Scholarly record

PETROGRAPHIC AND LITHOGEOCHEMISTRY OF GRANITES AND ASSOCIATED DIATEXITES (WNW OF PORTUGAL)

Daniel Teixeira, Maria dos Anjos Ribeiro, Helena C.B. Martins

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.

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DOI resolver: doi.org/10.5593/sgem2017/11/s01.053

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