The felsic dikes of La Gomera (Canary Islands): identification of cone sheet and radial dike swarms

On the northern part of La Gomera there exists a great abundance of trachytic–phonolitic dikes showing a broad diversity in dip and strike. Several methods have been applied in order to separate these dikes in different sets, localise the area from where they derive, and reconstruct the geometry of the swarms. The oldest dikes correspond to a radial swarm dated at 8 Ma. The felsic activity migrated then southwestwards and a second radial swarm and a cone sheet complex were developed between 7.5 and 6.4 Ma ago. The cone sheet complex is 10 km in diameter and shared its centre with that of the second radial structure. The cone sheets exhibit an outward decrease of dip angle whilst every individual sheet maintains a constant inclination. This geometry reflects the existence of an ancient single dome-shaped shallow magma chamber situated some 1650 m below present sea level. The eastern radial swarm represents a felsic episode that could mark the ending of the Lower Old Basalts, the earlier subaerial activity of La Gomera. The two other dike swarms represent a younger episode coeval with the Upper Old Basalts.