Multidisciplinary studies integrating, U–Pb geochronology, whole-rock geochemical data, isotope geochemistry, anisotropy of magnetic susceptibility (AMS) studies and gravimetry were carried out on the Vila Pouca de Aguiar and the Águas Frias-Chaves porphyritic biotite granite plutons. Both plutons occur independently in a distance of about 20 km. The Vila Pouca de Aguiar and Águas Frias-Chaves plutons are examples of late to post-orogenic felsic Variscan granites in northern Portugal (NW Iberian Peninsula). The U–Pb zircon analyses yield a consistent age of 299 ± 3 Ma which is considered to be the emplacement age of the two plutons. These granites are weakly peraluminous, show high HREE and Y (and low P) contents which are consistent with them being I-type. This is also supported by their weakly evolved isotopic compositions, 87 Sr/ 86 Sr i = 0.7044–0.7077 and εNd = − 2.0 to − 2.6, as well as by the whole rock oxygen isotope ( δ 18 O VSMOW) ranging from + 9.7‰ to + 11.0‰. The emplacement of granite magma took place after the third Variscan deformation phase (D 3 ) in an extensional tectonic regime, large scale uplift and crustal thinning. The integration of different data suggests that both plutons have the same feeding zone aligned within the Penacova-Régua-Verin fault (PRVF) and that both have the same structure which is related to late Variscan phases. The thicker shape for the Águas Frias-Chaves pluton comparing to that of the Vila Pouca de Aguiar pluton is compatible with different depths of PRVF sectors. The available data led us to propose a model of partial melting of a meta-igneous lower crustal source rather than an open-system of mantle–crust interaction. The interaction between the continental crust and invading mafic magmas could have been limited to mere heat transfer and, perhaps, local intermingling. Keywords Variscan granites I-type Geophysics Isotopic data Emplacement 1 Introduction The Variscan orogeny, a major event in the tectonic evolution of Western Europe, is currently explained by an obduction–collision orogenic model ( Ribeiro et al., 1990; Matte, 1991; Dias and Ribeiro, 1995 ). In NW of the Iberian Peninsula, three main ductile deformation phases have been identified in this part of the Variscan belt ( Ribeiro, 1974; Noronha et al., 1979; Ribeiro et al., 1990 ). The last ductile deformation phase (D 3 ), Namurian–Westphalian in age, is followed by a brittle deformation phase (post-D 3 ), late Carboniferous to Permian in age which is characterised by a set of conjugate strike slip faults (NNW-dextral and NNE-sinistral), pointing to a late-Variscan main compression around N–S ( Ribeiro, 1974; Arthaud and Matte, 1975 ). The D 3 and the post-D 3 deformation phases are related to the post-thickening extensional tectonic regime ( Lagarde et al., 1992; Dias and Ribeiro, 1995 ). During this post-collisional stage a continuous magmatic activity (mainly granitic) took place in the Central Iberian Zone and consequently in Northern Portugal. Based on several geological data and U–Pb emplacement ages related to this third Variscan phase D 3 ( Ferreira et al., 1987; Dias et al., 1998; Martins, 1998 ), the post-collisional granites were divided into the following groups: synorogenic (sin- late- and late to post-D 3 ; 320–300 Ma) and late to post-orogenic (post-D 3 ; 299–290 Ma). The emplacement of the Vila Pouca de Aguiar and the Águas Frias-Chaves granite plutons, located in the Central Iberian Zone, Northern Portugal, ( Farias et al., 1987 ), was controlled by the late brittle deformation phase, post-D 3 ( Martins, 1998 ). The Penacova-Régua-Verin fault (PRVF) is one of the late Variscan deep crustal lineations, which belongs to the NNE–SSW trending brittle system that crosscuts the whole of Northern Portugal. The PRVF was nucleated on D 3 and reactivated latter as a sinistral strike-slip fault with transtensional component. The granites presented in this paper are spatially related with this late strike slip fault, PRVF, and belong to the group of late to post-orogenic (post-D 3 ) granites. This fault, still tectonically active ( Cabral, 1995 ), and in its NE branch presents several CO 2 rich thermal water springs. Multidisciplinary studies were carried out in order to compare structural and genetic features of the Vila Pouca de Aguiar and the Águas Frias-Chaves plutons and to point out the relation with PRVF. The aim of this work is to characterize the shape of pluton at depth, by geophysical measurements, to investigate the age of emplacement and discuss the potential source materials, by major and trace elements geochemistry and isotope data, from representative post-D 3 Variscan plutons in northern Portugal. 2 Geological setting The most common Portuguese Variscan granitoids in the Central Iberian Zone are biotite granodiorites and monzogranites and were emplaced during the post-collisional stage of Variscan orogeny. In the Vila Pouca de Aguiar and the Águas Frias-Chaves plutons, biotite granites are the main variety and a detailed petrographic characterisation of these granites is present in Almeida et al. (2002) . The Vila Pouca de Aguiar and the Águas Frias-Chaves porphyritic biotite granite plutons are separated ca 20 km one from the other in independent outcrops and were emplaced into the major Régua-Verin fault, which belongs to the post-D 3 NNE-trending fault system ( Fig. 1 ). The Águas Frias-Chaves granite pluton is a small body, more or less regular in outcrop, which occupies an area of 30 km 2 and it is composed of a marginal porphyritic biotite-rich coarse-medium grained granite, the Águas Frias granite (AFG), and by a central two-mica medium-grained granite, the S to António de Monforte granite (SAMG) in which sampling is very difficult to obtain due...

