Genesis and emplacement of felsic Variscan plutons within a deep crustal lineation, the Penacova-Regua-Verin fault: An integrated geophysics and geochemical study (NW Iberian Peninsula)

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Genesis and emplacement of felsic Variscan plutons within a deep crustal lineation, the Penacova-Régua-Verín fault: An integrated geophysics and geochemical study (NW Iberian Peninsula) H.C.B. Martins ⁎, H. Sant'Ovaia, F. Noronha Department of Geology/Geology Centre, Faculty of Sciences, University of Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal abstractarticle info Article history: Received 29 February 2008 Accepted 17 October 2008 Available online 5 November 2008 Keywords: Variscan granites I-type Geophysics Isotopic data Emplacement 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. © 2008 Elsevier B.V. All rights reserved. 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 NWof 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 postthickening 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 (sinlateand 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 Lithos 111 (2009) 142–155 ⁎Corresponding author. Tel.: +351 22 0402461; fax: +351 22 0402490. E-mail address: [email protected] (H.C.B. Martins). 0024-4937/$ –see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2008.10.018 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Author's personal copy 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 postD 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 twomica medium-grained granite, the S to António de Monforte granite Fig. 1. (a) Geological distribution of Variscan syn to post-orogenic granitoids in Central Iberian Zone. 1. Post Palaeozoic; 2. Post-orogenic biotite granites; 3. Late-orogenic biotite granites; 4. Synorogenic two-mica granites; 5. Synorogenic biotite granites; 6. Metasedimentary rocks; 7. Faults. Inserted rectangles: studied plutons. (b and c) Sketch maps of the Águas Frias-Chaves and the Vila Pouca de Aguiar plutons. Geographical coordinates: UTM kilometric system. PSG: Pedras Salgadas Granite; VPAG: Vila Pouca de Aguiar Granite; AFG: Águas Frias granite; SAMG: S to António de Monforte granite. PRVF: PenacovaRégua-Verin Fault. 143H.C.B. Martins et al. / Lithos 111 (2009) 142–155 Author's personal copy (SAMG) inwhich sampling is very difficult to obtain due to the scarcity of outcrops and weathering effects. The Vila Pouca de Aguiar pluton (ca 200 km 2 ), has a NNE–SSW elongated shape, is a composite pluton with two main different biotitic granite facies: a peripherical biotiterich granite, the Vila Pouca de Aguiar granite (VPAG) and a central biotite granite, the Pedras Salgadas granite (PSG). These granites define a more or less concentric zoning and the field relationships suggest a nearly synchronous magmatic emplacement. The VPAG and AFG are mediumto coarse-grained granites characterized by a relative abundance of biotite and by a porphyritic texture composed of abundant light coloured K-feldspar megacrysts. ThePSGisahomogeneousgranite,alsoporphyritic,butmoreleucocratic than VPAG and AFG, and shows globular quartz in a mediumto finegrained groundmass and relatively scarce K-feldspar megacrysts not larger than 2.5 cm. Both plutons are intruded into two-mica peraluminousgranitesascribedtotheD 3 Variscanevent,and Upper Ordovicianto Lower Devonian metasedimentary sequence of the “Peritransmontano” Domain (Ribeiro,1974; Ribeiro,1998; Ribeiro et al., 2004)characterised by D 3 N120°trendingfoldswithsubhorizontalaxes andsubverticalaxial planar foliation (Ribeiro et al., 1990). The contacts are sharp, intrusive and discordant in relation to the general trending of the earlier Variscan structures which allow us to consider that the two plutons are late to post-orogenic or post-D 3 . These granites produced a metamorphic contactaureole(1kmwidth)withtheassemblagequartz+K-feldspar+ muscovite+cordierite±andalusite (Brink, 1960; Ribeiro, 1998), indicating a shallow crustal emplacement level. Scarce rounded microgranular mafic enclaves of granodioritic or more rarely tonalitic composition,varyinginsizefrom 10 to20cm, arealsoobservedspecially in the VPAG (Neiva and Gomes,1991). The three granites are monzogranites in composition and they contain 30 