Geochronology and geochemistry of the Pola de Allande granitoids (northernSpain): their bearing on the Cadomian-Avalonian evolution of northwest Iberia

The Pola de Allande pre-Variscan tonalite-granodiorite plutons are located in the Narcea Antiform, at the boundary zone between the Cantabrian and wes t Asturian Leonese zones of the Iberian Variscan belt. These granitoids wer e intruded into a Neoproterozoic siliciclastic sedimentary sequence with su bordinate volcanic intercalations and were subsequently overprinted by Vari scan thrust-related shear deformation. U-Pb laser ablation inductively coup led plasma - mass spectrometry (ICP-MS) dating of zircons from two plutons yielded concordant ages of intrusion of 605 +/- 10 and 580 +/- 15 Ma. To th e authors' knowledge, this is the first reported U-Pb Cadomian-Avalonian ag e for igneous rocks in this section of the Iberian Variscan belt. These int rusions are coeval with the main episode of dominantly calc-alkaline magmat ic activity related to Cadomian-Avalonian subduction. Major and trace eleme nt composition of the granitoids is characteristic of I-type high-K calc-al kaline granitoids generated in continental are settings, and are comparable to those of coeval granitoids in other areas of the Cadomian-Avalonian bel t. Sr and Nd isotopic signatures indicate that the genesis of the Pola de A llande granitoids involved either mixing of mantle melts of Cadomian extrac tion with an older enriched crust (Eburnean-Icarthian, i.e., ca. 2 Ga old c rust) or melting of a mafic infracrustal protolith with a Grenville age (ca , 1.2 Ga) mantle extraction. The Neoproterozoic sediments, hosting the Pola de Allande granitoids and present in large areas of northwest Iberia, may represent the back-are basin of the subduction complex in which the Avalon composite terrane constituted the main magmatic are.