In the Central Iberian Zone synorogenic (syn- and late-D-3, the last deformation phase) composite batholiths of biotite-rich granitods with peraluminous character are well represented. We report here U-Pb zircon and monazite ages, geochemical, Sr-Nd-O isotopic data and Anisotropy of Magnetic Susceptibility (AMS) studies for the late-D-3 Vieira do Minho pluton, in northern Portugal. AMS reveals the paramagnetic behaviour of these granites and the magnetic fabric enhances the major role of NW-SE regional anisotropies on controlling the ascent and emplacement of the magmas. Magnetic anisotropy and medium temperature microstructures also point out the evolution of regional tectonics during crystallization of the magma. U-Pb zircon and monazite analyses yield consistent ages of 310 +/- 2 Ma and 312 +/- 2 Ma, interpreted as emplacement age. The Vieira do Minho pluton consists of two peraluminous monzogranites, the VMG and the MRG. Both have similar epsilon(Nd) values (VMG= -52 to -5.7; MRG= -4.98 to -5.96) but the VMG is slightly enriched in Sr-87/(86)(i)=0.7087-0.7098 as well as in delta O-18 in the range of 10.6 parts per thousand to 11.0 parts per thousand than the MRG with Sr-87/(86)(i)=0.7064-0.7075 and delta O-18=9.9 parts per thousand-10.5 parts per thousand. These granites are associated with coeval scarce grabroic intrusions and/or mafic microgranular enclaves which are not considered as mafic precursors of the associated granitic magmatism. Instead, a lower metaigneous crustal source, at different levels, is proposed based on the available data set

Studies of Anisotropy of Magnetic Susceptibility (AMS) and U-Pb geochronology, in zircon fractions, were carried out in a magnetite type-granite, the Lavadores granite, which is an example of a late to post-orogenic Variscan granite in northern Portugal (NW Iberian Peninsula). The U-Pb zircon analyses yield an age of 298 ± 11 Ma that is considered to be the emplacement age of this granite and thus suggesting a post-tectonic emplacement. This is also supported by the magmatic nature of the microstructures. Magnetic lineations are subvertical and magnetic foliations display a general WNW-ESE subvertical pattern consistent with a dextral kinematic. These AMS data suggest that the emplacement of the Lavadores granite was controlled by a major mechanical anisotropy formed by transtensional structures associated with the Porto-Tomar shear zone.

In northern Portugal, large volumes of granitoids were emplaced during the last stage (D ₃ ) of the Variscan orogeny and display a wide range of petrological signatures. We studied the morphologies and internal structures of zircons from syn-, late- and post-D ₃ granitoids. The sin-D ₃ granitoids include the Ucanha–Vilar, Lamego, Felgueiras, Sameiro, and Refoios do Lima plutons, the late- and post-D ₃ granitoids are represented by the Vieira do Minho and the Vila Pouca de Aguiar plutons, respectively. Typological investigations after Pupin (1980) along with scanning electron microprobe imaging reveal that the external morphology of zircon changes consistently with a decrease in the crystallization temperature. Zircon populations from the Refoios do Lima and the Vieira do Minho granites show gradual changes in the internal morphologies and their typologic evolution trends are consistent with their mainly crustal origin. The Sameiro, Felgueiras, Lamego and Ucanha-Vilar granites have more complex internal and external morphology and typological evolution trends that cross the domain of the calc-alkaline to the aluminous granites compatible with a mixing process. Finally, the morphological types of the Vila Pouca de Aguiar granites are found both in calc-alkaline and sub-alkaline granites and their typological evolutionary trends follow the calc-alkaline/sub-alkaline trend, suggesting crustal sources with some mantle contribution.