A single Devonian subduction event for the HP/HT metamorphism of the Cabo Ortegal complex within the Iberian Massif

Abstract

Ion-microprobe data are reported for zircons from ultramafic, mafic and metasedimentary high-pressure (HP) rocks of the Cabo Ortegal allochthonous complex (Galicia, NW Spain). The new data suggest that the protoliths of eclogites (MORB-type and Mg- and Al-rich) and mafic granulites have formed in Early-Ordovician times (495±11; 469±9 and 478±12Ma, respectively) involving oceanic and/or continental mantle sources. We have found no evidence for a HP metamorphic event at 480Ma and for the previously proposed second HP event in the Silurian. Instead, a single subduction-induced PT-t loop with a climax around 390Ma (Early Devonian) can be deduced from the relevant domains of zircons from the following rocks: two different types of eclogites, two HP paragneisses, one garnet pyroxenite and one spinel harzburgite. Rapid cooling and exhumation (minimum rates of ca. 90°C/Ma and ca. 5.8km/Ma) are derived for the time immediately after the peak conditions. After cooling to ca. 500°C, slower cooling and exhumation rates (ca. 15–18°C/Ma and ca. 0.18km/Ma) are probably artefacts due to an episode of reheating at ca. 350Ma. This interpretation is supported by the scatter of various mineral ages and the presence of synkinematic granitoids. A subduction related Acadian (Early Devonian) HP metamorphism followed by an Early Carboniferous collision can thus be demonstrated for the first time also for the Iberian part of the European Variscides.