The Tamanhos, Maceira and Casal Vasco biotite gneiss-granites plot essentially within the field of the monzogranites in the R1–R2 diagram (la Roche et al., 1980) (Fig. 5). Their compositions vary from slightly peraluminous granodiorites to highly peraluminous monzogranites (SiO2 = 60 - 72%; A/CNK = 1.0 - 1.37) (Fig. 6) and are characterized by low Al2O3/TiO2 and high CaO/Na2O ratios (Fig. 7). As shown by Sylvester (1998), the CaO/Na2O ratio in peraluminous melts is predominantly controlled by the plagioclase/clay ratio of the source being therefore unlikely that granite melts with high CaO/Na2O ratios could be produced by simple partial melting of mature sedimentary protoliths (plagioclase-poor metapelites). A more immature quartzofeldsphatic-rich (greywackes) and/or metaigneous (tonalites-granodiorites) crustal source may therefore be proposed. However, high CaO/Na2O ratios can also result from mixing of strongly peraluminous crustal melts with basaltic magmas (Sylvester, 1998).
Figure 5. R1-R2 variation diagrams
R1-R2 variation diagrams for the Beiras granitoids (a) syn-kinematic biotite-bearing granodiorites-monzogranites and highly peraluminous two-mica granitoids; (b) late-post-kinematic basic and intermediate rocks, biotite monzogranites and biotite-muscovite peraluminous granites. sgab - syenogabbros; sd - syenodiorites; ne s - nepheline syenites; s - syenites; mz - monzonites; mzq - quartz monzonites; mzd - monzodiorites; gab - gabbros; gabno - gabbro norites; d - diorites; to - tonalites; gd - granodiorites; mzg - monzogranites; sg - syenogranites; alk g - alkaline granites.
Figure 6. A/CNK vs. SiO2 variation diagrams
A/CNK vs. SiO2 variation diagrams for the Beiras granitoids: (a) syn-kinematic biotite-bearing granodiorites-monzogranites and highly peraluminous two-mica granitoids; (b) late-post-kinematic basic and intermediate intrusive rocks, biotite monzogranites and biotite-muscovite peraluminous granites. Symbols as in figure 5.
Figure 7. CaO/Na2O vs Al2O3/TiO2
CaO/Na2O vs Al2O3/TiO2 for the Beiras syn-kinematic biotite-bearing granodiorites-monzogranites and highly peraluminous two-mica granitoids Symbols as in figure 5.
According to the same author, the Al2O3 contents in the melts derived from anatexis of both pelites and psammites remain nearly constant during the partial melting event due to the stability of aluminous refractory phases, whilst the concentrations of TiO2 tend to increase with increasing temperature as a result of the progressive breakdown of Ti-bearing phases (biotite and ilmenite). The low Al2O3/TiO2 ratios (13-41) and high CaO/Na2O ratios (0.3-1.0) displayed by these granitoids suggest generation at high temperatures (875-1000°C).
Compared to the other granitoid suites of the Beiras batholith, the syn-D3 granodiorite-monzogranites exhibit high to very high Ba and REE contents (Ba = 549-2670 ppm; ?REE = 481-681 ppm), low Rb/Sr ratios (<0.4), significant LREE enrichment (LaN/YbN = 34-46) and small to moderate Eu negative anomalies (Eu/Eu* = 0.66-0.24) (Fig. 8). Trace element normalized patterns for representative samples of the Maceira granodiorites and monzogranites are illustrated in Figure 9. The trace element composition for one sample of the lower crust felsic granulite xenoliths scavenged by early Mesozoic alkaline dykes in the Iberian Massif (Villaseca et al., 1999) was also plotted in the diagram of Figure 9. The Maceira granodiorites and monzogranites are significantly enriched in LILE, LREE, Zr and Hf and depleted in HREE, Ti and Y relative to the felsic granulites. These features are easily reconciled with a petrogenetic model involving low degrees of partial melting of a feldspar-rich orthogneissic source, leaving behind a residue similar to the felsic xenoliths (garnet-, rutile and ilmenite-rich and plagioclase/biotite poor).
