U-Pb zircon SHRIMP evidences of Cambrian volcanism in the Schistose Domain within the Galicia-Tras-os-Montes Zone (Variscan Orogen, NW Iberian Peninsula)

Abstract

SHRIMP U–Pb zircon analyses have shown the complexity of dating volcanic rocks due to the presence of inner cores within zircon crystals. Using the cathodoluminescence studies assisting ion microprobe analyses allow us to conclude that: the two low-grade metavolcanic samples from the Schistose Domain of the Galicia-Trás-os-Montes Zone in the northeast limb of the Verín-Bragança synform (NW Spain and NE Portugal) yield ages of 488.7 ± 3.7Ma and 499.8 ± 3.7Ma (lowermost Ordovician-Upper Cambrian). The Schistose Domain had been traditionally considered as a parautochthonous tectonic unit, i.e. as the stratigraphic continuation of the autochthonous underlying rocks, only locally or moderately detached from them as a result of strong dragging forces from large allochthonous units above it. Current interpretation of the Schistose Domain suggests that this domain formed the outboard edge of the Iberian terrane. Important Arenig, felsic magmatism with similar geochemical signature to the volcanic bodies in the Schistose Domain of the Galicia-Trás-os-Montes Zone (GTMSD) series is present also in the adjacent Ollo de Sapo Domain of the Central Iberian Zone. This contemporary nature of magmatic events provides an additional argument to support the “Iberian” affinity of the Schistose Domain of the Galicia-Trás-osMontes Zone. However, the Cambro–Ordovician facies are very different in the Schistose Domain with respect to the autochthonous unit, the Central–Iberian Zone, suggesting that the Schistose Domain must be considered as a major allochthonous unit with a displacement of over several tens of kilometers.