to 32% modal quartz, 20 to 24% perthitic K-feldspar (orthoclase and microcline) and 37 to 42% plagioclase with normal zoning, oligoclase-andesine in the VPAG+AFG and albite-oligoclase in the PSG. Biotite (modal average of 5% in PSG and 9% in VPAG+AFG) is present as the only ferromagnesian phase. Accessory minerals include zircon, apatite, allanite, xenotime, ilmenite, sphene and rare monazite in PSG; some muscovite and rare cordierite are present in the AFG. Although the AMS and the gravimetry studies were carried out on thetwoplutonsasawhole,thegeochemicaldatapresentedcorresponds only to the main biotitic granitic facies (VPAG, PSG and AFG). 3. Analytical methods 3.1. Geophysics 3.1.1. Anisotropy of magnetic susceptibility The AMS study was carried out to acquire a complete data set of the fabrics of the studied granites. At each site, four oriented cores (25 mm in diameter and 60 to 70 mm in length) were collected, each core being then sawed in two (eventually three) 22 mm long specimens. At least eight specimens per station were available for magnetic measurements. A total of 943 rock-cylinders were prepared for magnetic measurements for the Vila Pouca de Aguiar pluton. In the Águas Frias-Chaves pluton, the sampling grid is less dense and 125 rock-cylinders were measured. Measurements were performed using a KLY-3 Kappabridge susceptometer (±3.8×10 −4 T; 920 Hz) (in “Laboratoire des Mécanismes et Transferts en Géologie”da Universidade Paul Sabatier de Toulouse) and in a KLY-4 S model in the Geology Centre, Department of Geology, Faculty of Sciences (Porto University). A sequence of several measurements along different orientations of each specimen allowed computing orientation and magnitude of the three main axes k 1 ≥k 2 ≥k 3 of the anisotropy of magnetic susceptibility ellipsoid. With ExAMS program of Saint Blanquant (Unpublished) the mean susceptibility K m of each site which is the mean of the six individual arithmetic means k 1 +k 2 +k 3 /3 was calculated. The three axes K 1 ≥K 2 ≥K 3 , were also calculated, which are the vectorial means of the k 1 ≥k 2 ≥k 3 axes of the eight specimens. K 1 , the long axis of the mean ellipsoid, is the magnetic lineation of the site and K 3 , the short axis, is the normal to the magnetic foliation. P, the magnetic anisotropy ratio, corresponds to K 1 /K 3 . We have used in this study the parameter Ppara%=((K 1 −D/K 3 −D)−1)×100 and D(=−1.4 × 10 −5 SI), the diamagnetic component carried by the quartz and feldspars (Rochette, 1987). This parameter Ppara% is more convenient for the rocks displaying a low susceptibility from which it is necessary to subtract Dwhich is constant and isotropic (Bouchez et al., 1987). 3.1.2. Gravimetry Amongst the geophysical tools applied to granite bodies, gravimetry measurements are best suited to investigate the shape of the plutons at depth. Through the inversion of gravity data, which is particularly sensitive to density contrasts, the shape at depth of the pluton, and depth of its floor, may be obtained with good confidence. The understanding of the 3 dimensional shapes of the granite bodies and of their floor's depth can be used to find the feeder zones of the plutons. This study deals with the characterization of 3 dimensional shapes of the Vila Pouca de Aguiar and the Águas Frias-Chaves plutons, using the interpretation of the gravity data and the modelling of the residual anomaly obtained. In the Águas Frias-Chaves pluton gravity measurements were performed over 3649 closely spaced stations homogeneously distributed within an area of 379 km 2 , between the meridians 620 and 641 km and the parallels 4615 and 4632 km of the U.T.M. Kilometric System. In the Vila Pouca de Aguiar pluton 2027 measurements, were performed, corresponding to an area of 825 km 2 , between the meridians 605 and 630 km and the parallels 4586 and 4619 km of the U.T.M. Kilometric System. The raw gravity data were obtained with a gravimeter Lacoste and Romberg, G model, with a precision of ±0.01 mGal and with temperature and pressure compensation. Elevations were determined using a precise (±1 m) baro-altimeter that was calibrated several times a day. The treatment of raw gravity data was comprised of several stages: gravimetric corrections, subtraction of the regional effect and