Figure 8. REE chondrite normalized patterns
REE chondrite normalized patterns for the Beiras granitoids: (a) syn-kinematic biotite-bearing granodiorites-monzogranites and highly peraluminous two-mica granitoids; (b) late-post kinematic basic and intermediate intrusive rocks; (c) late-post kinematic biotite monzogranites and (d) late-post kinematic biotite-muscovite peraluminous granites. Symbols as in figure 5. Normalization constants from Evensen et al. (1978). Shadowed areas represent the range of REE chondrite normalized values for each granitoid type.
Figure 9. Primordial-mantle
Primordial-mantle normalized trace element patterns for the Beiras granitoids: (a) syn-kinematic biotite-bearing granodiorites-monzogranites; (b) highly peraluminous two-mica granitoids; (c) late-post kinematic basic and intermediate intrusive rocks; (d) late-post kinematic biotite monzogranites and biotite-muscovite peraluminous granites. For comparison, trace element compositions for lower crustal felsic granulites (Villaseca et al., 1999), upper crust Late Proterozoic-Cambrian metasediments (CXG) (Beetsma, 1995) and average upper crust (Taylor and McLennan, 1985) were also plotted. Normalizing constants from Sun and McDonough (1989).
The samples of the Maceira and Casal Vasco granites show relatively unradiogenic Sr and Nd isotopic signatures (87Sr/86Sri = 0.707-0.710; ?Ndi = -3 to -7) and define a steeply-sloped negative correlation on the ?Sri-?Ndi diagram (Fig. 10). The lack of Sr and particularly Nd isotopic homogeneity both within and between the individual units of the suite is not consistent with an origin by partial melting of a single metasedimentary and/or felsic metaigneous crustal protolith and appears to imply mixing of, at least, two distinct source components with contrasting Sr-Nd compositions. The isotopic data for several crustal protoliths of the Iberian pre-Variscan basement (age-corrected to 310 Ma) were also plotted in figure 10. Although there is a close match between the Sr-Nd isotopic compositions of the lower crust felsic metaigneous granulites and those of the Maceira and Casal Vasco granites, it is also possible to interpret the isotopic signature of this granite suite in terms of crustal contamination of a mantle-derived magma by lower crustal metapelitic granulites.
Figure 10. Nd initial vs. Srinitial variation diagram
Nd initial vs. Srinitial variation diagram for the Beiras granitoids: (a) syn-kinematic biotite-bearing granodiorites-monzogranites and highly peraluminous two-mica granitoids; (b) late-post-kinematic basic and intermediate intrusive rocks, biotite monzogranites and biotite-muscovite peraluminous granites. Symbols as in figure 5. The fields of potential crustal protoliths are also shown. Source materials include upper crustal Late Proterozoic-Cambrian metasediments (CXG) (Beetsma, 1995, Tassinari et al., 1995); orthogneisses from Ollo de Sapo complex (Beetsma, 1995); lower crustal felsic granulites (Villaseca et al., 1999); lower crustal pelitic granulites (Villaseca et al., 1999) and lower crustal basic granulites (Peucat et al., 1990).
As such, two alternative petrogenetic models may be proposed: (a) mixing between mantle-derived gabbroic magmas (low Rb/Sr - high Sm/Nd end-member) and lower crust pelite materials (high Rb/Sr - low Sm/Nd end-member); (b) partial melting of lower crustal felsic metaigneous granulites. Both models can yield similar geochemical signatures and are not clearly distinguished on the basis of Sr-Nd isotopic data. At the presently level, there is no evidence of basic intrusive rocks associated with the Maceira and Casal Vasco granites. Nevertheless, in a scenario of high temperature collision such as the Variscides, where upwelling asthenosphere was able to rise and invade the crust, the possible contribution of mantle-derived magmas to granite petrogenesis cannot be ruled out.
