Combined U–Pb geochronology and Lu–Hf isotope systematics by LAM–ICPMS of zircons from granites and metasedimentary rocks of Carrazeda de Ansiães and Sabugal areas, Portugal, to constrain granite sources

Abstract

The Carrazeda de Ansiães and Sabugal areas in the Central Iberian Zone of the Iberian Variscan Belt contain abundant S-type granites formed during the post-thickening stage of the Variscan orogeny (syn- to post-D3, i.e. the last ductile deformation phase). Magmatic zircons and inherited zircon cores from two granites (G and G5), magmatic zircons from three other granites (G1, G4 and G6) and detrital zircons from a chlorite phyllite and a metagraywacke were dated by LAM–ICPMS. Hf isotope data were obtained by the same method to constrain the sources for granites. Some granites have zircons with negative εHft values (G: from −8 to −4; G1: from −8 to −1; G4: from −6 to −3), suggesting an origin from heterogeneous crustal anatectic melts or, from mantle-derived magmas contaminated in the crust. Magmatic zircons from granites G5 and G6 show a larger variation, yielding also some positive εHft values (G5: from −8 to +6; G6: from −4 to +4) indicating crystallization from mildly contaminated mantle-derived magmas or from a lower crustal source. U–Pb isotope data of inherited zircon cores from granites G and G5 show that Neoproterozoic ages dominate, but only the inherited zircon cores from granite G have similar initial 176Hf/177Hf ratios to those from a chlorite phyllite of the Formation of Ervedosa do Douro (Carrazeda de Ansiães area), suggesting that this metasedimentary unit could have been involved in the genesis of the granite G. On the other hand, the Lu–Hf data suggest that the metagraywacke from Sabugal area should not be considered a component involved in the partial melting process that formed the granite G5. The occurrence of Mesoproterozoic detrital zircons in both metasediments suggests that these rocks are likely to have evolved from a peri-Amazonian realm, close to West Avalonian terranes, during the Neoproterozoic/Early Palaeozoic. However, a West African Craton provenance could also be considered, if a long distance river transportation of Mesoproterozoic zircons, from the southwest Baltica or Arabian–Nubian Shield, is admitted. The U–Pb isotope data obtained in detrital zircons from both metasedimentary rocks suggest that their maximum depositional ages are ca. 578Ma for the chlorite phyllite and ca. 577Ma for the metagraywacke.