Abstract
The Beata Ridge is a prominent SSW-trending topographic structure in the central Caribbean basin. It is characterized by unusually thick oceanic crust (up to 20 km) and is believed to form part of the Caribbean oceanic plateau. Samples recovered by submersible during the Nautica-Beata cruise show the ridge to be composed mainly of gabbros, dolerites, and rare pillow basalts. Textures, which vary significantly, reflect differences in cooling rates and suggest a subsurface, hypabyssal environment. Major-element compositions of gabbros and dolerites plot on simple trends that correspond to fractional crystallization of olivine, clinopyroxene, and plagioclase. Trace-element ratios are close to chondritic [(Nb/Zr)(N) 0.85-1.1] and rare earth clement patterns (REE) are almost flat [(La/Yb)(N) 0.63-1.02]. The source, however, was isotopically depleted (epsilon (Nd) +7.4 to +9.5). To explain these geochemical features, we propose that the magmas formed through pooling of fractional melts of spinel peridotite. The rare basalts recovered have higher trace-element ratios and enriched REE patterns [(Nb/Zr)(N) 3.45; (La/Yb)(N) 28-30]. They possibly formed through lower-degree melting of an isotopically less depleted source (epsilon (Nd) +5). Several samples were dated by the 40Ar-39Ar: method, either on whole rocks or separated plagioclases. Most samples have ages between 80 and 75 Ma, which are consistent with previous ages within the province, but others are surprisingly young, around 55 Ma. The chemical signature of the gabbro-dolerite group is very similar to that of basalts from other parts of the Caribbean and from other oceanic plateaus. The persistence of this signature raises questions about the validity of generally accepted mantle-plume models for the formation of oceanic plateaus. Alternative hypotheses are evaluated in the light of geodynamic reconstructions of the Caribbean plate. Two geodynamic models may account for the geochemical and isotopic characteristics of the Beata Ridge samples. In one interpretation, the Caribbean plateau formed similar to 80-90 Ma in the Pacific south of the Galapagos hot spot, possibly above the Sala y Gomez hot spot. In this model, the 76-Ma episode is related to the Galapagos plume. In the second interpretation, the Galapagos plume was responsible for the main plume-related magmatic event at 90 Ma and the 76-Ma episode is attributed to lithospheric thinning. In both interpretations, the 55-Ma episode is related to lithospheric thinning localized on the Beata Ridge.