The Albian-Turonian Island Arc Rocks of Tobago, West Indies: Geochemistry, petrogenesis, and Caribbean plate tectonics

An elemental and radiogenic isotope study of Cretaceous island arc rocks on Tobago, West Indies, reveals the magmatic processes taking place at the eastern edge of the Pacific-derived Caribbean Plate during development of the Greater Antilles Arc. The ∼110–103 Ma Volcano-Plutonic Suite comprises the ultramafic–intermediate Tobago Pluton and genetically related Tobago Volcanic Group. The volcanic rocks (breccias, tuffs, and mafic–intermediate lavas) have undergone shallow-level fractional crystallization involving plagioclase, clinopyroxene, olivine, and Fe–Ti oxides, but also preserve trace element evidence for ‘cryptic’ amphibole fractionation. The suite is inferred to have formed from a spinel lherzolite mantle wedge source fluxed largely by slab- and recycled volcanogenic sediment-derived fluids. A tonalitic mega-dyke intruding the pluton resembles high-silica adakites, and geochemical constraints indicate a likely origin by partial melting of the arc crust. A mafic dyke swarm (∼103–91 Ma) is partly coeval with the volcanic rocks, but some, perhaps the youngest dykes, are derived from isotopically distinct arc mantle sources compared with the volcanic rocks. Rare Nb-enriched and high-Nb dykes may relate to melting of a high field strength element-enriched source. Current Caribbean tectonic models involve the continuation of east-dipping Farallon Plate subduction beneath the proto-Caribbean seaway either until an Early Cretaceous initiation of proto-Caribbean subduction, or collision of the Caribbean Oceanic Plateau with the Greater Antilles Arc at ∼90–80 Ma. Both models may be compatible with the tectono-magmatic history of Tobago, wherein Tobago is thought to have detached from the fore-arc of the Caribbean arc system during Eocene intra-arc extension, the growth of the Grenada Basin, and inception of the Lesser Antilles Arc. Tobago- or La Désirade-like Mesozoic arc crust underlies much of the present-day Lesser Antilles Arc and not, as has recently been proposed, portions of the plume-derived Caribbean Oceanic Plateau