Emplacement, geochronology and petrogenesis of the syntectonic biotite-granitoids associated with the Vigo-Régua shear zone (Hercynian Central Iberian Zone, Northern Portugal) 

PhD thesis presented at University of Minho and University of Nancy (France) by Pedro Pimenta Simões, 2000

This work presents a study of the Hercynian syn-tectonic granitic plutons associated to the Vigo-Régua shear zone, enhancing their structure, mineralogy, geochemistry and isotopic geology (U-Pb, Rb-Sr, Sm-Nd). This study had in view the characterization of the granitoids, the precision of the emplacement ages and the definition of the origin and evolutionary processes of the magmas.

The studied granitoids are situated in the Central Iberian Zone (Northern Portugal), along the Vigo-Régua shear zone and are designated, from North to South, as Refoios do Lima, Sameiro, Felgueiras, Lamego and Ucanha-Vilar granites. These are biotite granodiorites and monzogranites that belong to the syn-F3 (the last Hercynian ductile deformation phase) biotite granitoid group. They present a porphyritic texture (potassium feldspar megacrysts) and mafic microgranular enclaves that decrease in frequency from South to North. The granites are composed of quartz + potassium feldspar + plagioclase (andesine/oligoclase) + biotite + zircon + monazite + apatite + ilmenite ± muscovite. In the Ucanha-Vilar, Lamego and Felgueiras granites also occur allanite ± titanite ± epidote, while in the Refoios do Lima granite cordierite + sillimanite ± tourmaline ± garnet were observed.

The granitic plutons present an internal structure that is essentially magmatic and that is marked by the orientation of the potassium feldspar megacrysts, biotite and mafic microgranular enclaves. The field observations and structural data indicate that the fabric is the result of the magma deformation by flattening with a reduced rotational component in a tectonic regime dominated by a NE-SW sub-horizontal compressive component, compatible with the F3 deformation phase, without significant movement along the Vigo-Régua shear zone. This shear zone was active in the final phase of granitoid emplacement as evidenced by N140&Mac251; dextral ductile shears. The ascent and emplacement of magmas was facilitated by the deformation of the metasedimentary host rock, together with the fracturation associated to the shear zone, with in situ lateral expansion. They are considered early sin-F3 plutons.

The studied granodiorites-monzogranites are moderately peraluminous, [(A/KNC)m:1.19-1.39], with SiO2 contents between 62 and 70%. The whole rock geochemical data show that the granites present a narrow range of compositions defining independent series marked by a decrease in Al2O3, CaO, MgO, Fe2O3t, TiO2, P2O5, Ba, Sr, Zr and La with the increase in SiO2, with the exception of the Sameiro granite that does not present a significant internal evolution. These series are also distinct in terms of the biotite composition, namely in Al and/or Mg contents. They present aluminopotassic characteristics (Refoios do Lima granite) and calc-alkaline to aluminopotassic characteristics (Sameiro, Felgueiras, Lamego and Ucanha-Vilar granites). All the granitoids present high Ba contents, with averages between 981 and 1522 ppm. The granites present rare earth patterns with significant enrichment in LREE relative to the HREE [(La/Yb)N=33-59] and moderate Eu anomalies (Eu/Eu*=0.57-0.66).

The typological evolutionary trend (T.E.T.) of zircon populations indicate a crustal origin for the Refoios do Lima granite. For the other granites the T.E.T. cross the domain of the calc-alkaline granites to the crustal granites. In the geochemical study, the zircons of the Refoios do Lima granite reveal a magmatic phase with HfO2 > 1.2% and Y2O3 < 0.5%. These geochemical and typological characteristics associated with the presence of numerous relict cores indicate a crustal origin for this granite, originated from an essentially anatectic magma. For the other granites the geochemical data of zircons, namely HfO2 < 1.2% and predominance of Th over U in the initial magmatic phase, indicate a calc-alkaline signature. However, the existence of a HfO2 compositional discontinuity in the magmatic phase suggests that during the evolution of the calc-alkaline magmas a mixing process occurred with anatectic magmas. Also the fact that the T.E.T. of the Ucanha-Vilar, Lamego, Felgueiras and Sameiro granites cross the calc-alkaline domain to the domain of the granites of crustal origin may de interpreted as indicating a mixing process between two compositionally distinct magmas.

For the U-Pb geochronological study zircon and monazite fractions from the Ucanha-Vilar, Lamego, Sameiro and Refoios do Lima granites were selected. The zircon isotopic data define direct discordia giving upper intercepts at 315 ± 5 Ma (MSWD = 3.6) for Ucanha-Vilar, 319 ± 4 Ma (MSWD = 2.1) for Lamego and 319 ± 13 Ma (MSWD = 5.8) for Refoios do Lima granite. For the Sameiro granite the zircon fractions define a reverse discordia (MSWD = 8.3), with a lower intercept at 314 ± 4 Ma (interpreted as a minimum age for the granite emplacement) and an upper intercept at 1932 ± 126 Ma; this reverse discordia is interpreted as indicating the existence of inherited cores in the zircons of the Sameiro granite, which is confirmed by BSEM. The monazites ages are almost concordant giving very similar 207Pb/235U ages: 317 ± 3 Ma for Ucanha-Vilar, 317 ± 5 Ma for Lamego, 318 ± 2 Ma for Sameiro and 317 ± 3 Ma for Refoios do Lima granite. The narrow range of age values is interpreted as representing an almost synchronous emplacement of the different plutons at about 317 Ma.

The Sr isotopic study reveals initial 87Sr/86Sr ratios (317 Ma) of 0.7072-0.7073 for Ucanha-Vilar, 0.7082-0.7085 for Lamego, 0.7083-0.7084 for Felgueiras, 0.7082-0.7084 for Sameiro and 0.7104-0.7106 for Refoios do Lima granite. The eNd (317 Ma) values are -4.39 to -4.75 for Ucanha-Vilar, -5.03 to -5.09 for Lamego, -5.02 to -5.43 for Felgueiras, -4.59 to -5.03 for Sameiro and -6.01 to -6.28 for Refoios do Lima granite. The Ucanha-Vilar and Refoios do Lima granites present the most extreme Sri and Nd values while the other granites reveal an intermediate but similar isotopic composition. The isotopic signatures are progressively more evolved from South to North. The high Sri and low Nd values indicate an important contribution of crustal components in the magmas. This crustal contribution is greater in the Refoios do Lima granite and less significant in the Ucanha-Vilar granite.

The geochemical and isotopic differences between the various granites exclude the possibility of a unique homogeneous source for the origin of all studied granites. The presence of gabbros in the region where the Sameiro granite outcrops, originated from an enriched mantle, and 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, support a mixing model for their genesis, involving a basic magma (probably a mantle-derived magma) and a felsic crustal magma. A crustal component equivalent to the Refoios do Lima granite is suggested, probably derived from felsic metaigneous sources in lower crust. This petrogenetic model of mixing is also in accordance with the zircon morphological and chemical characteristiques.

Subsequently to the dynamic process of mixing, the magmas evolved by crystallisation with distinct internal evolutions, with the exception of the Sameiro granite whose internal evolution is limited. The application of a quantitative model of fractional crystallisation to the others magmas suggests the fractionation of biotite + plagioclase ± potassium feldspar + apatite ± ilmenite in various proportions.

Theses abstracts