modelling (inversion techniques). In a gravity survey, several effects are produced by sources, which are not of direct geological interest for the purpose of this study. Once these effects are removed by correcting the raw data to a datum (topography, elevation, latitude) and also from the tidal and instrumentation variations, the Bouguer anomaly values are determined. With these values, a grid can be computed and a Bouguer anomaly map is drawn. The combination of the isovalue contour line gradient of the Bouguer anomaly map and geological knowledge yields a first interpretation for the geometry of the granite body. Our raw gravity data were corrected for the intrinsic constant of the apparatus and tidal effects and also for the usual topography, latitude and elevation corrections. The Bouguer correction was performed assuming a density of 2.70 and was interpolated by kriging along a one-kilometer sided grid. The residual anomaly for each one of the plutons was calculated from the Bouguer anomaly map by subtracting the regional gravity trend, which was modelled by a polynomial adjustment. According to Vigneresse (1990), the convenient residual anomaly map is obtained when the zero contour level of this map best outlines the contour of the granite body. The residual anomaly was inverted using an iterative procedure. Among the several possible inversion methods, our modelling was performed using the 3-D iterative procedure derived from Cordell and Henderson (1968) adapted to small-scale gravimetric investigations by Vigneresse (1990). In this process (Ameglio et al.,1997), the source was roughly modelled by small prisms each having a constant density and a progressive adjustment was performed until the calculated gravity field fitted the observed data. A map of surface densities for the main granite types of the two plutons and surrounding rocks, have been incorporated in the computation, in order to better constrain the nominal densities of the prisms. Data inversion was also tested with 144 H.C.B. Martins et al. / Lithos 111 (2009) 142–155 Author's personal copy Table 1 Major (wt.%), trace and rare earth (ppm) elements data of samples from Vila Pouca de Aguiar granite (VPAG), Pedras Salgadas granite (PSG) and Águas Frias granite (AFG), northern Portugal. VPAG PSG AFG Samples 74-3 74-12 74-15 74-16 74-9 74-5 74-20 74-4B 46-2 61-13 61-6A 61-6B 60-14 60-15 60-16 60-11 60-12 60-17 60-13 60-10 60-18 60-19 61-6 74-8 74-7 74-11 60-1 60-2 34-4 CV1 CV2 CV-8 CV-11 CV-13 SiO 2 70.72 71.52 72.24 71.96 72.89 71.72 70.65 71.84 71.77 71.36 70.74 70.91 71.04 72.35 70.65 70.47 71.18 73.77 73.62 74.24 73.00 74.47 74.40 74.12 73.56 73.95 72.40 73.72 72.77 73.11 72.19 71.96 72.71 73.85 TiO 2 0.39 0.35 0.31 0.4 0.3 0.3 0.43 0.38 0.34 0.37 0.36 0.36 0.39 0.34 0.36 0.32 0.36 0.16 0.17 0.15 0.19 0.12 0.15 0.17 0.18 0.24 0.17 0.20 0.34 0.28 0.32 0.31 0.30 0.29 Al 2 O 3 14.51 14.57 13.35 13.66 13.73 13.59 14.11 13.46 13.56 14.07 14.36 14.11 14.03 13.51 13.56 14.41 13.83 13.53 13.38 13.35 13.57 13.33 13.48 13.65 14.08 13.90 13.87 13.98 13.82 13.61 14.07 13.90 13.84 13.31 Fe 2 O 3 t 3.08 2.47 2.22 2.62 2.09 2.8 3.22 2.97 2.72 2.9 2.74 2.94 2.9 2.72 2.87 2.59 2.95 1.62 1.77 1.62 1.76 1.41 1.62 1.56 1.45 1.51 1.63 1.55 2.48 2.26 2.30 2.24 2.27 2.16 MnO 0.06 0.06 0.05 0.06 0.06 0.05 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.05 0.04 0.04 0.03 0.04 0.03 0.04 0.03 0.03 0.05 0.07 0.08 0.05 0.06 0.05 0.05 0.05 0.05 0.05 0.05 MgO 0.79 0.63 0.51 0.64 0.61 0.49 0.79 0.7 0.64 0.71 0.65 0.68 0.69 0.61 0.65 0.58 0.63 0.25 0.32 0.26 0.32 0.25 0.26 0.42 0.32 0.33 0.40 0.28 0.70 0.62 0.69 0.65 0.64 0.60 CaO 1.99 1.63 1.54 1.75 1.5 1.71 1.93 1.62 1.67 1.79 1.9 1.75 1.83 1.67 1.79 1.61 1.58 1.18 0.86 1.03 1.18 0.83 1.04 0.88 1.12 0.98 1.05 1.26 1.51 1.65 1.80 1.66 1.63 1.56 Na 2 O 3.68 3.7 3.54 3.5 3.62 3.52 3.5 3.34 3.47 3.54 3.66 3.58 3.47 3.45 3.47 3.58 3.32 3.50 3.45 3.54 3.52 3.55 3.45 3.52 3.45 3.33 3.61 3.38 3.49 3.53 3.61 3.54 3.50 3.39 K 2 O 4.41 4.62 4.47 4.2 4.25 4.26 4.24 4.25 4.3 4.26 4.26 4.36 4.33 4.2 3.95 4.82 4.54 4.59 4.67 4.55 4.52 4.67 4.47 4.91 4.64 4.79 4.61 4.64 4.08 4.05 4.21 4.09 4.01 3.96 P 2 O 5 0.12 0.06 0.14 0.16 0.07 0.11 0.13 0.11 0.11 0.15 0.13 0.12 0.13 0.1 0.11 0.12 0.13 0.06 0.07 0.06 0.07 0.05 0.07 0.04 0.05 0.10 0.02 0.05 0.15 0.13 