Journal of the Virtual Explorer

The early, syn-D₃ granodiorite-monzogranite suite is represented by three small plutons intruded syntectonically along the Juzbado-Penalva sinistral shear zone: the Tamanhos, Maceira and Casal Vasco granites (Fig. 3). These granites show heterogeneous deformation that may result in the development of a gneissic foliation. Based on zircon and monazite U-Pb geochronological data (Valle Aguado et al., 2005), the crystallization ages for the Maceira and Casal Vasco intrusions were estimated at 314 ± 5 Ma and 311 ± 1 Ma, respectively (Fig. 4).

Figure 4. U-Pb zircon/monazite ages

U-Pb zircon/monazite ages for the Beiras granite batholith (Mota Leite et al., 2005 and Valle Aguado et al., 2005).

Despite their strong deformation fabric, the Maceira and Casal Vasco biotite gneiss-granites still preserve remnants of igneous granitic textures. The main facies range from medium-grained equigranular to K-feldspar porphyritic. The medium-grained equigranular varieties dominate in the Maceira massif whereas the porphyritic facies are the major lithological type in the Casal Vasco pluton. Mineralogically, the granitoids contain quartz (20-30%) + plagioclase (An34-An18) (30-35%) + K-feldspar (? 20%) + biotite (> 10%) + apatite + zircon + monazite + opaques ± muscovite. Fibrolite needles and andalusite grains were occasionaly found in some samples of the Casal Vasco granitoids. Mafic microgranular enclaves (MME) are present in both rock units.

The syn-D₃ peraluminous granitoids of the Junqueira – Serra da Freita massif crop out as broadly concordant mesozonal plutons in the core of a NW-SE trending, D₃ megascopic antiformal structure - the Porto-Viseu Belt (Fig. 3).

Syntectonic intrusion is inferred from the development of an early NW-SE trending magmatic flow fabric evolving locally into a post-solidus foliation. The main granite facies is a medium-grained equigranular two-mica granite, hosting numerous metasedimentary xenoliths (micaceous restites with sillimanite). It is a typical S-type granite and consists of 27 to 35% quartz, 22 to 35% plagioclase (An7-An4), 15 to 30% perthitic K-feldspar, 2 to 8% biotite and 6 to 12% muscovite. Sillimanite, apatite, zircon, monazite and ilmenite are present as accessory phases. Monazite microfractions from the Junqueira granite yielded slightly reversely concordant points at an average 207Pb/235U age of 307.8 ± 0.7 Ma (Valle Aguado et al., 2005) (Fig. 4).

The Cota-Viseu coarse porphyritic biotite monzogranites occur as a large, discordant, irregularly shaped intrusion, more than 75 km long and 35 km wide (Fig. 3), bearing little or no petrographical evidence of solid state deformation. The granite is spatially associated with minor bodies of basic and intermediate rocks (gabbro-norites, monzodiorites, quartz monzodiorites and granodiorites) and contains abundant mafic microgranular enclaves (MME).

Mineralogically, the Cota-Viseu granite consists of K-feldspar megacrysts up to 8 cm long set in a medium- to coarse–grained groundmass of quartz, plagioclase (An15-32), K-feldspar (mostly perthitic microcline) and biotite (high-Al, low-Mg). Accessory minerals include apatite, zircon, monazite, ilmenite and rare xenotime. The lobate nature of the contacts between the monzogranite and the small stocks of more mafic rocks suggests a synchronous emplacement for the different plutonic units.

The quartz-bearing gabbronorites have medium-grained, hypidiomorphic granular textures and consist of plagioclase (An50-69), clinopyroxene (augite), orthopyroxene, magnesio hornblende, high-Mg biotite, interstitial quartz and accessory apatite, magnetite, ilmenite and zircon. Sericite, talc, chlorite, calcite and uralitic amphibole form the hydrothermal assemblage. In the quartz-diorites, quartz-monzodiorites and granodiorites, plagioclase ranges from andesine to oligoclase (An15-46), amphibole (magnesio and ferro-hornblende to actinolite) and high-Mg biotite are the dominant mafic phases. The amounts of quartz and perthitic K-feldspar increase towards the more evolved members of the sequence.

