SHALLOW-LEVEL MIGMATIZATION OF GABBROS IN A METAMORPHIC CONTACT AUREOLE, FUERTEVENTURA BASAL COMPLEX, CANARY-ISLANDS

Migmatization of gabbroic rocks at 2-3 kbar has occurred in the metamo rphic contact aureole of a mafic pluton in the Fuerteventura Basal Com plex (Canary Island;). Migmatites are characterized by a dense network : of closely spaced millimetre-wide leucocratic veins with perfectly p reserved igneous textures. They are all relatively enriched in Al, Na I: Sr Ba, Nb, Y and the rare earth elements compared with the unaffect ed country rock beyond the aureole. Migmatization under such low-press ure conditions war possible because of the unusual tectonic and magmat ic contact in which ii occurred. Multiple basic intrusions associated with extrusive volcanic activity created high heat flow in a small are a. Alkaline and metasomatized rocks present in the country rock of the intruding pluton were leached by high-temperature fluids during conta ct metamorphism. These enriched fluids then favoured partial melting o f the host gabbroic rocks, and contaminated both the leucosomes and me lanosomes. A transpressive tectonic setting at the time of intrusion c reated shearing along the contact between the intrusion and its host r ock. This shearing enhanced circulation of the fluids and allowed segr egation of the nea-formed melts from their restite by opening tension veins into which the melts migrated. Depending on the relative timing of melt segregation and recrystallization leucosomes range in composit ion from a 40-60% mixture of clinopyroxene (+/- amphibole) and plagioc lase to almost pure feldspathic veins. Comparable occurrences of gabbr os migmatized at low pressure are expected only at a snail scale in lo calized areas of high heat flow in the presence of fluids, such as in. mid-ocean ridges or ocean-islands.