40Ar–39Ar geochronology of Santo Antão, Cape Verde Islands

Santo Antão, the northernmost island of the Cape Verde Archipelago, consists entirely of silica-undersaturated volcanic products and minor intrusions. 40Ar–39Ar incremental heating experiments have been carried out on 24 samples that cover the entire exposed chronological sequence. The oldest lavas (7.57±0.56 Ma), representing an older volcanic basement, are exposed about 620 m above mean sea level. After an interval of quiescence of up to 4.3 Ma the volcanic activity resumed and continued at low eruption rates. The older basement is unconformably overlain by a ca. 810-m-thick lava sequence that spans an age range from 2.93±0.03 to 1.18±0.01 Ma. This sequence is cut by many dykes and sills. Simultaneous volcanic activity occurred in the northeastern, central and eastern part of the island. A phonolitic pumice deposit that forms a noteworthy feature over most of the island has an estimated age of 0.20 Ma. This predates volcanic activity that formed the highest point of the island (Tope de Coroa) which has an age of 0.17±0.02 Ma. The most recent eruption on the island formed nephelinitic lavas in the Porto Novo region at 0.09±0.03 Ma. The oldest volcanism exposed on Santo Antão, which took place about 7.6 Ma ago, was simultaneous with waning activity on Maio at the eastern end of the Cape Verde Archipelago.