Structural and stratigraphical significance of U–Pb ages from the Mora and Saldanha volcanic complexes (NE Portugal, Iberian Variscides)

Abstract

Abstract Geological mapping and structural investigations were carried out in the eastern rim of the Morais Allochthonous Complex in NE Portugal (NW Iberian Massif) along the first-order tectonic boundary between the lower part of the Galicia–Trás-os-Montes Zone (the parautochthonous Schistose Domain) and the autochthonous Central Iberian Zone. The aim was to correlate the Palaeozoic stratigraphic pile of both domains, and to establish their pre- and synorogenic Variscan evolution. An Early Palaeozoic volcanic complex forms part of the pre-orogenic record of the parautochthonous Schistose Domain. It includes two volcanic units, the oldest of which is described for the first time in this work and is named the Mora Volcanics. It is a bimodal volcanic suite with basic to acid rocks which has yielded a late Cambrian age ( c. 493 Ma). Above the Mora Volcanics, the Saldanha Volcanics show petrographic, geochemical and age ( c. 483 Ma) similarities to the Ollo de Sapo Formation, a volcano-detritic ensemble found in the autochthonous domain of NW and Central Iberia. The stratigraphic record and the geochemical signatures of both volcanic units indicate that they were originated by fast partial melting induced by extension along the northern Gondwana margin at the Cambro-Ordovician boundary, coeval with the opening of the Rheic Ocean.