The Junqueira syn-D3 two-mica granitoids show little compositional variation and a strong peraluminous character (A/CNK = 1.15 - 1.50) (Figs. 5 and 6). Silica contents range from 71 to 74%, Ca, Mg, Ti, Ba, Sr, ?REE and HFSE contents are low (CaO = 0.3-0.6%; MgO = 0.2-0.6%; TiO2 = 0.12-0.36%; Ba = 215-380 ppm; Sr = 40-70 ppm; ?REE = 35-295 ppm), Rb/Sr ratios are high (2-6) and P2O5 is high but variable. Their REE patterns vary from LREE enriched with large negative Eu negative anomalies (LaN/YbN = 47; Eu/Eu* = 0.24) towards less fractionated patterns with slightly negative Eu negative anomalies (LaN/YbN = 8; Eu/Eu* = 0.6) (Fig. 8).
The close association between these granites and the migmatites of the high-grade Porto-Viseu metamorphic belt suggests an origin by widespread dehydration partial melting of metasedimentary crustal sources comparable to the CXG metapelite-metagreywacke units presently exposed in the Central Iberian Zone. Estimates of P-T conditions at the time of partial melting range between 8-3 Kbar and 800-600*C (Escuder Viruete et al., 2000). Compared to experimental peraluminous melts produced from plagioclase-poor pelites (Patiño Douce and Johnston, 1991) and plagioclase-rich psammitic gneisses (Patiño Douce and Beard, 1995; Skjerlie and Johnston, 1996), under similar P-T conditions, the Junqueira granites show CaO/Na2O, Al2O3/TiO2 and Rb/Sr ratios (CaO/Na2O < 0.3; Al2O3/TiO2 = 42-130; Rb/Sr = 2-6) more compatible with a derivation from feldspar-poor sources at relatively high temperatures (Sylvester, 1998) (Fig. 6).
In trace element variation diagrams, normalized to primordial mantle (Fig. 9), the compositions of the Junqueira granites do not differ significantly from those of the low-grade metapelites and metagreywackes of the CXG. This suggests that the strongly peraluminous two-mica granites could have been produced by moderate degrees of partial melting of CXG metasediments at middle crustal levels. The relative depletions in Nb, Ta, Ti, Y and HREE point to the presence of Fe-Ti oxides ± rutile + garnet + biotite in the residual mineral assemblage.
There is a good agreement between the initial ?Nd values of the syn-D3 two-mica granitoids (-5.7 to -3.5) and the inferred Nd isotope compositions for both the CXG metasediments (?Ndi = -3.7 to -6.4) and the Ollo de Sapo orthogneisses (?Ndi = -4.6 to -6.3) at the time of granite generation. However, the Sr isotopic signature of the Junqueira granites is highly variable and clearly less radiogenic than those of their presumed crustal sources. The observed spread of Sr initial ratios is coupled with a very narrow range of ?Ndi values and can be ascribed to a wide number of causes: partial re-equilibration of the Rb-Sr system during deformation (Page and Bell, 1986), post-magmatic alteration (Kwan et al., 1992) and disequilibrium partial melting (Allègre and Minster, 1978). Assuming that the Sr isotopic ratios of both granites and protoliths could have been disturbed by any of these processes, the geochemical and isotopic characteristics of the Junqueira granites are well accounted for by moderate degrees of partial melting of the CXG metasediments under vapour absent conditions as previously proposed by Beetsma (1995).
