Late Cenozoic alkaline volcanism in the northwestern Caribbean: tectonic setting and Sr isotopic characteristics

A province of alkaline volcanism has developed over the last 10 m.y. in the northwestern part of the Caribbean plate. Most of the volcanism is Quaternary in age and follows an apparent halving of the spreading rate at the Cayman Rise spreading center 2.4 m.y. ago. Intraplate deformation in Central America and the Nicaraguan Rise has produced a series of north-south orientated grabens. This extensional tectonism is associated temporally and spatially with some of the alkaline magmatism. Strontium isotopic ratios of rocks from sixteen of these centers of volcanism enable three separate areas with different isotopic characteristics to be identified. The largest area corresponds to the Nicaraguan Rise and is characterized by low⁸⁷Sr/⁸⁶Sr ratios (0.7026–0.7031). A more concentrated area of alkaline magmatism in northeastern Costa Rica has intermediate⁸⁷Sr/⁸⁶Sr ratios (0.7036–0.7038) which are within the range shown by the adjacent calc-alkaline volcanoes. In central Hispaniola high⁸⁷Sr/⁸⁶Sr ratios (0.7047–0.7063) are found in strongly alkalic rocks and in rocks that are transitional to calc-alkaline in nature. In both Costa Rica and Hispaniola the increased radiogenic strontium may have come from volatile-rich fluids escaping from adjacent subducting slabs of oceanic crust. The isotopic differences between the two areas may be related to the relative longevity and high rate of subduction in Costa Rica compared to Hispaniola. The Costa Rican alkaline rocks overlie a segment of the Cocos plate which is being subducted at a smaller angle (～ 35°) than at the rest of the Central American arc.