???=???0.7045???0.7049, are unusual in the rocks related to the Deccan Traps and identify a new end-member in this province, that could be identified as ???Lewisian-type??? lower crust and/or enriched mantle. The melting episode that generated these alkaline rocks likely occurred close to the base of the ca. 100???km-thick Indian lithosphere, very shortly after the main eruption of the Deccan tholeiites.