Scholarly record
PETROGRAPHIC AND LITHOGEOCHEMISTRY OF GRANITES AND ASSOCIATED DIATEXITES (WNW OF PORTUGAL)
Abstract
The studed area, a WNE-ESE cross-section Espinho-LobпїЅo, is located is the westhern boundary of Central Iberian Zone and Ossa Morena Zone, near the NW cost line of Portugal. This is an axial sector of the Iberian Variscan Orogen from which several geodynamic interpretations have been proposed. The aim of this work is a contribution to the petrogenese and variscan evolution of the (meta) igneous and metasedimentary units, constrain by macro, microstructural, petrographic and lithogeochemical data. The metasedimentary units are the Schist-Greywacke Complex (Lordelo do Ouro Unit) and Espinho and Lourosa Units. Both are intrud by igneous rocks classified as diatexite gnaisso-migmatites and granites. The samples analyzed in this preliminary study belong to Espinho and Lourosa Units. The diatexites have a granitic composition with a macroscopic anisotropy marked by schlieren, which sometimes show a convoluted geometry around quartz-feldspar lenses. This anisotropy defines microlithons which have an earlier foliation inside. The schlieren are essentially of micaceous composition, predominantly biotite over muscovite, associated with sillimanite. The biotite presents mainely a decussed texture. The granites are biotitic, with a coarse-grain phaneritic texture, and are generally isotropic. They are leucocratic to mesocratic due to the large percentage of biotite. They are very rich in accessories minerals, including zircon, apatite, rutile, oxides and hydroxides. Quartz is the most abundant mineral in all samples and appears often to be subhedral to anhedral with regular or slightly wavy extinction. Goticular quartz inclusions in plagioclase and potassic feldspar are frequents. The plagioclase and the potassic feldspar present high degree of sericitization with abundant secondary muscovite. The biotite is often chloritized, especially on the edges of the crystals with abundant rutile needles. Texturally the abundance of myrmekites must be highlighted, as well as the intergrowth of the edges of plagioclase and potassium feldspar grains. The whole rock geochemistry highlights the strong siliceous nature (SiO2>65%) of the diatexites and granites. The granites exhibit high content of light rare earth elements (LREE) and quite lower content of heavy rare earth elements (HREE). They have a strong fractionation La/Sm and a flattened spectrum in the heavier rare earths (Er/Lu). They have a low negative anomaly (Eu/Eu* varies between 0.82 and 0.94). The diatexitic rocks have greater fractionation of LREE and lower fractionation of HREE, and a negative Eu anomaly, ranging between 0.58 and 0.80.
Publication Impact Profile
Publication details
References14
Chaminé H.I., Estratigrafia e estrutura da faixa metamórfica de Espinho–Albergaria-a-Velha (Zona de Ossa-Morena): implicações geodinâmicas. Universidade do Porto, Portugal, 2000.
Mendes M.H., Contribuição para o estudo das rochas metamórficas aflorantes entre Ovar e Espinho, Universidade de Aveiro, Portugal, 1988.
Ribeiro A., Munhá J., Dias R., Mateus A. Pereira E., Ribeiro M.L., Fonseca P., Araújo A., Oliveira J.T., Romão J., Chaminé H., Coke C., Pedro J.C., Geodynamic Evolution of the SW Europe Variscides, Tectonics, 2007, Vol. 26, Issue 6.
Teixeira C., Perdigão J., Carta Geológica de Portugal 1/50 000: Notícia explicativa da folha 13-A (Espinho), Serviços Geológicos de Portugal, Portugal, 1962.
Romão J., Rodrigues J.F., Pereira E., Ribeiro A., Relações tectonoestratigráficas entre o terreno Ibérico. Pags. In: Mirão, J. & Balbino, A. (Eds), VII Congresso Nacional de Geologia, 2006, Portugal, pp 123 - 126.
Dias R., Ribeiro A., Romão J., Coke C., Moreira N., A review of the arcuate structures in the Iberian Variscides; constraints and genetics models, Tectonophysics, 2016, 681, pp170 – 194
Gutiérrez-Alonso G., Collins A.S., Fernández-Suárez J., Pastor-Galán D., González-Clavijo E., Jourdan F., Weil A.B., Johnston S.T., Dating of lithospheric buckling: 40Ar_39Ar ages of syn–orocline strike–slip shear zones in northwestern Iberia, Tectonophysics, 2015, 643, pp 44 – 54.
Cândido de Medeiros A., Carta Geológica de Portugal 1/50 000: Notícia explicativa da folha 13-B, Serviços Geológicos de Portugal, Portugal, 1964.
Ferreira N., Iglésias M., Noronha F., Pereira E., Ribeiro A., Ribeiro, M.L.,. Granitoides da Zona Centro Ibérica e seu enquadramento geodinâmico, Bea, F., Carnicero, A., Gonzalo, J. C., Lópes Plaza, M., Rodríguez Alonso, M. D. (Eds), Geologia de los granitoides y rocas associadas del macizo hespérico: libro homenaje a L. C. Garcia de Figuerola. Editorial Rueda, Madrid, 1987, pp37 – 51
LNEG-LGM, Carta Geológica de Portugal, na escala de 1:1 000 000. 3ª edição. Laboratório Nacional de Engenharia e Geologia, Portugal, 2010.
Oliveira J.T., Pereira E., Ramalho M., Antunes M.T., Monteiro J.H.,. Carta Geológica de Portugal (parte norte), escala de 1:500 000. Serviços Geológicos de Portugal, Portugal, 1992.
Debon F., Le Fort P., A chemical-mineralogical classification of common plutonic rocks and associations. Transactions of the Royal Society of Edinburgh, Earth Sciences, 1983, 73, pp 135-149.
Evensen N.M., Hamilton P.J., O´Nions R.K., Rare earth abundances in chondrite meteorites. Geochimica et Cosmochimica, 1978. Acta, 42, pp 1199-1212.
Bea F., Residence of REE, Y, Th and U in Granites and Crustal Protoliths; Implications for the Chemistry of Crustal Melts. Journal of Petrology, 1996, 37 (3), pp 521-552.
View or Download full articleAccess options
SWS access login
Login as SWS Scientific CommitteeLogin as SWS Scientific PartnerLogin as SWS AuthorAuthors and approved SWS contributors will read and export their own linked papers after identity matching by SWS profile, email and SGEM GlobalID.
For librarian assistance: [email protected]
Purchase Instant Access
- Article can be downloaded after successful payment.
- Article may be used according to SWS library access terms.
- Article cannot be redistributed.
