Tectonically assisted exhumation and cooling of Variscan granites in an anatectic complex of the Central Iberian Zone, Portugal: constraints from LA-ICP-MS zircon and apatite U–Pb ages

Abstract

Understanding the exhumation of middle to lower crustal rocks is of utmost importance to unravel intracrustal mass transfer processes during orogenic build-up. The Figueira de Castelo Rodrigo–Lumbrales Anatectic Complex (FCR–LAC) is located within the autochthonous terrane of the Variscan Central Iberian Zone and is an example of the association between S-type granites and migmatites. The anatectic complex contacts to the north and south with low-grade metamorphic units through the Huebra and Juzbado–Penalva do Castelo shear zones, respectively. Integration of new U–Pb zircon and apatite age data allowed us to obtain Variscan crystallization ages, inherited zircon ages and unprecedented cooling rates for different facies of the FCR–LAC granites. The zircon crystallization ages mostly cluster around 313–317 Ma for the syn-tectonic granites, whereas the dated late-tectonic granite provided an age of 300 Ma. The cooling rates range from 13 to 35 °C Ma−1, which implies fast exhumation (0.3–0.84 mm a−1) and shallow emplacement (ca. 8 km deep), compatible with exhumation facilitated by large crustal-scale shear zones. Inherited zircon in the granites reveals melting of Cadomian metasediments (650–550 Ma), Upper Cambrian–Lower Ordovician (495–470 Ma) metaigneous rocks (Ollo de Sapo formation) and of minor older components, suggesting protolith affinity with the Northern Domain of the Central Iberian Zone.