This group is spatially related to the Vigo–Régua shear zone, and includes, from south to north, the Ucanha–Vilar, Lamego, Felgueiras, Sameiro and Refoios do Lima plutons.

They are porphyritic (orthoclase phenocrysts) medium-grained biotite granodiorites/monzogranites and contain quartz + plagioclase (andesine/oligoclase) + perthitic orthoclase + biotite + zircon + monazite + apatite + ilmenite ± muscovite ± allanite (+ cordierite + sillimanite in the Refoio do Lima pluton) (Dias et al., 2002; Simões, 2000). The Ucanha–Vilar is spatially associated with hm-sized granodioritic stocks. Mafic microgranular enclaves are common, although they are rare, in the Refoios do Lima pluton. The U–Pb zircon geochronological data for Ucanha–Vilar, Lamego, Sameiro and Refoios do Lima granites have given crystallization ages between 313 and 321 Ma, with almost concordant monazite ages of 317 and 318 Ma.

They are interpreted as moderately peraluminous with magnesian and alkali-calcic affinity (Frost et al., 2001), being the Refoios do Lima the most aluminous. SiO₂ contents range from 62 to 70% and are characterized by rather high Ba (720–2181 ppm), high LREE contents (La = 77–167 ppm), highly fractionated patterns (La_N/Yb_N = 32–78) and moderate negative Eu anomalies (Eu/Eu* = 0.52–0.72). On the other hand, these granitoids display ⁸⁷Sr/⁸⁶Sr initial ratios and ɛ_Nd values varying in the ranges 0.7072–0.7116 and –4.4 to–6.3, respectively (Dias et al., 2002; Simões, 2000).

Late-D₃ biotite-dominant granitoids are represented by the Vieira do Minho pluton. This pluton is composite and consists of two different granite units (Almeida et al., 2002; Martins et al., 2013): the Vieira do Minho granite (VM) is a coarse-grained monzogranite and the Moreira de Rei (MR) granite a medium-grained monzogranite.

The two granites are porphyritic and present the following mineral association: quartz + perthitic K-feldspar (orthoclase or microcline) + plagioclase (oligoclase-andesine) + biotite ± muscovite. Andalusite and cordierite were also observed, but only in one sample from the Vieira do Minho granite. Apatite, zircon, titanite, ilmenite, monazite (only in VM) and thorite + allanite (in MR) can be found as accessory phases. The gradational contacts between the Vieira do Minho and Moreira de Rei granite indicate an almost synchronous emplacement. The U–Pb isotopic analyses carried out on zircon and monazite fractions from the Vieira do Minho granite indicate a crystallization age of 310 ± 2 Ma (Martins et al., 2013). The Moreira de Rei granite has been dated by Dias et al. (2002) yielding an age of 308 ± 4 Ma. Geochemically (Frost et al., 2001), this group is represented by peraluminous granites with alkali-calcic signature. However, the VM granite plots mostly in the ferroan field and the MR in the magnesian field. The VM and the MR granites are characterized by a high and restricted SiO₂ range from 67 to 73 wt. %, and they present a moderate REE fractionated patterns (La_N/Yb_N = 13–22) and moderate negative Eu anomalies (Eu/Eu* = 0.42–0.61). On the other hand, both granitoids have similar ɛ_Nd values (VMG = −5.2 to −5.7; MRG = −4.98 to −5.96), but the VM granite is slightly more enriched in (⁸⁷Sr/⁸⁶Sr)_i = 0.7087–0.7098 as well as in δ¹⁸O in the range from 10.6‰ to 11.0‰ than the MR granite with (⁸⁷Sr/⁸⁶Sr)_i = 0.7064–0.7075 and δ¹⁸O = 9.9‰–10.5‰ (Martins et al., 2013).

