Geochronology and geochemistry of the Pola de Allande granitoids. Their bearing on the Cadomian/Avalonian evolution of NW Iberia. J. Fernández-Suárez, G. Gutiérrez-Alonso, G.A. Jenner, S.E. Jackson, 1998, Canadian Journal of Earth Sciences, 35, 1439-1453. DESCARGAR-DOWNLOAD.
Abstract: The Pola de Allande pre-Variscan tonalite-granodiorite plutons are located in the Narcea Antiform, at the boundary zone between the Cantabrian and West-Asturian Leonese zones of the Iberian Variscan belt. These granitoids were intruded into Neoproterozoic siliciclastic sedimentary sequence with subordinate volcanic intercalations and were subsequently overprinted by Variscan thrust related shear deformation. U-Pb Laser Ablation ICP-MS dating of zircons from two plutons yielded concordant ages of intrusion of 605±10 and 580±15 Ma. To the authors’ knowledge, this is the first reported U-Pb Cadomian-Avalonian age for igneous
rocks in this section of the Iberian Variscan belt. These intrusions are coeval with the main episode of dominantly calc-alkaline magmatic activity related to Cadomian-Avalonian subduction. Major and trace element composition of the granitoids is characteristic of I-type high-K calc-alkaline granitoids generated in continental arc settings, and are comparable to those of coeval granitoids in other areas of the Cadomian-Avalonian belt. Sr and Nd isotopic signatures indicate tha the genesis of the Pola de Allande granitoids involved either mixing of mantle melts of Cadomian extraction with an older enriched crust (Eburnean-Icarthian) or melting of a mafic infracrustal protolith with a Grenville age (ca. 1.2 Ga) mantle extraction. The Neoproterozoic sediments, hosting the the Pola de Allande granitoids, and present in large areas of NW Iberia represent the back-arc basin of the subduction complex in which the Avalon composite terrane constituted the main magmatic arc.