SIGNIFICANCE OF MORB-DERIVED AMPHIBOLITES FROM THE ARACENA METAMORPHIC BELT, SOUTHWEST SPAIN

The Aracena metamorphic belt, in the southwest Iberian Massif, is char acterized by the presence of MORB-derived amphibolites and continental rocks deformed and metamorphosed during the Hercynian orogeny. Geoche mical 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 b asalt protolith was extruded Neodymium isotopic ratios are typical of MORB, and oxygen isotopes indicate that these amphibolites have been d erived from the uppermost part of the oceanic crust. Thermal evolution , revealed from the study of chemical variations in the amphibole chem istry, is interpreted as resulting from subduction in a low-pressure r egime in which the thermal structure of the continental hanging-wall p layed an important role. This continental wall was previously heated b y subduction of a slab window resulting from migration of a triple jun ction along the continental edge during plate convergence. Three petro logic arguments support this tectonic model. These are: (I) the low-pr essure inverted metamorphic gradient of amphibolites of the oceanic do main; (2) the high-temperature low-pressure metamorphism of the contin ental hanging wall; (3) the early intrusion of boninites into the cont inental domain.