The post-D₃ Vila Pouca de Aguiar pluton has a NNE–SSW (N20 to N30) elongated shape, with the same orientation of the Late Variscan D₄ brittle structure, the Régua–Verin fault. It is also a composite massif with two main different biotite-rich granite units: the Vila Pouca de Aguiar granite (VPA), medium-coarse-grained with some mafic microgranular enclaves, and the Pedras Salgadas granite (PS), medium to fine grained. Both granites are porphyritic and contain quartz + perthitic K-feldspar (orthoclase or microcline) + plagioclase (oligoclase–andesine) + biotite + ilmenite + apatite + zircon + allanite ± monazite ± xenotime. The field relationships indicate a synchronous magmatic emplacement of these granites. The U–Pb isotopic analyses of zircon fractions from the Vila Pouca de Aguiar granite give an emplacement age of 299 ± 3 Ma (Martins et al., 2007, 2009). These granites are characterized by a weakly peraluminous character and by calc-alkaline to alkali-calcic signatures (Frost et al., 2001). The SiO₂ content ranges from 71 to 74% and they have wing-shaped patterns with (La/Yb)_N ranging from 3 to 9, with moderate negative Eu anomaly (Eu/Eu* = 0.29–0.50). These granites display weakly evolved isotopic compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.7044–0.7077 and ɛ_Nd = −2.0 to −2.6), and a whole-rock oxygen isotope (δ¹⁸O VSMOW) ranging from +9.7‰ to +11.0‰ (Martins et al., 2009).

5.1.1 Syn-D₃ biotite granitoids

The typological study using the methodology of Pupin (1980) shows that for Refoios do Lima (REF) granite, the main subtypes of zircons are S1, S2, S6, S7, S11, and S12 (Fig. 2), with a predominance of {211} pyramids on {101}, indicating an aluminous magmatic environment (Pupin, 1980), giving a low IA index in the range from 274 to 286 (Fig. 3a). This aluminous environment is also evidenced by the presence of cordierite and sillimanite in the granite. Most of the zircons present equally well-developed {100} and {110} prisms, with a slight predominance of the latter, giving a mean IT index in the range from 371 to 440 (Fig. 3a). According to their distribution, the population of zircons plot in the intrusive aluminous monzogranites–granodiorites domain, although one of the samples is situated in an area overlapping multiple typological domains (Fig. 3a).

5.1.2 Late-D3 biotite-dominant granitoids

According to their typological distribution, the zircons of the late-tectonic granites are dominantly of subtypes S2, S3, S7, S12, and S17 (Fig. 2) for the Vieira do Minho granite, having an almost equally well-developed {100} and {110} prisms, with a slightly predominance of the latter, which give a mean IT index in the range of 384 to 405 (Fig. 4a). Most of zircons present the {211} pyramid more developed than the {101} one, and accordingly a low IA index, 321–331 (Fig. 4a). In the Moreira de Rei granite, we can distinguish two different typological distributions: one is dominantly of subtypes S12, S13, and S17, typical of calc-alkaline granites (Pupin, 1988), and the other belong to the S3 and L3 types (Fig. 2). These typological distributions imply a similar dominance of the two prisms and the two pyramids, as found in the Vieira do Minho granite, but with some minor development of the {110} prism and the {101} pyramid. Therefore the MR granite has mean index values of IT = 452 and of IA = 349, slightly higher than those recorded for the VM granite (Fig. 4a). The zircon population from the Vieira do Minho granite reveals characteristics of aluminous monzogranites, but overlapping several typological domains (Fig. 4). On the other hand, the morphological characteristic of zircons from the Moreira de Rei granite seems to indicate also typological characteristics typical of aluminous monzogranites–granodiorites (Fig. 4), but closer to those of zircons from calc-alkaline granites (Pupin, 1988).

5.1.3 Post-D3 biotite granitoids

The study carried out by BSE reveals that the zircons have internal structures that are typically magmatic, with magmatic zoning and nebulitic structures.

The nebulitic structures are characterised by diffuse structures (Fig. 5a), which are more prevalent in less evolved types (subtypes S24, S18).

Magmatic zoning is thin, sometimes wavy, and occur in zircons of all granites (Fig. 5b–d). In the Refoios do Lima granite, zircons are characterised by a thin magmatic zoning, with rare nebulitic structures.

Zircons with elongated and short prisms reveal identical structures (magmatic zoning and/or nebulitic structures) (Fig. 5b), while the subspherical multifaceted zircons have poorly defined nuclei, surrounded by a magmatic zoning, sometimes weak.

The zircons with lamellar form, common in Ucanha–Vilar and Lamego granites, reveal a thin magmatic zoning with frequent inclusions of biotite.