0.14 0.15 0.15 0.13 LOI 0.62 0.67 0.61 0.83 0.82 0.9 0.77 1.09 1.18 0.63 0.93 0.93 0.95 0.81 1.23 1.04 1.23 1.10 1.15 1.02 1.12 1.10 0.85 0.76 0.76 0.90 0.76 0.61 0.80 0.64 0.54 0.59 0.69 0.76 Total 100.4 100.3 98.98 99.78 99.94 99.45 99.81 99.8 99.8 99.82 99.77 99.78 99.8 99.8 98.69 99.58 99.79 99.79 99.50 99.85 99.29 99.81 99.82 100.1 99.61 100.1 98.57 99.66 100.19 99.94 99.93 99.14 99.80 100.07 Ba 330 401 362 331 266 264 375 332 267 339 344 373 345 275 284 416 453 302 289 265 295 271 277 272 250 200 330 225 288 267 308 319 300 257 Rb 233 234 251 247 214 245 236 243 263 222 242 222 232 226 222 245 221 225 254 276 237 273 237 277 278 312 237 287 228 237 241 227 231 216 Sr 69 124 113 101 102 36 108 90 86 98 103 100 103 83 88 100 103 69 71 65 70 58 66 44 75 60 85 75 89 82 98 103 90 82 Cs 23.8 17.5 18.1 –13.8 –18.9 18.8 19.4 –– –18.6 26.2 17.0 35.2 18.0 17.7 23.9 22.6 16.0 19.0 22.0 –19.9 20.2 –19.2 24.5 22.1 19.2 17.8 20.6 23.8 Co 5.00 3.90 3.40 –36.50 –5.23 4.49 4.33 4.00 4.22 4.31 4.64 4.27 4.26 3.94 4.00 1.89 1.96 2.04 2.00 1.60 1.70 –1.00 1.00 –1.20 4.21 3.53 3.96 3.84 3.00 3.00 Cr 31.0 29.0 33.0 –29.0 –72.7 65.0 87.3 76.0 82.3 99.0 48.9 79.4 73.1 62.8 85.5 72.7 74.0 75.8 54.0 68.8 48.0 –26.0 28.0 –32.0 56.1 114.0 47.5 51.8 49.0 41.0 Zn 54.0 46.0 47.0 –13.0 –54.8 51.6 46.3 46.0 42.6 58.6 51.4 44.8 49.5 46.0 55.0 33.3 28.9 35.0 33.0 34.0 37.9 –36.0 42.0 –27.0 63.5 36.0 61.6 65.2 53.0 36.0 Sn 15.00 2.00 13.00 –20.00 –11.30 12.00 13.60 –– –10.70 14.00 15.10 10.90 12.00 10.60 11.00 17.00 9.96 15.90 18.00 –21.00 57.00 –26.00 13.05 12.66 12.75 13.44 13.00 12.00 W 6.80 2.10 0.80 –––2.43 2.00 2.29 –– –1.43 1.90 3.15 6.92 5.00 2.84 4.89 2.80 2.06 2.49 3.42 –1.50 1.60 –3.60 1.81 1.13 –––– Nb 16.30 15.70 15.70 –14.50 –14.20 14.90 14.40 13.60 ––13.30 13.60 14.30 12.80 13.80 12.10 13.00 14.00 13.00 13.00 14.00 –15.20 15.90 –15.60 17.89 14.70 15.94 16.29 17.00 15.00 Zr 169 162 153 –126 –196 189 157 173 170 172 155 150 158 148 189 99 110 108 106 87.8 100 –96 98 –97 147 122 144 136 134 117 Ga 24.00 24.00 23.00 –24.00 –22.00 21.70 21.30 20.00 21.70 21.30 20.90 19.70 20.00 22.00 21.80 18.00 19.00 20.70 19.00 19.70 19.00 –25.00 26.00 –27.00 20.93 18.65 20.02 21.02 19.00 18.00 Th 16.15 19.82 18.72 –17.60 –18.00 20.60 21.60 20.80 19.00 24.20 15.00 16.50 12.90 17.00 26.50 21.00 21.50 20.70 22.90 19.40 19.00 –20.69 20.84 –23.26 19.07 15.63 17.65 18.08 17.00 14.50 Y 36.00 37.00 38.00 –36.00 –38.00 41.80 48.40 39.90 41.30 40.70 36.00 38.40 37.30 26.60 40.80 33.00 40.00 37.00 39.80 43.60 27.00 –35.00 40.00 –35.00 32.93 29.82 32.10 30.48 34.00 30.00 V 30.00 22.00 20.00 –19.00 –35.60 29.60 27.70 31.60 28.80 28.50 29.00 26.40 26.20 24.00 26.00 10.80 13.70 11.00 12.90 10.00 10.70 –10.00 9.00 –11.00 –––––– Be 6.00 5.00 6.00 –5.00 –6.57 5.83 7.77 5.69 6.38 4.07 5.80 6.98 7.00 6.79 5.00 5.15 5.57 8.45 5.74 8.78 6.00 –6.00 8.00 –10.00 7.00 6.00 6.00 6.00 5.00 5.00 U 5.51 6.81 9.29 –6.69 –8.35 8.47 9.07 –– –7.51 10.70 5.08 6.85 8.53 7.86 12.60 12.50 7.46 10.00 13.50 –4.43 4.83 –12.53 5.78 12.40 16.64 5.93 4.80 5.80 Hf 5.10 5.10 4.60 –4.10 –––––––––––––––––––3.60 3.60 –3.70 4.87 4.15 4.77 4.17 4.40 3.90 La 28.77 –27.32 –––32.72 30.53 26.08 36.02 30.77 33.10 25.84 26.05 27.72 33.59 39.99 27.81 27.92 26.24 26.11 25.46 24.40 –– –31.50 –33.10 24.16 27.47 31.39 30.90 25.60 Ce 66.48 –66.23 –––70.23 65.51 57.05 73.04 66.47 69.21 54.85 55.33 59.97 71.38 83.18 57.57 56.23 56.34 55.40 52.85 48.24 –– –70.50 –67.78 50.60 57.73 64.03 65.20 54.40 Nd 27.47 –25.12 –––31.75 30.43 27.68 32.08 30.07 31.58 24.61 28.04 27.60 29.69 37.40 23.63 23.64 23.68 25.52 22.67 21.07 –– –25.21 –28.64 22.55 25.51 27.08 27.80 23.40 Sm 6.45 –6.05 –––7.42 7.15 6.65 7.43 6.98 6.88 6.23 7.10 6.54 5.81 7.59 5.62 5.39 5.56 6.12 5.64 4.88 –– –5.95 –6.28 5.21 5.91 5.96 6.80 5.90 Eu 1.04 –0.94 –––1.03 0.81 0.77 0.74 0.81 0.74 0.80 0.89 0.78 0.77 0.88 0.54 0.50 0.58 0.70 0.47 0.55 –– –0.72 –0.64 0.56 0.67 0.68 0.65 0.55 Gd 6.11 –5.64 –––6.30 6.11 6.32 5.87 6.86 6.56 5.47 6.00 5.83 4.99 7.22 4.87 4.91 5.09 5.60 5.36 4.16 –– –5.44 –6.01 5.17 5.70 5.56 6.00 5.20 Dy 5.91 –5.68 –––6.42 6.63 7.22 6.21 6.46 6.29 5.89 6.63 6.23 4.37 6.34 5.59 5.75 5.61 6.56 6.59 4.68 –– –5.50 –5.96 5.23 5.68 5.46 6.00 5.20 Er 3.28 –3.26 –––3.29 3.72 4.53 3.50 3.90 3.83 3.23 3.89 3.72 2.28 3.65 3.33 3.50 3.42 3.97 4.10 2.66 –– – 