Stratigraphy, volcanic processes and evolution of the Martin Vaz archipelago

Abstract

The volcanic Martin Vaz Archipelago consists of four Plio-Pleistocene small islands (total area of 0.37 km2) in the South Atlantic Ocean in front of Brazil's coast. Together with Trindade Island, the archipelago corresponds to the easternmost portion of the Vitória-Trindade Seamount Chain (VTC).We performed geological mapping, coupled with X-ray diffraction, petrography, scanning electron microscopy, and lithogeochemistry analyses at Martin Vaz samples. Four lithostratigraphic units were identified (i) Martin Vaz Formation records older explosive phonolitic activity. It is interpreted as resulting from hydromagmatic volcanism; (ii) Subvolcanic phonolites consist of a dome (cryptodome) and plug morphologies, which represent the most voluminous rocks at Martin Vaz Archipelago; (iii) Mirante Formation records hyper-concentrated density currents of ultrabasic rocks; and (iv) Atobá Formation is composed of ultrabasic to intermediate lavas of basanitic to phonolitic composition, interpreted as a result of Strombolian (Fountain Fed)/Hawaiian volcanism.The volcanic rocks described at Martin Vaz Archipelago and Trindade Island represent the final members of the nephelinite-phonolite series, with a significant absence of intermediate phases. Basanites of the Martin Vaz, Mirante, and Atobá formations are geochemically similar to the nephelinites of Trindade Island. In the same way, the geochemical characteristics of phonolites from subvolcanic bodies and the Bandeira Member are similar to those observed on the phonolites of Trindade Island. Geochemical and thermodynamic modeling suggest cogeneticity between ultrabasic and phonolitic magmas. Fractional crystallization about 0.5 GPa and at low water pressures explain the chemical evolution trends observed. Geochemical characteristic points to melts of a metasomatized source composed of garnet wehrlite and/or hornblendite.The new data allowed for the elaboration of a novel conceptual model for the geological evolution of the Martin Vaz Archipelago, one of the last volcanic manifestations in the Brazilian territory.