Sometimes we can see internal morphological evolutions shown by different kinds of magmatic zoning. In zircon 5b (Sameiro granite), in the inner zone, the pyramidal face is {101}, evolved to {211} in the direction of the outer zone (S9 → S2), indicating a reaction with the melt.

Some zircons also record complex histories involving fracturing and consolidation of broken crystals, like in zircon 5c (Felgueiras granite).

In Sameiro, Felgueiras, Lamego and Ucanha–Vilar granites, zircons are characterised mainly by two stages of formation (Fig. 5b and c):

• • an inner zone, with nebulitic or without internal structure in less developed morphological types or a zoning in the more evolved morphological types, with rounded shapes in the area of the pyramidal faces, sometimes with the zoning truncated;
• • an outer zone, with magmatic zoning.

This type of structure indicates a reaction step with the magmatic environment.

Relic cores were also identified like in Sameiro granite, where some zircons show relic cores, indicating the presence of inherited zircons (Fig. 5d). The presence of these inherited zircons in Sameiro granite is responsible for the existence of a reverse discordia according to a geochronological study on U–Pb analysis of multi-grain zircon and monazite fractions (Dias et al., 1998). Relic nuclei were also identified in zircons from Refoios Lima, Sameiro, Felgueiras and Lamego granites through the observation of their internal structure by cathodoluminescence. Zircons with inherited cores were not detected in the Ucanha–Vilar granite. The occurrence of these relict nuclei are more frequently in the zircons of the Refoios do Lima granite and less in the Lamego granite, showing a geographic decrease from north to south.

According to their morphology, four different populations of zircon have been recognized in these granitoids: prismatic zircons (short, long and acicular), lamellar zircons and a less common subspherical multifaceted zircon. The internal structures (BSE images) of the zircons from both granites are very similar and present typical magmatic patterns. Generally they consist of an unzoned to weakly zoned core surrounded by a rim of euhedral regular fine oscillatory zoned zircon (Fig. 5e and g). Zircon e is a crystal from the VM granite that shows a well-developed {110} prism and two pyramids {101} and {211}, with the {211} pyramid more dominant, corresponding to subtype S2/S3. Zircon g (MR granite) has an inner well-developed {110} prism and {211} pyramid. The {100} prism and the {101} pyramid are subordinate (subtype S3/S4) and are overgrown by a single {110} prism and an almost equal development of the {211} and {101} pyramids, subtype L2/L3.

In some zircons, that rim is in turn surrounded by a weakly zoned outer rim (Fig. 5f), which is interpreted to have formed by recrystallization of the primary zircon grown during the cooling of the granite (Pidgeon et al., 1998). Zircon f (VM granite) has an inner zone with equal development of the two prisms {110} and {100}, and the {211} pyramid slightly predominant over {101} belongs to subtype S12. This morphological type is surrounded by a S7/S2 subtype with a {100} prism more subordinate. The BSE images also show a clear growth discontinuity between a partially resorbed inner zone with faint zoning, and a finely zoned outer zone (Fig. 5h). These structures are interpreted as magmatic, probably developed in two crystallization steps and revealing some disequilibrium between the early generation and the liquid. These structures are common in prismatic zircons. Zircon h (MR granite) is a subtype S7/S2, in which there is a certain equilibrium in the development of the two pyramids, with predominance of the {110} prism. The acicular and the lamellar zircons are very homogeneous crystals devoid of cores and showing generally a faint zoning with dominantly oscillatory internal structures. The BSE imaging of the subspherical type reveals the presence of inherited cores, surrounded by finely zoned rims.

Morphological study has allowed the identification of different types of zircon, prismatic zircons (short, long and acicular) and lamellar zircons.

Zircons from this group of granites show a much more regular zoning than the other groups. In the Vila Pouca de Aguiar, the BSE images of the prismatic zircons revealed an inner zone surrounded by a well-developed magmatic oscillatory zoning as a result of heterogeneous trace element distribution (Martins et al., submitted) (Fig. 5i and j). Zircon i (VPA granite) is a crystal showing an inner zone with a well-developed {100} prism and equally well-developed {211} and {101} pyramids (subtype S23) overgrown by a S15 subtype, whereas zircon crystal j (VPA granite) is a P1 subtype, and thus shows a well-developed {110} prism and a single {101} pyramid.