3.12 –3.41 3.02 3.23 3.05 3.60 3.30 Yb 3.35 –3.59 –––3.70 4.05 5.41 3.89 4.51 4.19 4.13 4.94 4.67 2.44 3.90 4.23 4.00 4.38 4.91 5.93 3.55 –– –3.53 –3.50 3.19 3.41 3.08 3.30 3.10 Lu 0.63 –0.67 –––0.58 0.62 0.76 0.54 0.69 0.64 0.58 0.74 0.66 0.33 0.53 0.64 0.55 0.67 0.76 0.85 0.55 –– –0.71 –0.52 0.47 0.52 0.45 0.48 0.44 145H.C.B. Martins et al. / Lithos 111 (2009) 142–155 Author's personal copy respect to sensitivity of results to the density contrasts. A map of the calculated depth of the pluton floor was finally obtained. Densities weremeasured using the cylindricalspecimens that were collected for the AMS study. Densities were determined with the pycnometer technique as described by Vigneresse and Cannat (1987): the cores were set under vacuum, impregnated by water and weighed, then slowly dried to avoid thermal cracking, and weighed again. 3.2. Geochemistry 3.2.1. Whole rock geochemistry Representative majortraceand rare-earth element data of the three granites studied, the VPAG, the PSG and the AFG, are reported in Table 1. Whole-rock chemical compositions from the VPAG and the PSG were analysed by inductively coupled plasma emission spectrometry, ICP-AES, and ICP-MS, mass spectrometry using the method of Govindaraju and Mevelle (1987) at CRPG, Nancy, France. The precision of the analyses was about 1% for major elements and most trace elements. Some trace elements (W, U, Th, Ta and Hf) were analysed by neutron activation, INAA, (Hofmann, 1992). Majortraceand rareearth element compositions from the AFG were obtained by ICP-MS at Activation Laboratories (Canada). Precision and accuracy of ICP-MS analyses are commonly within 10%. The validity and usefulness of any compositional data are crucially dependent upon the quality of the sample collected in the field, so thirty four representative samples, 12–15 kg each, were collected in outcrops and some quarries taking into account that all of them were as fresh and unweathered as possible. 3.2.2. Sr–Nd isotopes Thirteen samples were selected for Rb–Sr and nine of these for Sm–Nd isotopic analysis (Table 2). Rb–Sr and Sm–Nd isotopes from the VPAG and the PSG were analysed at CRPG, Nancy, France, using techniques described by Michard et al. (1985). The concentrations of Rb, Sr and Sm, Nd were determined by isotopic dilution and Rb isotopic ratios were measured using a Cameca TSN 206 mass spectrometer, while Sr, Sm and Nd isotopic ratios were measured with a Finnigan MAT 262 mass spectrometer. The 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios were corrected for mass fractionation effects to 86 Sr/ 88 Sr=0.1194 and 146 Nd/ 144 Nd=0.7219 using NBS 987 Standard. The maximum uncertainties in 87 Rr/ 86 Sr and 147 Sm/ 144 Nd were 1.5% and 0.5% respectively, at the 95% confidence level (2σ). Measurements of Sr and Nd isotope values from the AFG (three samples) were carried out at the Laboratoire Magmas et Volcans, Clermont-Ferrand (France). Chemical procedures for sample preparation are described in Pin et al. (1994) and Pin and Santos Zalduegui (1997). Sm and Nd concentrations were determined by isotope-dilution TIMS using a mixed 149 Sm150 Nd tracer. The 147 Sm/ 144 Nd values are precise to +/−0.2% at the 95% confidence level. 143 Nd/ 144 Nd ratios were measured by TIMS in a ThermoFinnigan Triton instrument in static multicollection mode, and corrected for mass fractionation by normalization to 146 Nd/ 144 Nd=0.7219. The JNdi isotopic standard measured under the same conditions gave 143 Nd/ 144 Nd=0.512099(1). 87 Sr/ 86 Sr ratios were measured by TIMS using a modified VG54E instrument in static multicollection mode, and corrected for mass fractionation by normalization to 86 Sr/ 88 Sr=0.1194. The reported 87 Sr/ 86 Sr ratios were adjusted to the NIST SRM 987 standard 87 Sr/ 86 Sr=0.710244(20), at the 95% confidence level (2σ). In the calculations of ɛN dT , 143 Nd/ 144 Nd CHUR =0.512638 and 147 Sm/ 144 Nd CHUR =0.1967 (Jacobsen and Wasserburg, 1984)have been used. Regression lines on a 87 Sr/ 86 Sr vs 87 Rb/ 86 Sr plot have been calculated using the least-squares method as implemented in the Isoplot program (Ludwig, 2003). 