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

Ion-microprobe data are reported for zircons from ultramafic, mafic and met asedimentary high-pressure (HP) rocks of the Cabo Ortegal allochthonous com plex (Galicia, NW Spain). The new data suggest that the protoliths of eclog ites (MORB-type and Mg- and At-rich) and mafic granulites have formed in Ea rly-Ordovician times (495 +/- 11; 469 +/- 9 and 478 +/- 12 Ma, respectively ) involving oceanic and/or continental mantle sources. We have found no evi dence for a HP metamorphic event at 480 Ma and for the previously proposed second HP event in the Silurian. Instead, a single subduction-induced PT-t loop with a climax around 390 Ma (Early Devonian) can be deduced from the r elevant domains of zircons from the following rocks: two different types of eclogites, two HP paragneisses, one garnet pyroxenite and one spinel harzb urgite. Rapid cooling and exhumation (minimum rates of ca. 90 degreesC/Ma a nd ca. 5.8 km/Ma) are derived for the time immediately after the peak condi tions. After cooling to ca. 500 degreesC, slower cooling and exhumation rat es (ca. 15-18 degreesC/Ma and ca. 0.18 km/Ma) are probably artefacts due to an episode of reheating at ca. 350 Ma. This interpretation is supported by the scatter of various mineral ages and the presence of synkinematic grani toids. A subduction related Acadian (Early Devonian) HP metamorphism follow ed by an Early Carboniferous collision can thus be demonstrated for the fir st time also for the Iberian part of the European Variscides. (C) 2001 Else vier Science B.V. All rights reserved.