Genesis of roque nublo formation: A special kind of ignimbritic eruptions in Gran Canaria

A series of comagmatic volcanic materials originated by subaerial eruptions that happened during the Neogene in Gran Canaria (Canary Islands) are studied in this work. The sequence consists of flows (basalts, basanites, tephrites) underlying volcanic agglomerate sheets. The whole unit, which reaches a maximum thickness greater than 700 m, is cut by a number of phonolite plugs. Flows, agglomerate sheets and plugs are genetically related, forming a differentiation series whose evolution has been rather complex: crystal fractionation, amphibole resorption, changes in oxygen pressure and gaseous transfer have played a role in the genesis of these volcanics. From an evolutionary point of view, the Roque Nublo Formation can be described as an alkaline series with two different undersaturated zones (tephritic flows and phonolitic domes) separated by a maximum of saturation (the agglomerate matrix is chemically a trachyte). In this respect, the Roque Nublo Formation is similar to the alkaline Cantal series, although in Gran Canaria the relations between trachytes and phonolites seem to be better defined. As for the agglomerates, their uncommon characteristics (heterogeneous and very poorly sorted boulders predominating over a vitroclastic welded matrix) lead one to think that they were produced by an unusual kind of cruption; certainly not by the « nuée ardente » types which have been repeatedly postulated before. The author’s suggestion is that these agglomerates (« Roque Nublo type ») were formed in ignimbritic-style eruptions of higy-viscosity magmas contained in very high-pressure chambers.