3.2.3. Oxygen isotope Oxygen isotope data were performed on seven samples analysed for Sr and Nd isotopes at the Stable Isotopic Laboratory of Salamanca. Oxygen was extracted from rocks by laser fluorination techniques, quantitatively converted to CO 2 by the reaction with a heated carbon rod and analyzed for 18 O/ 16 O ratio with a dual inlet VG SIRA-II Mass Spectrometer. The analytical data are reported in the familiar δnotation referenced to SMOW. Two or more extractions were made on each sample; the reproducibility of isotopic analyses is ±0.1‰. NBS28 yielded an average δ 18 O value of 9.5‰VSMOW. The isotopic results, along with the calculated initial 87 Sr/ 86 Sr and ɛNd values are given in Table 2. 3.2.4. U–Pb dating The U–Pb isotopic analyses were carried out also at CRPG, Nancy, France, using a conventional U–Pb method on multigrain zircon fractions. The zircons were recovered by crushing the sample and sieving, followed by heavy liquid and magnetic separations and finally hand picking. Four zircon fractions were selected according to their morphology, colour, and lack of inclusions, fractures and metamictisation. Some of these fractions were submitted to air-abrasion (Krogh, 1982) to eliminate the external zones of the crystal where Pb loss may have occurred. All zircon fractions were observed on a backscattered scanning electron microscopy (BSEM). Table 2 Sr, Nd and δ 18 O isotopic data selected from Vila Pouca de Aguiar granite (VPAG), Pedras Salgadas granite (VPAG), Pedras Salgadas granite (PSG) and Águas Frias granite (AFG), northern Portugal. Rb tot Sr tot 87 Rb/ 86 Sr 87 Sr/ 86 Sr 87 Sr/ 86 Sr i Nd Sm 147 Sm/ 144 Nd 143 Nd/ 144 Nd ɛNd δ 18 O (ppm) (ppm) ±(2σ) (299 Ma) (ppm) (ppm) ±(2σ) (299 Ma) ‰ VPAG 60-12 b 217.25 106.69 5.906 0.732084 (30) 0.7069 33.21 8.39 0.1529 0.512423 (8) −2.6 10.3 60-16 b 202.09 91.00 6.444 0.734166 (41) 0.7067 ––– – – – 46-2 b 261.65 86.52 8.779 0.744242 (22) 0.7069 24.31 5.8 0.1443 0.512410 (9) −2.5 10.3 74-3 b 222.39 114.79 5.617 0.730929 (25) 0.7070 26.52 5.95 0.1357 0.512392 (12) −2.5 – 74-12 b 232.19 99.33 6.781 0.735986 (29) 0.7071 ––– – – – PSG 60-10 b 275.89 67.35 11909.000 0.755493 (24) 0.7048 20.56 5.05 0.1431 0.512435 (7) −2.0 10.5 60-13 266.27 71.53 10814.000 0.750709 (25) 0.7047 21.33 4.87 0.1381 0.512423 (12) −2.0 11.0 60-17 253.36 71.43 10302.000 0.748267 (20) 0.7044 ––– – – – 60-19 290.98 58.58 14465.000 0.766501 (29) 0.7050 19.44 4.77 0.1484 0.512444 (7) −2.0 10.3 74-8 272.49 109.6 7255.000 0.735876 (37) 0.7052 ––– – – – AFG 34-4 228.46 88.63 7.437 0.73934(10) 0.7077 29.10 6.05 0.1257 0.512371(5) −2.5 9.7 CV-2 241.49 97.65 7.135 0.737973(13) 0.7076 29.90 6.26 0.1265 0.512373(4) −2.5 9.8 CV-13 215.95 82.01 7.645 0.740401(15) 0.7079 27.30 5.89 0.1303 0.51238(5) −2.5 – 146 H.C.B. Martins et al. / Lithos 111 (2009) 142–155 Author's personal copy Chemical preparation of U and Pb analysis includes: (1) HNO 3 3N warm washing of the zircon; (2) HF digestion at 240 °C and HCl 3 N dissolution of the fluorides at 180 °C in a Teflon bomb (Parrish,1987); (3) separation of Pb and U by elution on anionic resin of two aliquots (one with the addition of a mixed 208 Pb235 U spike) following Krogh (1973). Common Pb blanks varied from 30 and 80 pg during this study. The atomic ratios were corrected for initial common lead composition blanks (Stacey and Kramers, 1975) and mass fractionation using NBS 983 Standard. The U–Pb ages with 2σerrors were calculated using a version of Isoplot program (Ludwig, 2003). The decay constants used for age determinations are from Steiger and Jäger (1977). The U–Pb zircon analytical data are presented in Table 3. 4. Results 4.1. Geophysics 4.1.1. Anisotropy of Magnetic Susceptibility (AMS) In the Vila Pouca de Aguiar pluton, the susceptibility magnitudes rang from 40 to 220×10 −6 SI according to a well-defined zoning with an average value of 101.0×10 −6 SI (also see Sant'Ovaia et al., 2000). In the Águas Frias-Chaves pluton, the susceptibility range from 40.0 to 103.0×10 −6 SI with an average value of 80.7×10 −6 SI (Sant'Ovaia and Noronha, 2005a). Such low susceptibilities which characterize magnetite-free granites (see Rochette, 1987) are typical of paramagnetic granites (Bouchez, 1997). In the latter the iron is dominantly carried by the silicates, principally biotite in our case. In both plutons magnetic susceptibility values characterize perfectly each one of the granite types: higher average values of 135.5×10 −6 