Tobago, West Indies, a fragment of a Mesozoic oceanic island arc: petrochemical evidence

Abstract

The pre-Cenozoic rocks of Tobago, West Indies, represent an exceptional cross-section through a portion of a composite Mesozoic arc system, other parts of which may be found in the Villa de Cura klippe, the Aruba-Blanquilla island chain, and perhaps in the Western Cordillera of Colombia and Ecuador. The geochemical data from Tobago presented in this paper provide a basis from which a regional view of the petrologic character of the accreted Mesozoic arc of the Southern Caribbean Mountain system can be deduced. The Mesozoic rocks of Tobago can be divided into four main groups: the North Coast Schist, the ultramafic to mafic plutonic complex, the Tobago Volcanic Group, and late mafic dykes. Rocks of the North Coast Schist served as wallrock for the plutonic–volcanic–dyke complex, which evolved during the mid-Cretaceous. Geochemical and Nd and Sr isotopic data presented demonstrate the oceanic island arc character of all the pre–Cenozoic rocks of the island. All suites span the calc-alkaline and tholeiitic regions on discrimination diagrams. Rare Earth element (REE) patterns for the North Coast Schist, Tobago Volcanic Group and plutonic suite are essentially identical and are moderately LREE-enriched (La/Yb = 1.1–2.7). The dykes exhibit the most unevolved REE patterns, and are interpreted as most closely representative of parental liquid compositions. Initial ɛ Nd (+6.6 to +9.4) and 87 Sr/ 86 Sr (0.70303–0.70364) for all samples except North Coast Schist and Tobago Volcanic Group argillites are similar in value and overall range to other oceanic island arcs. Limited isotopic data from argillite samples from Tobago suggest an important change in the continental-derived sediment contribution during Mesozoic arc growth. Nd isotopic data from North Coast Schist siliceous argillite indicate virtually no continental detritus in this Neocomian sediment, while Nd isotopic data from the Tobago Volcanic Group argillite suggest a definite continental contribution during Albian arc growth.