A new felsic cone-sheet swarm in the Central Atlantic Islands: The cone-sheet swarm of Boa Vista (Cape Verde)

Abstract

The island of Boa Vista is one of the oceanic islands with the largest amount of felsic rocks reported in the world (50% of the total outcropping igneous rocks of Boa Vista). The felsic rocks are trachytes and phonolites generated within the second main stage of the volcanic activity that lasted about four million years. Three consecutive felsic episodes are distinguished: the first occurred between 14.5Ma and 13.5Ma, the second between 13.6 Ma and 12.0 Ma, and the third and last between 12.0 Ma and 10.4Ma. Their geochemical composition reveals the presence of crystal fractionation processeswith participation of feldspars andamphibole aswell as different accessory phases involved in each episode. The Boa Vista felsic cone-sheet swarm, the youngest third episode, is formed by several hundreds of peralkaline ne-phonolitic sheets distributedwithin a circular surface about 11 kmin radius. The cone-sheets conforma single general pattern characterized by a decreasing dip outward the structure, together with a constant dip of each individual sheet. The average inclination of sheets is around 40° in the sectors next to the centre of the structure whereas it is only about 30° next to its periphery. The magmatic focus of the sheets is located over 3 km deep almost coinciding with the convergence centre of a contemporary basic radial dike swarm.