Cadernos Lab. Xeolóxico de Laxe Coruña. 2011. Vol. 36, pp. 177 - 206 ISSN: 0213-4497 Las pegmatitas de Barroso-Alvão, Norte de Portugal: anatomía, mineralogía y geoquímica mineral Pegmatites from Barroso-Alvão, Northern Portugal: anatomy, mineralogy and mineral geochemistry MARTINS, T.1 and LIMA, A.2 (1) Geology Centre-Porto University; Rua do Campo Alegre, 687; 4169-007 Porto, Portugal ([email protected]) (2) Department of Geosciences, Environment and Spatial Planning, Rua do Campo Alegre, 687; 4169- 007 Porto, Portugal ([email protected]) Recibido: 13/12/2010 Revisado: 5/02/2011 Aceptado: 20/02/2011 Abstract The Barroso-Alvão pegmatite field is located in the Variscan belt, in the western part of the Iberian Peninsula, Northern Portugal and it is recognised for its numerous and varied aplite- pegmatite intrusions. This is a rare-element aplite-pegmatite field with enrichment in Li, Sn, Nb>Ta, Rb, and P. Several hundreds of pegmatite bodies were identified and described in this field area intruding a variety of rock types including different metasedimentary, and granitic rocks. In this study we present the geology and mineralogy, and mineral geochemistry of different types of aplite-pegmatites bodies found at Barroso-Alvão. Their description was based on field observa- tion, mineralogy, emplacement of the bodies and geochemical data. There were identified five dif- ferent groups: intragranite pegmatites with major quartz, feldspar, muscovite, biotite, and minor tourmaline, beryl and garnet; barren pegmatites with quartz, feldspar, muscovite, and minor bi- otite, apatite, and beryl, among other accessories; spodumene pegmatites with spodumene, Nb-Ta minerals, and Mn-Fe-Li phosphates, along with other accessory mineral phases; petalite pegma- 178 Martins and Lima CAD. LAB. XEOL. LAXE 36 (2011) tites with petalite, cassiterite, Nb-Ta minerals, Mn-Fe-Li-Ca-Al-Ce-U phosphates; and lepidolite pegmatites with albite, lepidolite, cassiterite and Sr-Al phosphate minerals. Keywords: pegmatite, lithium, Barroso-Alvão, Hercinian orogeny, Portugal CAD. LAB. XEOL. LAXE 36 (2011) Las pegmatitas de Barroso-Alvão 179 INTRODUCTION its entirety, including its origins, petrogen- esis and the distribution of different intru- The Barroso-Alvão pegmatite field has sive phases (MARTINS, 2009). Particular been studied for more than twenty years, interest was given to the following aspects: and it is recognised for a great number of the pegmatite’s relationship with the hosting aplite-pegmatite intrusions. The pegmatite rock, their internal structure and morphol- field, located in the Trás-os-Montes e Alto ogy; petrography and mineralogical descrip- Douro region, Vila Real district of North- tion; mineral geochemistry and evolution of ern Portugal, covers portions of four 1/50 the pegmatite bodies. 000 scale geological maps: 6A-Montalegre, In this paper we divide the pegmatites 6B-Chaves, 6C-Cabeceiras de Basto, and of the Barroso-Alvão area into different 6D-Vila Pouca de Aguiar. groups and describe their anatomy, relation Barroso-Alvão potential was first rec- with the country rock, mineralogy and min- ognised during a regional mapping and eral geochemistry. We use the mineral geo- granite petrology programme where several chemistry to evaluate the pegmatite degree spodumene aplite-pegmatite dykes were de- of fractionation and the evolution trend in scribed (NORONHA, 1987). Several studies this pegmatite field. focussed on the mineralogy, geochemistry, petrology and exploration aspects of these GEOLOGICAL SETTING spodumene-amblygonite bearing intrusions (e.g. CHAROY and NORONHA, 1988; The Barroso-Alvão pegmatite field is DÓRIA et al., 1989; NORONHA and located in the Variscan belt, in the western CHAROY, 1991; CHAROY et al., 1992; portion of the Iberian Peninsula (Figure 1). PIRES, 1995; AMARANTE et al., 1999; It belongs to the Galicia-Trás-os-Montes FARINHA and LIMA 2000; LIMA, 2000; geotectonic zone (FARIAS et al., 1987), CHAROY et al., 2001). close to the thrust that represents the south- Later it was identified a suite of aplite- ern boundary with the Central Iberian geo- pegmatite bodies in which petalite is a tectonic zone. The studied pegmatite field dominant phase (LIMA et al., 2003 a,b,c). is positioned to the West of the Penacova- Following this discovery a more expansive Régua-Verin fault, one of the major NNE- project was undertaken in order to under- SSW Variscan faults that affect the Iberian stand the Barroso-Alvão pegmatite field in Peninsula. 