Abstract
The Nordeste volcanic complex of Sao Miguel island is one of the oldest known complexes (4.01 to 0.95 m.y.) of alkaline volcanism in the eastern Azores. It contains about 1,300 m of exposed volcanics and hypabyssal intrusives that have been subdivided into five stratigraphic sequences: lower basalts, Nordeste ankaramites, upper basalts, tristanites, and trachytes. The bulk of the complex is predominantly of basaltic composition and is confined to the three lowermost sequences: lower basalts, olivine-clinopyroxenerich alkali basalts and trachybasalts; Nordeste ankaramites, relatively thick ankaramite flows; and upper basalts, olivine-alkali basalts, and aphyric trachybasalts and tristanites. The two youngest sequences, tristanites and trachytes, are limited to small flows and hypabyssal intrusions and to thin veneers of tophra. Numerous mafic dikes of varied composition and relative ages cut the complex.
Feldspar compositions range from An86 to potash feldspar or Or56 Olivines range from Fo85 to Fo40, and clinopyroxenes range from diopsidic-augitcs to aluminous titanaugites to aegirine augites. Based on the Al-Ti content of the clinopyroxenes, two trends are recognized: high silica basalts (transitional) with clinopyroxenes enriched in Si, Ca, and Cr and a low-silica alkaline trend (ankaramite trachyte) with clinopyroxenes enriched in Al and Ti. Coexisting titanomagnetites and ilmenites indicate consolidation temperatures from 958 °C to 695 °C-and oxygen fugacites between 10-12.1 to 10-14.2.
Plots of 60 chemical analyses on magnesia and alkali-silica variation diagrams show the bulk of the Nordeste volcanics to be a potassic, mildly alkaline, undersaturated to saturated, alkali basalt trachybasalt tristanite trachyte series. The oldest flows of the complex, characterized by low TiO2, high SiO2, and clinopyroxenes poor in TiO2 and Al2O3, are interpreted as representative of chemistries transitional between tholeiitic and alkalic suites. Comparison of the Nordeste volcanics and Terceira (Azores) volcanics show the latter to be more undersaturated and sodic in lieu of potassic. Compared to other Atlantic islands, Sao Miguel is most similar to Gough Island, differing in having higher Ti, K, Fe, and P, but lower Na and Mg, and late differentiates which are characteristically quartz normative.
The chemical characteristics of the Nordeste Complex, as interpreted from phase chemistry and magnesia variation diagrams, and tested using the Wright and Doherty (1970) least-squares computer method for liquid lines of descent, support a crystal fractionation model. The magnesian (> 7.0% MgO) porphyritic-ankaramitic alkali basalts are accumulative in origin. The trachybasalts trachyte series represent fractionated liquids generated by the removal of titanaugite, plagioclase, and olivine in the early stages, followed by abundant plagioclase, augite, and minor opaques in the late stages, from “parental” alkali olivine basalt melts in crustal reservoirs (5 to 10 km). The predominance of plagioclase in the fractionation scheme and its rarity as an xenocrystic phase suggest the Azores platform to be underlain by gabbroic and dioritic layered complexes.
