The older volcanic complexes of Sao Miguel, Azores: Nordeste and Povoacao

摘要

The oldest part of S. Miguel is to the east of Furnas. Previous research argued that these volcanics belong to a construct called the Nordeste Volcano, a heavily eroded shield which not only extends to the east coast of the island but also underlies Furnas Volcano in the west. On the basis of geomorphological mapping, we argue that Nordeste comprises two volcanic systems: an older Nordeste construct (the Nordeste Volcanic System); and the younger Povoação Volcano which straddles the Nordeste shield on its western margin. The Nordeste Volcanic System consists of the Lower Basalts which constitute the overwhelming majority of its subaerial products which are exposed in coastal cliff sections. Above the Lower Basalts is a surficial drape of Ankaramites and the Upper Basalts. There is no evidence of large explosive trachytic eruptions from Nordeste Volcanic System. Povoação Volcano comprises an early shield construct, after which the volcano experienced caldera collapse. Post-caldera deposits are poorly exposed, but include basaltic, mugearitic and trachytic lavas intercalated by cut and fill sequences. Radiometric dating has yet to resolve fully the absolute ages of the Nordeste and Povoação volcanic systems, but morphology indicates that the former is much older than the latter.