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

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Ancochea Soto, Eumenio and Hernán Reguera, Francisco and Huertas Coronel, María José and Brandle, J.L. (2012) A basic radial dike swarm of Boa Vista (Cape Verde Archipelago); its significance in the evolution of the island. Journal of volcanology and geothermal research, 243-244 . pp. 24-37. ISSN 0377-0273

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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: 17–16 Ma) is equivalent to the shield building stage of Hawaii and the later
(the Recent Volcanics (8–4 Ma) is in some aspects comparable to the post-erosional stage. An important intermediate
essentially felsic stage (the Trachytic–Phonolitic Complex: 14.3–12.8 Ma) 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 5 m. The individual
dikes have an observable mean length of about 300 m. 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 11.5 Ma) 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.


Item Type:Article
Uncontrolled Keywords:Basic dikes, Radial dikes, Oceanic islands, Boa Vista Island, Cape Verde Islands
Subjects:Sciences > Geology > Petrology
ID Code:17871
Deposited On:23 Jan 2013 13:35
Last Modified:23 Jan 2013 13:35

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