Structure, composition and age of the small islands of Santa Luzia, Branco and Raso (Cape Verde Archipelago)

Abstract

The northern alignment of Cape Verde Archipelago contains three small islands in its central sector: Santa Luzia, Branco and Raso on which there is very scarce geological, petrological or geochronological information available. The three islands along with the island of São Vicente arise on a continuous underwater relief less than 50 m deep and about 70 km long that formed from separate volcanic edifices. The southernmost island, Raso is formed by accumulation of several tens of lava flows and some remaining strombolian cones. An erosion surface covered by sediments allows the separation of two different episodes of growth: the lower one older than 2 Ma, and the upper one between 0.9 and 1.2 Ma. The smaller island of Branco is the remnant of a large stratovolcano about 6 km in radius, 1000 m of altitude and between 5 and 6 Ma in age. Santa Luzia, the northernmost and largest island of this particular group is essentially a remaining rift zone between 4 and 7 Ma old, at least 600 m high, about 10 km long and 7 km wide. The SE extreme of the island is occupied by rocks of an older partially submarine stage of growth which, also include gabbroic intrusions crossed by a dense network of dikes whose age may be estimated in 8 Ma or more. On the contrary, the NW end shows the only remains of late post-erosion activity that is of much less volumetric importance and it has been dated between 0.8 and 1.4 Ma.