The felsic complex of the Vallehermoso Caldera: interior of an ancient volcanic system (La Gomera, Canary Islands)

A mid-to-upper Miocene felsic cone sheet dike complex, caldera collapse and dome intrusion, located around the Vallehermoso and Tamargada areas (North of La Gomera Island) is described. It constitutes one of the less-known volcanic episodes in the Canary Archipelago and especially in La Gomera Island. Based on detailed field and structural evidence, it is suggested that the cone sheet shows, at the surface, an almost circular geometry truncated to the NW sector. After the cone sheet dike complex formed, decompression of the interior of an assumed shallow magma chamber occurred due to the subsequent migration of felsic magma throughout the dikes of the cone sheet. This last event caused the roof of the magma chamber to collapse, leading to the formation of a 3- to 4-km-wide caldera. A breccia deposit formed during the collapse and filled a depressed central area which finally was affected by an episode of magmatic resurgence, leading to a later almost circular dome intrusion. The association of cone sheet, breccia deposits and domes constitutes the so-called "Trachytic-Phonolitic Complex" (TPC) and should be viewed as the root of an ancient intensely eroded volcanic edifice made of trachytes, nepheline phonolites and minor Habyne phonolites as the dominant materials. A description of the structural, petrological and geochemical features of the Trachytic-Phonolitic Complex and a discussion about their origin are considered in this paper. We also consider the correlation between another felsic intrusion of alkali gabbros and syenites (called Tamargada syenites) that also crops out in the Vallehermoso-Tamargada area with respect to the Trachytic-Phonolitic Complex. Our data suggest that this alkaline intrusion of alkali gabbros and syenites belongs to an earlier intrusive episode. On the basis of petrological, geochemical and detailed field studies, both trachytes and nepheline-phonolites included in the Trachytic-Phonolitic Complex most likely evolved from alkali mafic magmas from the Upper Old Basalt formation through a dominant fractional crystallization process. Additionally, hauyne phonolites could develop from local and later magmatic processes such as gas transfer that altered and changed a small part of the previously mentioned nepheline phonolites to hauyne phonolites. (C) 2004 Elsevier B.V. All rights reserved.