A complex history for the Caribbean plateau: Petrology, geochemistry, and geochronology of the Beata Ridge, south Hispaniola

The Beata Ridge is a prominent SSW-trending topographic structure in the ce ntral Caribbean basin. It is characterized by unusually thick oceanic crust (up to 20 km) and is believed to form part of the Caribbean oceanic platea u. Samples recovered by submersible during the Nautica-Beata cruise show th e ridge to be composed mainly of gabbros, dolerites, and rare pillow basalt s. Textures, which vary significantly, reflect differences in cooling rates and suggest a subsurface, hypabyssal environment. Major-element compositio ns of gabbros and dolerites plot on simple trends that correspond to fracti onal crystallization of olivine, clinopyroxene, and plagioclase. Trace-elem ent ratios are close to chondritic [(Nb/Zr)(N) 0.85-1.1] and rare earth cle ment patterns (REE) are almost flat [(La/Yb)(N) 0.63-1.02]. The source, how ever, was isotopically depleted (epsilon (Nd) +7.4 to +9.5). To explain the se geochemical features, we propose that the magmas formed through pooling of fractional melts of spinel peridotite. The rare basalts recovered have h igher 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 isot opically less depleted source (epsilon (Nd) +5). Several samples were dated by the Ar-40-Ar-39: method, either on whole rocks or separated plagioclase s. Most samples have ages between 80 and 75 Ma, which are consistent with p revious 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 oceani c plateaus. The persistence of this signature raises questions about the va lidity of generally accepted mantle-plume models for the formation of ocean ic plateaus. Alternative hypotheses are evaluated in the light of geodynami c reconstructions of the Caribbean plate. Two geodynamic models may account for the geochemical and isotopic characteristics of the Beata Ridge sample s. 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 G omez hot spot. In this model, the 76-Ma episode is related to the Galapagos plume. In the second interpretation, the Galapagos plume was responsible f or 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 epi sode is related to lithospheric thinning localized on the Beata Ridge.