THE VOLCANIC ROCKS OF SÃO JORGE ISLAND, AZORES (PORTUGAL): PETROGRAPHY AND GEOCHEMISTRY

Abstract

The São Jorge Island is part of the Central Group of the Azores Archipelago (Portugal), located near the triple junction, of the North American, Euro-Asian and African lithospheric plates, in the North Atlantic region. The island volcanostratigraphy is composed of three volcanic complexes called Topo, Rosais and Manadas. According to the bibliography, the age of the São Jorge Island is 1.309 ± 0.0035 Ma (⁴⁰Ar/³⁹Ar). The rocks have medium to fine granulometry, bearing porphyritic, microporphyritic, glomeroporphyritic, intergranular, pilotaxytic, trachytic and intersertal textures. They were chemically classified as basanites, alkaline basalts and hawaiites, composed of olivine, augite, plagioclase and opaque minerals that occur as phenocrysts or inserted in the matrix. Some samples of alkaline basalt and hawaiite show kaersutite, interpreted as a xenocrystal. Reabsorption textures, embayment, zoning and reaction edges are common features in phenocrystals and indicate that the magma underwent rapid decompression, with sudden variation of temperature and pressure. The geochemical interpretation allowed concluding that the lavas have sodic alkaline affinity and were generated by low partial melting rates, from an enriched mantle-derived magma. The geochemical data are compatible with those of magmas generated in an oceanic intraplate (OIB) geotectonic setting.