Geochemistry and age of the Tobago igneous rocks

Abstract

The island of Tobago, situated at the SE corner of the Caribbean plate, is made up of groups of volcanic, plutonic and metamorphic rocks. Thc volcanics are mainly calc-alkaline island-arc rocks which range from basalts to rhyotlacites. They have been affected by metamorphism of the lower zeolite zone facies. The plutonic rocks, which intrude the volcanics range from high-Al gabbros to biotite granodiorites, are much fresher. The metamorphics arc of the greenschist facies and are overlain by the volcanics. All these rocks were hitherto believed to be of Cretaceous age, but new K-Ar dates from the freshest available samples, including a mineral determination, show a Lower Cretaceous age for the pluton. It therefore indlicates that the overlying volcanics are at least of Lower Cretaceous age. Upper Cretaceous ages obtained for the volcanics appear to be artificially young due to the effects of metasomitic alterations on these rocks. It also appears that the Tobago metamorphic rocks may be of a Jurassic age, similar to that of equivalent rocks in neighbouring Trinidad.