PhD thesis presented at University of Minho by Anabela Mendes, 2002
Abstract
The Peneda-Gerês granitic massif is a representative example of a post-tectonic Hercynian massif. It outcrops over the border between NW Portugal and the south of Galicia (Spain), therefore integrating the Central Iberian Zone of the Iberian Massif. This massif is composed of six main granitic units: the Gerês, Paufito, Illa, Carris, Calvos and Covas granites. The observed field relationships between these units revealed contacts that are either gradual or sharp and lobated, suggesting that the emplacement of these units was synchronous. The observation of the crystal shape preferred orientation of potassium feldspar megacrysts in the granitic units with porphyritic character permitted to decipher magmatic foliations that are clearly more perceptible near the borders of the massif. This planar structure is subparallel to the borders of the massif and dips steeply and dominantly towards its center. Therefore, the general structure seems to be that of a funnel and an important upper portion of the intrusion seems to already have been eroded.
All granitic units of this massif are peraluminous leucogranites, being the peraluminous character weaker in the Gerês, Paufito, Carris and Covas granites. In terms of magmatic affinity, the various typological indicators point out a subalkaline ferropotassic signature for the majority of the units (Gerês, Paufito-Illa, Carris and Covas). The Calvos granite is an exception for it presents characteristics of a typical S-type granite and aluminopotassic association.
The whole-rock geochemical study (major, minor and trace elements) revealed five distinct compositional domains for the six granitic units. The Carris granite is the only unit which is compositionally similar to another unit, namely to the most evolved samples of the Paufito granite. These data revealed also that the two most internal and concentrical units of the massif, the Paufito and Illa granites, although being compositionally distinct, present a compositional and evolutionary continuity with the most internal unit, the Illa granite, being the more evolved compositional term. The Gerês, Paufito, Illa and Calvos granites present also internal chemical-mineralogical evolutions. The samples of Gerês granite define a compositional evolution that, parallel to an increase in SiO2, is characterized by a decrease of Al2O3, K2O, CaO, Zr, Th, (La/Yb)N and an increase in Y, with no variation of Ba, Sr, Th and Rb. In the Paufito and Illa granites, parallel to the increase of SiO2 there is a decrease in Al2O3, Fe2O3t, MgO, TiO2, CaO, Ba, Sr, Zr, Th, light rare earths and Eu/Eu*. In the Calvos granite, with the increase in SiO2 there is a decrease in Al2O3, Fe2O3t, MgO, TiO2, P2O5, Ba, Sr, Zr, light rare earths, (La/Yb)N and Eu/Eu*.
The geochronological study undertaken revealed a set of very concordant data for the two isotope systems that were analysed. The Rb-Sr isotope system yielded an age of 296 ± 6 Ma for the Gerês granite (MSWD=0.08) and 292 ± 6 Ma for the Paufito and Illa granites (MSWD=0.26). The U-Pb isotopes yielded for the Gerês granite an age of 296 ± 2 Ma (monazite) and 297 ± 7 Ma (zircon). For the Paufito granite an age of 290 ± 2.5 Ma (zircon) was obtained. This set of data permits to consider an emplacement age of 290 to 296 Ma for the Peneda-Gerês massif.
During the U-Pb geochronological study and the zircon geochemical study, the observation of the internal structure of more than a hundred zircons by scanning electron microscope revealed the existence, in some cases, of inherited cores of two different types. In some cases they were dark cores similar to those observed in zircons of subalkaline granites of other regions and that were interpreted as relict cores indicating a hybridization process (Pupin and Persoz, 1999). The other type of cores are discordant cores that occur mostly in late-crystallizing zircons of the Illa granite and that are interpreted as the testimony of a late contamination of the magma.
The Sr, Nd isotope data revealed that the granites of the Peneda-Gerês massif have poorly evolved isotope compositions. The Gerês and Covas granites present the same isotope composition with (87Sr/86Sr)296 between 0.7030 and 0.7038, epsilon Nd296 between -1.51 and -1.57. The Paufito and Illa granites also present the same composition: (87Sr/86Sr)290 from 0.7061 to 0.7069 and epsilon Nd290 between -2.34 and -2.39. The Carris granite has an intermediate composition between these two groups: (87Sr/86Sr)290 between 0.7048 and 0.7053, epsilon Nd290 between -1.88 and -1.96. The Calvos granite possesses the most evolved isotope composition: (87Sr/86Sr)296 between 0.7076 and 0.7092, epsilon Nd296 between -3.49 and -3.92.
The model ages TDM are of 1.05 Ga for the Gerês and Covas granites, 1.11 Ga for the Paufito and Illa granites, 1.07 to 1.08 Ga for the Carris granite and 1.20 to 1.23 Ga for the Calvos granite. Aditionally, for the Carris granite, in the U-Pb geochronological study of zircons a reverse discordia yielded an upper intersection at 1.2 Ga. These results permit to conclude that, just as in other zones of the European Variscan chain, in this region a portion of continental crust was present at least since the Mid Proterozoic.
The set of available data permit to conclude that the Paufito and Illa granites are comagmatic. Based mainly on the Sr, Nd isotope data, a comagmatic origin can also be admitted for the Gerês and Covas granites. The distinct isotope signatures of these two sets of units and of the Calvos granite suggests that they derived from different sources. Relatively to the Carris granite, its isotope and mineralogical (biotite) composition is intermediate between the surrounding Gerês granite and the nearby Paufito granitic unit, although its whole-rock geochemistry is similar to the Paufito granite. The isotope signature of this granite may possibly not correspond to a primary character which would reflect the source composition. The original isotope composition of the Carris granite may have been modified by diffusive transfer between the original magma (probably the same as that of the Paufito granite) and the more evolved surrounding magma (that of the Gerês granite). It seems, therefore, that the granitic units of this massif are the result of the emplacement of at least three intrusive magmas (Gerês-Covas, Calvos and Paufito-Illa-Carris) that evolved independently.
The chemical-mineralogical characteristics of the internal evolutions presented by the Gerês-Covas granites, Calvos granite and Paufito-Illa granites may be interpreted as primary evolutions dominantly controlled by a process of fractional crystallization. This hypothesis is not contested by the results of the modelization of this process.
The comparison between late-Hercynian subalkaline granites of Portugal and of other regions of the Hercynian chain reveals that this type of plutonism presents an isotopic signature globally weakly evolved. The available data indicates that the composition is dominantly in the interval: Sri = 0.705-0.707 and epsilon Ndi = -1.04 to -2.54.
The origin of these magmas may be the partial fusion of lower crust material or instead the mixing between a mantle component and a crustal component. In the first case the crustal sources would have to possess a more depleted isotopic signature than the lower crustal xenoliths present in the central sector of the Iberian Peninsula. In case a mantle component is present, this component would have a distinct signature of the mantle component that is admitted by some authors to be in the origin of the syn-F3 and late-F3 granitoids of NW Portugal. According to Pupin and Persoz (1999), the origin of subalkaline ferro-potassic magmas envolves mixing between an alkaline and a calc-alkaline magma.