Geochronological, geochemical and geophysical investigations in the high-grade mafic-ultramafic complex at Cabo Ortegal and other pre-existing elements in the Hercynian basement of Galicia (NW Spain)

Mafic and ultramafic rocks from a high-grade complex at Cabo Ortegal in NW Spain were investigated in the field and in the laboratory by means of petrological, isotope-geochronological and geochemical methods. Isotope-geochronological methods were applied to orthogneisses from a high-grade complex near Mellid and from the blastomylonitic graben between Malpica and Tuy, a mica-bearing eclogite lens from the same graben, and to a two-mica granite from the area near La Guardia. A gravity survey was carried out in part of the belt of sub-circular complexes surrounding the Ordenes basin. The geochronological investigations revealed ages of about 500 Ma to 320 Ma for older elements in the Hercynian basement of Galicia. Geochemical investigations in the Cabo Ortegal complex showed that the granulites and eclogites form a group that was part of an older, pre-existing continental crust. The ultramafic rocks of the Cabo Ortegal complex may have been derived in two melting episodes from a proposed mantle-plume. It is possible that the metagabbros in the complex originated in the second melting episode. The gravity survey showed the existence of three positive anomalies, situated below high-grade complexes near Santiago de Compostela, near Mellid and near Sobrado, that can be interpreted as ultramafic diapirs. A model involving continental rifting, mantle-plume diapirism, rejuvenation of the lower crust and initial seafloor-spreading is proposed for the evolution of the Early Palaeozoic continental lithosphere of the northwestern Iberian Peninsula.