AN OCEANIC FLOOD-BASALT PROVINCE WITHIN THE CARIBBEAN PLATE

The thick oceanic crust of the Caribbean plate appears to be the tecto nized remnant of an eastern Pacific oceanic plateau that has been inse rted between North and South America. The emplacement of the plateau i nto its present position has resulted in the obduction and exposure of its margins, providing an opportunity to study the age relations, int ernal structure and compositional features of the plateau. We present the results of Ar-40-Ar-39 radiometric dating, major-, trace-element, and isotopic compositions of basalts from some of the exposed sections as well as drill core basalt samples from Leg 15 of the Deep Sea Dril ling Project. Five widely spaced, margin sections yielded ages ranging from 91 to 88 Ma. Less well-constrained radiometric ages from the dri ll cores, combined with the biostratigraphic age of surrounding sedime nts indicate a minimum crystallization age of similar to 90 Ma in the Venezuelan Basin. The synchroneity of ages across the region is consis tent with a flood basalt origin for the bulk of the Caribbean plateau (i.e., large volume, rapidly erupted, regionally extensive volcanism). The ages and compositions are also consistent with plate reconstructi ons that place the Caribbean plateau in the vicinity of the Galapagos hotspot at its inception. The trace-element and isotopic compositions of the similar to 90 Ma rocks indicate a depleted mantle and an enrich ed, plume-like mantle were involved in melting to varying degrees acro ss the plateau. Within the same region, a volumetrically secondary, bu t widespread magmatic event occurred at 76 Ma, as is evident in Curaca o, western Colombia, Haiti, and at DSDP Site 152/ODP Site 1001 near th e Hess Escarpment. Limited trace-element data indicate that this phase of magmatism was generally more depleted than the first. We speculate that magmatism may have resulted from upwelling of mantle, still hot from the 90 Ma event, during lithospheric extension attending gravitat ional collapse of the plateau, and/or tectonic emplacement of the plat eau between North and South America. Still younger volcanics are found in the Dominican Republic (69 Ma) and the Quepos Peninsula of Costa R ica (63 Ma). The latter occurrence conceivably formed over the Galapag os hotspot and subsequently accreted to the western edge of the platea u during subduction of the Farallon plate. (C) 1998 Elsevier Science B .V.