Los ortogneises peralcalinos del borde meridional de la Unidad Malpica-Tuy: caracteres geológicos, mineralógicos, geoquímicos y mineralizaciones de Y-Nb-Zr-TR asociadas

Abstract

This paper summarizes the main tectonic, mineralogical, petrological and geochemical features of the three peralkaline complexes located in the southern end of the Malpica-Tuy Unit, in Galicia (NW Spain). The Porriño and La Guía complexes occur in metasedimentary rocks, mainly graywackes, of the Iberian Massif, while the Piñeiro complex is hosted by a pre-Variscun biotite granite. New data concerning the chemical composition and boundaries of the various facies which form the Porriño complex are given here. This data allow to define up to eight petrological facies and reinterprete the origin of these peralkaline bodies. The content and distribution of the major, trace and RE elements in the three complexes are the result of a magmatic differentiation process which gave place to a progressive enrichmente in SiO₂ and a corresponding continuous decrease in Al₂O₃, Fe₂O₃, MgO, TiO₂ and P₂O₅. In the Porriño complex, this geochemical trend towards more evolved rocks is consistent with field observations which show an evolution in time from the earlier biotite-hornblende to the late riebec kite, aegyrine-riebeckite, and biotite facies. Likewise, the REE content is highest in the latest radioactive biotite orthogneiss in which some mineralogically complex Y-NbZr- REE ores have been found. From the genetic point of view, the geochemical and field data show that the orthogneisses forming these peralkaline complexes fall in the field of intraplate granitoides (WPG) and correspond to A-type granites originated by melting of an anhydrous continental crust. Later on, the igneous rocks were folded and overthrust on the Precambrian-Lower Cambrian basement during the Variscan orogeny.