The core of the Caribbean plate is comprised of oceanic crust that appears to be the remnants of an oceanic plateau (e.g., Duncan and Hargraves, 1984; Sinton et al., 1998). The study of this oceanic plateau has been predominantly centered on its obducted margins that are subaerially exposed along the perimeter of the plate (e.g., Klaver, 1987; Sen et al., 1988; Alvarado et al., 1997; Kerr et al., 1997; Sinton et al., 1997) because these areas are relatively accessible. However, the relationship between the obducted margins and the intact plateau has an element of uncertainty, so it is preferable to sample the plateau directly by drilling. Presently, the Caribbean crust has been penetrated by drilling during Leg 15 of the Deep Sea Drilling Project (DSDP) and most recently during Leg 165 of the Ocean Drilling Program.
DSDP Leg 15 drilled into the uppermost few meters of the oceanic crust at five sites: three in the Venezuelan Basin (Sites 146, 150, and 153), one from the Beata Ridge (Site 151), and one from the Nicaraguan Rise/Hess Escarpment (Site 152) (Fig. 1). The Colombian Basin basement was not sampled. The drilled units are thick, coarse-grained basaltic sills or flows overlain by, or intruding, foraminiferal limestone (Donnelly et al., 1973). Fossils within the overlying or intruded sediments are as old as Coniacian (89.0-85.8 Ma; Gradstein et al., 1994) in the Venezuelan Basin. The extraordinary thickness of the crust (as thick as 15-20 km in some areas) and the similar ages of sediments over the smooth basement in the Venezuelan Basin led Donnelly (1973) to conclude that the region had experienced a flood basalt event. Site 152 differs from the other DSDP sites in that it is located on thinned crust between the Hess Escarpment and the Beata Ridge and the recovered flows are surrounded by early Campanian-age sediments.
Although the main objectives of Leg 165 were to recover the sedimentary record within the Caribbean region, a secondary objective was to penetrate the igneous basement beneath the sediments. This objective was reached at Site 1001 in the southern-most edge of the lower Nicaraguan Rise along the Hess Escarpment (Fig. 1), approximately 40 km southwest of DSDP Site 152. Basaltic basement rocks were drilled in Hole 1001A for a total penetration of 37.65 m with 20.5 m recovered.
Detailed descriptions of the lithology and petrography of the basalts recovered at Site 1001 can be found in Sigurdsson, Leckie, Acton, et al. (1997). The basalts are overlain by Campanian limestones containing fossils that indicate that the basalts had erupted by at least 77 Ma. The basaltic sequence can be divided into 12 distinct units that likely represent individual lava flows and associated hyaloclastite beds. Flow margins are often highly vesiculated and glassy. The succession is dominated by aphyric basalt, but sparsely plagioclase-phyric and plagioclase and pyroxene-phyric basalts are also present.
Here we report the results of 40Ar-39Ar radiometric dating and electron microprobe analysis of flow-rim glass and plagioclase crystals from some of the recovered basalts. An average of three 40Ar-39Ar analyses indicate an eruption age of 81 ± 1 Ma. A single glass sample analyzed by microprobe analysis is tholeiitic and almost identical to the shipboard X-ray fluorescence (XRF) analyses of the whole rock. The slightly porphyritic basalts have at least two populations of plagioclase, groundmass, and glomerocrystic plagioclase laths that appear to be in equilibrium with the surrounding melt and corroded tabular phenocrysts that have a higher An content (An84-86).