Migmatization of gabbroic rocks at 2-3 kbar has occurred in the
metamorphic contact aureole of a mafic pluton in the Fuerteventura Basal
Complex (Canary Island;). Migmatites are characterized by a dense
network: of closely spaced millimetre-wide leucocratic veins with
perfectly preserved igneous textures. They are all relatively enriched
in Al, Na I: Sr Ba, Nb, Y and the rare earth elements compared with the
unaffected country rock beyond the aureole. Migmatization under such
low-pressure conditions war possible because of the unusual tectonic and
magmatic contact in which ii occurred. Multiple basic intrusions
associated with extrusive volcanic activity created high heat flow in a
small area. Alkaline and metasomatized rocks present in the country rock
of the intruding pluton were leached by high-temperature fluids during
contact metamorphism. These enriched fluids then favoured partial
melting of the host gabbroic rocks, and contaminated both the leucosomes
and melanosomes. A transpressive tectonic setting at the time of
intrusion created shearing along the contact between the intrusion and
its host rock. This shearing enhanced circulation of the fluids and
allowed segregation 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 composition from a 40-60% mixture of clinopyroxene (+/-
amphibole) and plagioclase to almost pure feldspathic veins. Comparable
occurrences of gabbros migmatized at low pressure are expected only at a
snail scale in localized areas of high heat flow in the presence of
fluids, such as in. mid-ocean ridges or ocean-islands.