A basic radial dike swarm of Boa Vista (Cape Verde Archipelago); its significance in the evolution of the island

Abstract

A basic radial dike swarm unrelated to other basic units of Boa Vista (Cape Verde Archipelago) has been localized and characterized in the central sector of the island.According to new radiometric data three main stages in the evolution of Boa Vista are distinguished: the earlier (the Old Volcanic Complex: 17–16Ma) is equivalent to the shield building stage of Hawaii and the later (the Recent Volcanics (8–4Ma) is in some aspects comparable to the post-erosional stage. An important intermediate essentially felsic stage (the Trachytic–Phonolitic Complex: 14.3–12.8Ma) followed the basaltic shield stage. This felsic stage has equivalents in some other oceanic islands as the Canary Islands, specially Gran Canaria, Tenerife, and La Gomera. The central sector of Boa Vista is also occupied by the Felsic Subvolcanic Complex, a unit consisting of phonolitic breccias, syenites and monzonites that represent the Trachytic–Phonolitic Complex hypabyssal roots. The felsic rocks as a whole constitute half of the total amount of igneous rocks on the island making up Boa Vista, the island with the highest percentage of felsic rocks in the Central Atlantic Ocean.More than 200 dikes of the basic radial swarm intruding the Felsic Subvolcanic Complex have been measured. The intensity of the multiple dike injection is sometimes rather high, roughly a dike every 5m. The individual dikes have an observable mean length of about 300m. The composition of these dikes is always foiditic (nephelinites, melilitites, and limburgites), slightly different in composition (more alkaline and richer in incompatible elements) to the other basic units of the island (the Old Volcanic Complex and the Recent Volcanics).The radial dikes converge in an area located NW of the geometrical center of Boa Vista, a zone where the hypothetical center of the Old Volcanic Complex and the Trachytic–Phonolitic Complex edifices must also have been situated. The ages obtained from the dikes (between 14.8 and 11.5Ma) indicate that the radial injections are contemporary with the phonolites and the trachytes of the Trachytic–Phonolitic Complex and yet are part of the intermediate evolutionary stage of Boa Vista.