SI and 89.2×10 −6 SI were found for the VPAG and the AFG respectively, and lower values of 67.8 and 42.2×10 −6 SI for the PSG and the SAMG. These values characterize the mineralogical difference between central facies and peripheric facies. Susceptibility is therefore directly correlated with the amount of biotite which increases from the centre to the periphery of the plutons (Martins et al., 2007). The anisotropy magnitudes are quite low, always lower than 2% for all the sampled sites: 1.4% for the Vila Pouca de Aguiar pluton and 1.6% for the Águas Frias-Chaves pluton. In VPAG pluton the highest anisotropy values are correlated with the least magnetic granites; in Águas Frias-Chaves pluton thereare no differences between centraland peripheric granites. AMS fabric patterns are very regular throughout the plutons. In the Vila Pouca de Aguiar pluton, magnetic foliations have moderate outward dips (the dip average is 34°) with strikes more or less parallel to the pluton elongation and are concordant for both granites. Magnetic lineations are subhorizontal and are also subparallel to the pluton elogation. However in the centre of the pluton, the foliations are WNW–ESE striking and are parallel to the magnetic lineation trends (Sant'Ovaia et al., 2000). In the Águas Frias-Chaves pluton magnetic foliations are parallel to the borders of the pluton with E–W strikes and with outward dips (ca 30°). Magnetic lineations are WNW–ESE trending and with shallow plunges (Fig. 2). Both, magnetic lineations and foliations are concordant for all the pluton. It must be noted that ASM lineations and foliations are similar in central area of the Vila Pouca de Aguiar pluton and the Águas Frias-Chaves pluton. 4.1.2. Gravimetry The isovalue contour line gradients of the Bouguer anomaly map (Fig. 3) for the Vila Pouca de Aguiar pluton clearly indicates that the plutonisboundedbyoverallinwarddippingcontactsurfaces.Aregionof pronouncedminimaispresentatthe northernendof thepluton,toward a still more pronounced one corresponding to the Águas Frias-Chaves pluton. The Bouguer anomaly map shows that the Águas Frias-Chaves pluton appears as a depression with anomalies ranging from −55 to −61 mGal. The pluton is well outlined by the −55 mGal contour line with a gradient inward to the pluton. A region of pronounced mínima (b63 mGal) is present at the southwestern border of the pluton, which corresponds to the alluvium deposits from Chaves graben (Fig. 3a). Fig. 2. Orientation diagram of magnetic foliations poles and lineations: (a) and (c) from the Águas Frias-Chaves pluton; (b) and (d) from the Vila Pouca de Aguiar pluton. Projection in Schmidt net, lower hemisphere (1, 2, 3, 4, 5, 6, 7 e 8% contours). Table 3 U–Pb isotopic data on zircon from lateto post-orogenic Vila Pouca de Aguiar granite (VPAG). Concentrations Isotopic ratios Apparent ages (Ma) Fractions Weight U Total Pb⁎ 206 Pb/ 204 Pb 206 Pb⁎/ 238 U 207 Pb⁎/ 235 U 207 Pb⁎/ 206 Pb⁎ 206 Pb⁎/ 238 U 207 Pb⁎/ 235 U 207 Pb⁎/ 206 Pb⁎ (shape) (mg) (ppm) (ppm) 2σ(%) 2σ(%) 2σ(%) 2σ2σ2σ 74-20/A (Z) na 0.26 1424 56.6 294 0.040655± 0.292862± 0.052245± 256.9± 260.8± 296± (Needle) 0.134 0.362 0.243 0.3 0.8 6 74-20/B (Z) na 0.25 2279 82.3 350 0.037278± 0.26829± 0.052198± 235.9± 241.3± 294± (Flat) 0.125 0.395 0.289 0.3 0.8 7 74-20/C (Z) a 0.12 1123.0 49.4 2092 0.045184± 0.325718± 0.052283± 284.9± 286.3± 298± (Short prisms) 0.093 0.189 0.985 0.3 0.5 2 74-20/D (Z) a 0.13 498.0 22.4 626 0.0453± 0.326624± 0.052294± 285.6± 287± 298± (Long prisms) 0.163 0.422 0.271 0.5 1.1 4 a: fraction submitted to abrasion; na: fraction not submitted to abrasion; Pb* radiogenic lead. 147H.C.B. Martins et al. / Lithos 111 (2009) 142–155 Author's personal copy In the Vila Pouca de Aguiar pluton the residual anomaly mapyields a negative signature for thepluton of about −4to−8mGalinamplitude, and a positive anomaly at the east and west of the area, correlates with the metasediments. The residual anomaly map for the Águas FriasChaves pluton yields a negative signature for the studied pluton of about 0 to −6 mGal in amplitude, and a positive anomaly at the east and west of the area, that can be correlated with metasediments. The residual anomaly map satisfactorily isolates the effect of the studied pluton, except at its southwestern part, where the