Some zircons have cores, which could correspond to an earlier magmatic crystallization (Pidgeon, 1992), sometimes partially resorbed and with a faint zoning. In the Pedras Salgadas granite zircons are characterised by an external well-developed magmatic oscillatory zoning, which is interpreted as typical of primary, magmatic zoning (Fig. 5k and l). Zircon k presents an inner zone with a well-developed {110} prism (subtype S24) evolving to the outer zone to an equal development of the two prims and a {101} pyramidal face more dominant (S15 subtype); zircon l has a well-developed {110} prism and a single {101} pyramid (subtype P1).

The acicular and lamellar zircons are in general unzoned to weakly zoned, and lack magmatic overgrowths.

Zircon grains from the Refoios de Lima granite show gradual changes in their morphology, whereas in Sameiro, Felgueiras, Lamego and Ucanha–Vilar granites they have a more complex morphology (internal and external), which reflects modifications in the composition of the magma and provides a qualitative record of magma evolution. The typological population and typological evolutionary trend (TET) of zircons from Refoios do Lima (REF) granite takes place over an area indicating that this granite has a mainly crustal origin (Fig. 3b), with equally well-developed {100} and {110} prism, with a predominance of the latter, indicating an aluminous environment of formation. This typological distribution is similar to what happens to monzogranites and granodiorites of Margeride type (Pupin, 1976, 1980) whose typological characteristics of zircon are identical to those of zircons from the Refoios do Lima granite. Crystalline groupings parallel to the crystallographic axis c are also frequent in the Refoios de Lima granite (Simões, 2000), reinforcing the hypothesis of a crustal origin. This is in agreement with mineralogical, geochemical and isotopic studies for Refois do Lima granite (Dias et al., 2002; Simões, 2000) that confirmed an origin from crustal-derived melts. In fact, the isotopic composition of Refoios do Lima granite (Sr_i = 0.7104–0.7106; ɛ_Nd = –6.0 to –6.3) shows a signature that is typical of crustal materials.

For zircons of Sameiro (SAM), Felgueiras (FEL), Lamego (LAM) and Ucanha–Vilar (U–V) granites, the typological evolutionary trends cross the domain of the calc-alkaline granites to the aluminous granites, contrary to what would be expected from a simple evolutionary crystallization process (Fig. 3b). This may be interpreted as indicating a mixing process between two compositionally distinct magmas, which could explain those typological evolutions. In fact, the presence of gabbros in the region where the Sameiro granite outcrops originated from an enriched mantle (Dias and Leterrier, 1994), and from the presence of granodiorites–quartz monzodiorites in the Ucanha–Vilar area, as well as the occurrence of mafic microgranular enclaves in the Sameiro, Felgueiras, Lamego, and Ucanha–Vilar granites, supports a mixing model for their genesis, involving a basic magma (probably a mantle-derived magma) and a felsic crustal magma. This is also supported by geochemical and isotopic studies of these granites (Dias et al., 2002; Simões, 2000). So, the typological evolutionary trends (TET) and the fact that some zircons are characterised by two stages of formation can be interpreted as a mixing process, which is in good accordance with mineralogical, chemical and isotopic composition of those granites (Dias et al., 2002; Simões et al., 1997; Simões, 2000).

The gradual change in morphology of the late-D₃ biotite-dominant granitoids suggests that zircon was crystallizing as the magma cooled and there is no evidence supporting abrupt changes in magma composition. Moreover, the internal structures shown in Fig. 5 as well as the evolutionary trends (Fig. 4b) within the zircon crystals mimic the TETs displayed by the zircon population to which they belong. Effectively, zircon f (Vieira do Minho granite) and zircon g (Moreira de Rei granite) are very good examples. Zircon f has an inner zone belonging to the subtype S12, which is surrounded by a S7/S2 subtype. Zircon g has an inner well-developed {110} prism and {211} pyramid (subtype S3/S4), and is overgrown by a single {110} prism and an almost equal development of the {211} and {101} pyramids, subtype L2/L3. The morphological signatures of these zircon grains, namely the mean point and typological evolutionary trends (Fig. 4), with almost constant IAs during their evolution, indicate a crustal or dominantly crustal origin for the VM and the MR granites. The evolution in zircon morphology suggests the crystallization and/or the fractionation of two magmas during a general decrease in temperature. There is no evidence of magma mixing in these samples and the whole process is consistent with control by fractional crystallization during cooling. This is also supported by geochemical and isotopic data, as discussed by Martins et al. (2013), who argued that the granites from the Vieira do Minho pluton were probably derived from crustal protoliths, although at different crustal levels.