The different intrusive units of the late-post-D3 Cota-Viseu batholith define a curvilinear array in the R1–R2 diagram (la Roche et al., 1980) with compositions ranging from gabbro-norites, diorites, monzodiorites, quartz monzodiorites and granodiorites to monzogranites (Fig. 5). The samples of Alcafache-Freixiosa-Dão biotite-muscovite granites plot mainly in the field of the syenogranites, at the extreme acid end of the same trend (Fig. 5). Taken together, these granitoids show a wide compositional range (SiO2 = 52-75%; MgO = 7.9-0.2%) and decoupled high-K calc-alkaline and peraluminous affinities. The least evolved members of the suite are dominantly metaluminous (A/CNK < 1.0), the Cota-Viseu coarse porphyritic biotite monzogranite is slightly peraluminous (A/CNK = 0.9-1.12) and the more evolved Alcafache-Freixiosa biotite-muscovite granites exhibit peraluminous signatures (A/CNK = 1.0-1.22) (Fig. 6).
In Harker diagrams, the full spectrum of rock types display almost continuous geochemical trends, indicating a strong genetic link between all units. For increasing SiO2 contents, K, Rb and Eu/Eu* increase, Ca, Al, Mg, Sr decrease and Fe, Ti, Ba, LREE, HFSE change from an incompatible to a compatible behaviour. There is a clear hiatus in SiO2 content (SiO2 = 53-56%) between the more primitive compositions (gabbro-norites and diorites) and the dominant biotite granodiorites and monzogranites. However, the occurrence of ubiquitous mixing-mingling relationships between these rock types suggests that open-system AFC processes may have played a major role in the petrogenesis of the suite. Despite their distinctive peraluminous character, the Alcafache-Freixiosa-Dão two-mica granites show significant compositional overlap with the more evolved members of the Cota-Viseu granodiorite-monzogranite sequence and could therefore have been produced from similar sources and processes.
Chondrite-normalised REE patterns for selected samples of these granitoids are presented in Figure 8. The gabbro-norites are characterized by low ?REE contents (48-78), very low LaN/YbN ratios (5-6) and absent or slightly positive Eu anomalies. In contrast, the rocks of intermediate composition exhibit distinctive Eu negative anomalies, highly variable ?REE contents (96-475) and more fractionated REE patterns (LaN/YbN = 9-18). The Cota-Viseu biotite monzogranites and the two-mica Alcafache-Freixiosa-Dão granites have subparallel REE profiles and show a steady decrease of LREE and increasing negative Eu anomaly with magmatic differentiation. The overall chemical variation of the suite appears to have been largely controlled by crystal-melt fractionation processes and can be accounted for by an AFC model involving contamination of gabbroic magmas by anatectic crustal melts plus fractionation of the following mineral assemblages: (a) plagioclase + orthopyroxene + clinopyroxene, (b) plagioclase ± clinopyroxene + amphibole + biotite, (c) plagioclase + biotite + K-feldspar ± apatite + monazite + zircon.
In trace element primordial mantle-normalised diagrams (Fig. 9), all rock units including the gabbro-norites show Nb-Ta-Ti troughs and Th-enrichment, probably reflecting inherited crustal signatures. The LILE, REE and to, a lesser extent, the HFSE increase from the basic to the intermediate rocks (SiO2 = 52-65%) whilst Sr decreases (Fig. 9c). For SiO2 contents greater than 65%, the LREE decrease and the negative anomalies of Sr, P and Ti tend to become increasingly more pronounced. It is also clear that the Alcafache-Freixiosa-Dão two-mica granites and the Cota-Viseu biotite granites overlap in composition for the same range of SiO2 contents (Fig. 9d). All these features point to an extensive involvement of fractional crystallization processes in their petrogenesis.
As expected from an AFC-style petrogenetic model, the Sr-Nd isotopic data for the different intrusive units tend to define a curved trend from crustal compositions towards positive ?Nd initial and low ?Sr initial values, typical of a depleted mantle component (?Sri = 65.2 to 1.4; ?Ndi = -4.0 to +0.6) (Fig. 10). Fractional crystallization and hybridization between coeval mantle- and crust-derived magmas is therefore proposed as a major mechanism for the production of abundant volumes of granite magmas during the last stages of the Variscan orogeny in Iberia.