Early attempts to date the Cota-Viseu granite using the Rb-Sr whole rock method yielded ages ranging between 282 ± 6 Ma (Pereira, 1991), 308 ± 11 Ma (Silva, 1995) and 315 ± 9 Ma (Azevedo and Nolan, 1998). Conventional multigrain U-Pb analysis of zircon and monazite grains from two samples of the Cota-Viseu coarse porphyritic biotite monzogranite have given ages in the range 305-308 Ma. The 207Pb/235U ages obtained in the monazite fractions of both samples are identical within error (306 ± 9 Ma and 306 ± 4 Ma, respectively) and appear to reflect the crystallization age of the granite (Valle Aguado et. al. 2005) (Fig. 4). Based on field relationships, a similar age is assumed for the basic and intermediate rocks.

The late-post-D₃ biotite-muscovite granite suite comprises two large plutonic complexes: the Alcafache-Freixiosa massif (in the centre) and the Dão massif (in the north) (Fig. 3) showing intrusive relationships with the Cota-Viseu coarse porphyritic biotite monzogranite. These granites span a wide spectrum of petrographic types (medium- and coarse-grained inequigranular to fine-, medium- and coarse-porphyritic) and have 25-33% plagioclase (An1-28), 26-32% quartz, 25-35% K-feldspar, 4-6% high-Al, low-Mg biotite and 2-5% muscovite as major rock-forming minerals. K-feldspar (mostly perthitic microcline) is present both as an interstitial groundmass phase and as subhedral megacrysts. However, the K-feldspar megacrysts are generally smaller than those of the coarse porphyritic biotite types. Another important difference between these granites and the Cota-Viseu coarse porphyritic biotite granitoids is the occurrence of muscovite as a significant modal phase. The various facies of the biotite-muscovite granitoids are generally enclave-free although they may contain, in places, sparse biotite schlieren and enclaves of igneous and metasedimentary origin.

Rb-Sr whole rock ages for these granites range from 291 ± 13 Ma and 293 ± 10 Ma in the medium grained and fine-medium porphyritic varieties of the Freixiosa-Alcafache massif (Azevedo and Nolan, 1998; Silva, 1995) to 287 ± 7 Ma, in the medium-coarse porphyritic biotite-muscovite types of the same massif (Silva, 1995).

In the northwest, the Cota-Viseu coarse porphyritic biotite monzogranite is intruded by the Castro Daire composite massif (Fig. 3). This massif, first described by Schemerhorn (1956), consists of five main granitoid units, defining a concentric zonation pattern of progressively less basic compositions from margin to core (Lamelas hornblende-biotite quartz monzodiorite; Castro Daire biotite monzogranite; Calde coarse porphyritic biotite-muscovite monzogranite, Alva and Lamas two-mica monzogranites).

The Calde coarse porphyritic biotite-muscovite granite is the dominant lithological type within the Castro Daire intrusion. It is predominantly composed of quartz, K-feldspar (mostly perthitic microcline) and plagioclase (oligoclase - albite) showing oscillatory and complex zoning. Biotite is the main mafic phase. Muscovite is present in variable amounts. Apatite, zircon, monazite, xenotime and opaques occur as common accessory phases. Andalusite and masses of fibrolite needles of restitic origin and late-stage garnet and tourmaline (schorl) are sporadically observed. Dark igneous enclaves and pelitic inclusions are unevenly distributed throughout this facies.

U-Pb zircon and monazite ages for the Calde granite were determined at ETH, Zurich, using conventional isotopic dilution techniques on single-grain fractions (Mota Leite et al., 2005). The zircon population consisted of three acicular to long prismatic grains and yielded overlapping 206Pb/238U ages of 294.8 ± 2.9 Ma. The three analysed monazite crystals yielded a 207Pb/235U age of 294.1 ± 3.5 Ma. In view of the good agreement of the monazite and zircon U-Pb ages, it is possible to date the emplacement of the Calde coarse porphyritic biotite-muscovite granites at 294 Ma (Fig. 4).