180 Martins and Lima CAD. LAB. XEOL. LAXE 36 (2011) Fig. 1. Simplified geology map of the Barroso-Alvão region according to the 1/50 000 scale geological maps: 6A-Montalegre, 6B-Chaves, 6C-Cabeceiras de Basto, and 6D-Vila Pouca de Aguiar. Metasediments and deformation Three deformation phases (D1 to D3) were identified in the metasedimentary The main hosts of the pegmatites from rocks in S1 to S3 superimposed schistosities. Barroso-Alvão are meta-pelitic, mica-shists, The S1, S2 and S3 foliations observed in the and rarely carbonaceous or graphitic shists field were produced during the three main of upper Ordovician to lower Devonian age. ductile deformation phases of the Hercyni- RIBEIRO et al., 2000 subdivided the sequence an orogeny (D1, D2 and D3) (NORONHA et into three distinct formations called unit Sa, al., 1981; DIAS and RIBEIRO, 1995). unit Sb, and unit Sc based on metamorphic First deformation phase (D1) generated conditions and association of lithologies: an axial planar foliation (S1) that is well pre- - Unit Sa is composed of phyllites and served at higher levels of the Galicia–Trás– mica schists intercalated with carbonaceous os–Montes Zone. The S1 foliation is parallel or graphitic schists and some quartz phyl- to the stratification (S1//S0) with main orien- lites and calcsilicate rocks. tation N120º. - Unit Sb consists of quartzites and The D2 is responsible for a sub-hori- quartz phyllites interlayered with phyllites zontal shear cleavage (S2) that is locally up- and some carbonaceous or graphitic schists. turned vertically by the D3 phase. Lutites and graphitic schists occur in smaller The D3 produced a locally strong, pen- proportions than in Unit Sa; etrative crenulation cleavage (S3) that is - Unit Sc is characterised by a relatively preferentially associated with ductile shear monotonous sequence of phyllites, mica zones (NORONHA et al., 1981). This was schists and quartz wackes with rare calcsili- produced during the final Hercynian duc- cate rocks. Although rare, these calcareous tile upright folding event that resulted in the rocks are important in differentiating the 120° trending folds, which overprint struc- third unit from the previous units. tures, related to phases D1 and D2. CAD. LAB. XEOL. LAXE 36 (2011) Las pegmatitas de Barroso-Alvão 181 Following D3, brittle deformation gener- A biotite isograd was also identified par- ated important faults that affect the entire allel to the andalusite isograde with three study area. The main trends of these fault mineralogical associations: systems vary from N020º - N030ºE to N-S, i) quartz + muscovite + biotite; and E-W to ENE-WSW. ii) quartz + muscovite + biotite + chlo- rite; Metamorphism iii) quartz + muscovite + biotite + albite +/- chlorite +/- garnet. The Central-Iberian Zone and the Gali- In addition to the regional metamor- cia–Trás–os–Montes Zone have experienced phism, we can distinguish a post-kynematic intense and widespread granitic plutonism contact metamorphism related with post-D3 and metamorphism. MARTINEZ et al. granite magmatism and local, hydrother- (1990) delineated isograds of several meta- mal alteration related to aplite-pegmatite morphic zones for the Iberian Massif. Their or quartz vein emplacement has also been distribution is related to the regional meta- recognised. It is inferred by a decussate tex- morphism, closely related to granite pluton- ture in the phyllosilicates and the develop- ism that has been complicated by subse- ment of aluminosilicate porphyroblasts of quent faulting events. andalusite and rarely sillimanite (fibrolite). Within the study area it is possible to Porphyroblasts of cordierite, garnet and bi- identify a synorogenic prograde metamor- otite are also common. All porphyroblasts phism of medium to low pressure and high are Late- to Post-D3, and some of them are temperature. The metamorphism isograds restricted to the presence of quartz veinlets are parallel to the contact of syntectonic (RIBEIRO et al., 2007). granites and to the lithostratigraphic units. The isograds result from the Pre to Syn-D3 Granitic rocks regional thermal peak, related to prograde dynamothermal metamorphism (NO- The region hosting the Barroso-Alvão RONHA, 1983; NORONHA and RIBEI- pegmatite field contains several types of gra- RO, 1983; RIBEIRO et al., 2000). nitic rocks. In this work it is going to be used Close to the contact with the syn-D3 Ca- the classification proposed by FERREIRA beceiras de Basto granite there are two dif- et al. (1987), which takes into account he re- ferent mineralogical associations defining lationship of the granitic rocks and the Her- the andalusite isograde: cynian orogeny. In the field area, it is possi- i) quartz + muscovite + biotite +/- anda- ble to identify Syn-D3 granitoids and late to lusite +/- staurolite; post tectonic granitoids (Post-D3). Figure 2 ii) quartz + muscovite + biotite +/- anda- shows a very simple and schematic division lusite +/- staurolite +/- garnet +/- plagioclase. of the granites found in Barroso-Alvão. 182 Martins and Lima CAD.