The Beiras granite batholith is located in the CIZ, well within the Iberian Authochthon terrane (Fig. 3). It was emplaced into metasediments of Proterozoic-Cambrian to Upper Carboniferous age, variably affected by the main Variscan deformation phases (D1, D2 and D3).
Figure 3. Simplified geological map
Simplified geological map and tectonic setting of the Beiras region. U-Pb ages of the Beiras batholith from Mota Leite et al. (2005) (Calde granite) and Valle Aguado et al. (2005) (Junqueira, Cota-Viseu, Casal Vasco and Maceira granitoids).
The pre-Ordovician lithologies constitute a thick and monotonous megasequence of metapelites and metagreywackes, generally referred to as the "Complexo Xisto Grauváquico ante-Ordovícico" (CXG) (Carrington da Costa, 1950; Teixeira, 1954). A late Proterozoic to Cambrian age has been generally assumed for the CXG.
The post-Cambrian Palaeozoic stratigraphic record is exposed in the core of the Porto-Sátão Syncline and includes a succession of Ordovician, Silurian, Lower Devonian and Upper Carboniferous deposits (Fig. 3). These megasequences can be grouped into two main units: a) the pre-orogenic to syn-orogenic materials of Lower Ordovician to Lower Devonian age and b) the post-orogenic unmetamorphosed, continental, coal-bearing deposits of Upper Carboniferous age, formed in a narrow intramontane limnic tectonic basin.
According to Dallmeyer et al. (1997) and Ábalos et al. (2002), Variscan crustal thickening started around 360 Ma in the CIZ autochthon, with the D1 deformation phase inducing prograde metamorphism of Barrovian type. During the Early-Middle Carboniferous, the D1 contractional structures were variably overprinted by a major syn-collisional D2 extensional event attributed to a large-scale gravitational collapse of the thickened continental crust (e.g. Escuder Viruete et al., 1994; Díez Balda et al., 1995; Valverde Vaquero, 1997; Ábalos et al., 2002; Valle Aguado et al., 2005).
Late stage D3 deformation is related to three crustal-scale transcurrent shear zones (Fig. 3): the Porto-Tomar dextral shear zone (PTSZ), the Douro-Beira sinistral shear zone (DBSZ) and the sinistral Juzbado-Penalva shear zone (JPSZ) (Ribeiro et al., 1980; Iglesias and Ribeiro, 1981; Dias, 1994; Valle Aguado et al, 2000). It is generally agreed that these three shear zones have accommodated part of the shortening related to the final stages of the continental collision. D3 deformation occurred under greenschist facies retrograde conditions at higher crustal levels whilst, in lower crustal levels, high temperatures could have locally persisted as a result of the high thermal gradients inherited from D2 and the intrusion of synkinematic granitoids (Dallmeyer et al., 1997; Ábalos et al., 2002).