PROVIDENCIA ISLAND: A MIOCENE STRATOVOLCANO ON THE LOWER NICARAGUAN RISE, WESTERN CARIBBEAN - A GEOLOGICAL ENIGMA

Providencia Island, located on the Southern Nicaraguan Rise in the western Caribbean, represents a deeply dissected stratovolcano. A new geological map of the island shows that the island is composed of 3 major (Mafic, Breccia and Felsic) and 5 minor (Trachyandesite, Conglomerate, Pumice, Intrusive and Limestone) stratigraphic units. The Felsic and Breccia units comprise a suite of rhyolitic and dacitic lavas and pyroclastic deposits respectively, with the latter also containing xenolithic mildly alkaline olivine basalt, hawaiite and trachyandesite clasts. The oldest exposed rocks belong to the Mafic Unit, best exposed as a sequence of megacryst-bearing basalt lavas, lithified base surge deposits and hyaloclastites, on the extreme southeast tip of the island and as a sequence of lava flows on the NW coast. Compositionally the Mafic Unit and the basic xenolithic inclusions belong to a Ne-normative suite, whereas the volcanic rocks from the Trachyandesite, Breccia and Felsic Units are all Q-normative. The significance of a bimodal magmatic suite, which has characteristic Sr, Nd and Pb mantle isotopic ratios for all rock types, will be discussed in terms of the island’s tectonic setting and geological evolution. This study also provides 11 new U/Pb zircon ages, that can be subdivided into two groups, one of Miocene age (11.9 to 10.8 million years) and one ranging in age from Paleozoic to Proterozoic (454 to 1661 million years). The presence of these old ages within a younger volcanic sequence will be evaluated at in terms of the overall geological evolution of Central America and the Caribbean Plate.