THE GEOCHEMISTRY AND PETROGENESIS OF THE LATE-CRETACEOUS PICRITES ANDBASALTS OF CURACAO, NETHERLANDS-ANTILLES - A REMNANT OF AN OCEANIC PLATEAU

The island of Curacao in the southern Caribbean Sea is composed mainly of a thick sequence (> 5 km) of pillow lavas, grading upwards from pi crites at the base of the exposed section, to basalts nearer the top. Modelling suggests that picrites are related to the basalts by fractio nal crystallisation. Initial radiogenic isotope ratios of the picrites have a restricted compositional range: epsilon(Nd) = +6.1 to +6.6, Sr -87/Sr-86 = 0.70296-0.70319; whereas the basalts display a wider range of compositions: epsilon(Nd) = +6.6 to +7.6, Sr-87/Sr-86 = 0.70321-0. 70671. This variation in isotope ratios between basalts and picrites m ay be due to the assimilation of altered oceanic crust (or possibly pa rtial melts of such crust) by a picritic magma along with fractional c rystallisation. The relatively narrow range of Nd and Pb isotopic comp ositions in the Curacao lavas suggests either that the source region w as homogeneous, or that melts from a heterogeneous mantle source were well mixed before eruption. Chondritic to slightly light rare earth el ement enriched patterns, combined with long-term light rare earth elem ent depletion (positive epsilon(Nd)), suggest that the lavas were form ed by polybaric melting of spinel Iherzolite, with small a contributio n from garnet Iherzolite melts. High-MgO lavas, the absence of a subdu ction related chemistry, and the chemical similarity to other oceanic plateaux, suggest a mantle plume origin for the Curacao lava successio n. The Curacao volcanic sequence is part of an oceanic plateau formed at about 88-90 Ma, fragments of which are dispersed around the Caribbe an as well as being obducted onto the western margin of Colombia and E cuador. The occurrence of high-Mg lavas throughout this Cretaceous Car ibbean-Colombian igneous province requires anomalously hot mantle (> 2 00 degrees C hotter than ambient upper mantle) over a large part of a putative plume head, which is inconsistent with some mantle plume mode ls.