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 50m deep and about 70km 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 2Ma, and the upper one between 0.9 and 1.2Ma. The smaller island of Branco is the remnant of a large stratovolcano about 6km in radius, 1000m of altitude and between 5 and 6Ma in age. Santa Luzia, the northernmost and largest island of this particular group is essentially a remaining rift zone between 4 and 7Ma old, at least 600m high, about 10km long and 7km 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 8Ma 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 1.4Ma.The three islands as a whole are formed by moderately or strongly alkaline, basic and ultrabasic rocks similar to those found in the neighboring island of São Vicente. Ultraalkaline rocks only appear in Raso and differentiate types as trachyandesites and phonolites are exclusively found in Santa Luzia. The similar age found for the main stages of growth recorded in the mentioned four islands and, even more, in the other two islands: Santo Antão and São Nicolau situated at the end of the Barlovento alignment, are not easily comprehensible with evolution models implying a simple spatial progression of the volcanic activity from east to west in this array of the archipelago. On the other hand, the temporal coincidence in periods of quiescence observed in the different islands seems to point to the existence of a cyclical activity for the ensemble.