Finally, the post-D₃ biotite granitoids show narrowly-spaced uninterrupted oscillatory patterns representing higher degrees of zircon saturation (supersaturation) (Hoskin and Schaltegger, 2003). According to Clairborne et al. (2010 and references therein), oscillatory zoning represents kinetic effects at the crystal-melt interface, dependent on the ordering in the melt by polymerization and often promoting local supersaturation and disequilibrium, rather than rapid changes in the composition or conditions in the bulk melt. The zircon typology of these granites is quite different from the previous ones, exhibiting significantly different mean points and TETs (Fig. 4). The typological evolutionary trends are characterised by increasing IAs for decreasing ITs. The morphological types are found in calc-alkaline and in sub-alkaline granites and both define typological evolutionary trends that begin in the calc-alkaline domain and extend towards the sub-alkaline one (Fig. 4b). The Pedras Salgadas IA index has a higher value than Vila Pouca de Aguiar, pointing to a more alkaline nature and accordingly the mean point indicates that the Pedras Salgadas granite belongs to the sub-alkaline granites. Thus, the zircon morphological characteristics of the VPA and the PS granites follow the calc-alkaline/sub-alkaline trend, and the source rock, according to Pupin’ scheme, could be a hybrid of mantle and crustal origin or mainly of mantle origin, typical of I-type granitoids (Fig. 4).

The internal structures show convincingly which type is changing continuously as the magma cooled and became more alkaline, as documented by one Vila Pouca de Aguiar crystal (Fig. 5i) and by others from the Pedras Salgadas granite (Fig. 5k). These crystals show a significant change in morphology from inner to outer zones (zircon i S23→S15; zircon k S24→S15), representing a change in the crystal morphology during crystal growth. Nevertheless, other crystals (Fig. 5j and l) have kept the same typology throughout the crystallization process.

The petrogenetic model proposed for the zircons typologically studied in these granites is in accordance with geochemical and isotopic data. The trends for major and trace elements in the Vila Pouca de Aguiar and the Pedras Salgadas granites are consistent with the fact they belong to the I-type. This is also supported by their weakly evolved isotopic compositions; the VPA granite presents a (⁸⁷Sr/⁸⁶Sr)₂₉₉ between 0.7067 and 0.7071, whereas εNd299 is around −2.5 (Martins et al., 2009). The PS granite yields the less evolved isotopic composition with (⁸⁷Sr/⁸⁶Sr)₂₉₉ ranging from 0.7044 and 0.7050 and εNd299 = −2.0. Therefore, an origin by mantle input with local intermingling with crustal sources has been proposed (Martins et al., 2009).

The geochemical and isotopic data measured in these granites are consistent with the typological study of their zircon populations. This fact shows that the morphology of zircons populations can be used with a high degree of certainty to determine the origin and evolution of granitic magmas in a quick and inexpensive way. Moreover, the evidence that internal evolution within a single crystal mimics the typological evolutionary trends in all granites studied corresponds to an important step in the knowledge of zircon growth within silicic melts.

The knowledge of the crystal morphology of zircons and of their internal structures is crucial for the interpretation of U–Pb zircon geochronology and Hf isotope studies, because zircons commonly show heterogeneous growth zones, and thus it is important to have the ages and Hf isotopes analyzed within the same zones. Therefore, their crystal morphology and their internal structures could be an important clue to understand which processes lead to the wide scatter of ages and Hf isotopes, as it has been highlighted in several papers (e.g., Belousova et al., 2009; Condie et al., 2005; Gerdes and Zeh, 2009; Hawkesworth and Kemp, 2006; Kurhila et al., 2010; Orejana et al., 2011; Shaw et al., 2011; Villaseca et al., 2012).