Significance of MORB-derived amphibolites from the Aracena metamorphic belt, southwest Spain

The Aracena metamorphic belt, in the southwest Iberian Massif, is characterized by the presence of MORB-derived amphibolites and continental rocks deformed and metamorphosed during the Hercynian orogeny. Geochemical relationships of these amphibolites indicate the existence of a multiple fractionation process from a set of parental magmas, implying the existence of a multi-chamber system beneath the ridge where the basalt protolith was extruded Neodymium isotopic ratios are typical of MORB, and oxygen isotopes indicate that these amphibolites have been derived from the uppermost part of the oceanic crust. Thermal evolution, revealed from the study of chemical variations in the amphibole chemistry, is interpreted as resulting from subduction in a low-pressure regime in which the thermal structure of the continental hanging-wall played an important role. This continental wall was previously heated by subduction of a slab window resulting from migration of a triple junction along the continental edge during plate convergence. Three petrologic arguments support this tectonic model. These are: (I) the low-pressure inverted metamorphic gradient of amphibolites of the oceanic domain; (2) the high-temperature low-pressure metamorphism of the continental hanging wall; (3) the early intrusion of boninites into the continental domain.