Age constraints to the relationships between magmatism, metamorphism and tectonism in the Aracena metamorphic belt, southern Spain

The Aracena metamorphic belt (AMB), southwest Iberian peninsula, is charact erized by the following geological elements: (a) a high-temperature/low-pre ssure (HT/LP) metamorphic belt a few kilometres wide and more than 200 km l ong; (b) a linear belt of oceanic amphibolites with a low-pressure inverted metamorphic gradient; (c) crustal-scale ductile shear zones; and (d) mafic , noritic intrusions of high-Mg andesite (boninite) composition. The relati onships between these elements led to the proposal of a model of ridge subd uction for this sector of the Hercynian belt of Europe. This interpretation is supported by the age relationships displayed between the main rock unit s considered representative of the main tectonic and petrological processes responsible for the geological elements mentioned previously. The results of a geochronological study (Ar-Ar, Rb-Sr and Sm-Nd) clearly support a Late Paleozoic tectonic evolution at an active continental margin, The time evo lution of the metamorphism in the oceanic domain, ranging from 342.6 +/- 0. 6 Ma in the west to 328.4 +/- 1.2 Ma in the east, over a distance of 70 km along the metamorphic belt, support a tectonic model of triple-junction mig ration responsible for the creation at depth of a slab-free window with dec isive consequences for the thermal evolution of the region. The origin of t he linear metamorphic belt of HT/LP regime may be explained by the migratio n along a continental margin of a punctual thermal anomaly induced by the c reation of a triple-junction at the continental margin.