zero contour level doesn't close. In this sector there is a separation between the zero contour level, elongated parallel to a graben (related to PRVF) and with aNNE–SSW trending outward the studied area limits. The negative anomaly of the Águas Frias-Chaves pluton is related to a lower density of thegranite thanthatof the surroundingcountry-rocks. In thegraben, we have also a strong negative residual anomaly (−12 mGal) due to the lower density of the alluvium deposits. Gravity modelling of each one of the plutons pointed out that their shapes are quite different (Sant'Ovaia et al., 2000; Sant'Ovaia and Noronha, 2005b)(Fig. 3b). For the Vila Pouca de Aguiar pluton the gravity modelling discloses that the pluton is laccolithic and does not exceed 1 km in thickness over more than 60% of its outcrop area. Within a triangular area facing the northern end of the pluton, and extending somewhat to the north under the northern cover rocks, the pluton's floor becomes deeper than 1 km under the present surface. The skeleton lines joining the deepest zones form a N-trending valley at the western side of this triangular area, and a NE-trending one at its eastern side. To the south, these two valleys merge into a single one extending south within the PSG type. Along the western valley, three narrow areas, circular in map view and up to 5 km deep, could be viewed as root-zones for this pluton. The southern one is located right in the center of the inner and least susceptible domain of the PSG. Gravity modelling of the Águas Frias-Chaves pluton suggests that its floor presents depth values which reach 12 km. In the central zone, Fig. 3. (a) Bouguer gravity anomaly map of the Vila Pouca de Aguiar and the Águas Frias-Chaves plutons and surrounding areas (in mGal). (Sketch map of the plutons in white); (b) Cross-section parallel to the PRVF of the Vila Pouca de Aguiar and Águas Frias-Chaves plutons obtained after gravimetric data inversion and AMS data. Fig. 4. U–Pb Concordia diagram showing analytical data for zircons fractions from one sample (74-20) of the Vila Pouca de Aguiar granite, northern Portugal. 148 H.C.B. Martins et al. / Lithos 111 (2009) 142–155 Author's personal copy under the outcrop of the two-mica granite, there is the main volume of the pluton, with floor depths reaching 16 km. In this zone it is located a deep conduit which can be assumed as a feeding root. At the south limit, the pluton floor seems to extent in SSW direction under the cover rocks with shallow depth values. The VPAG can be considered as a sill while the PSG 2–3 km thick over 2/3 of its outcrop area, is thicker and forms the main volume of the Vila Pouca de Aguiar pluton. The Águas Frias-Chaves pluton has a greater thickness (≈10 km) and belongs to the wedge-floored pluton type of Ameglio et al. (1997). Nevertheless, these data didn't underline any differences in shape of the two granites from the Águas FriasChaves pluton. Gravity data also suggest that the Águas Frias-Chaves pluton is more rooted than the Vila Pouca de Aguiar pluton and show that there is a connection of the two gravimetric anomalies in depth. 4.2. Geochemistry 4.2.1. U–Pb geochronology The U–Pb zircon data were carried out on four zircon fractions of one sample (74–20) from the VPAG. A typological study (Pupin,1980) has allowed the identification of several zircon types (Martins and Noronha, 2000). The zircons are very limpid, colourless or light yellow colour. The BSEM images of the long prismatic zircons revealed an inner zone surrounded by a complex magmatic zoning. Some of them have cores that could correspond to an earlier magmatic crystallization. The prismatic acicular and flat zircons are more homogeneous crystals devoid of cores and showing generally a faint zoning with dominantly oscillatory internal structures. The four zircon Fig. 5. A versus B parameters, in millications, (Debon and Le Fort, 1983) from the Vila Pouca de Aguiar, Pedras Salgadas and Águas Frias granites, northern Portugal; I–S boundary line from Villaseca et al. (1998a). Fig. 6. Variation diagrams of selected major (wt.%) elements vs. parameter B=Fe+Mg+Ti (in millications) from the Vila Pouca de Aguiar, Pedras Salgadas and Águas Frias granites, northern Portugal. 149H.C.B. Martins et al. / Lithos 111 (2009) 142–155