THE ACCUMULATION OF RARE-EARTH AND HIGH-FIELD-STRENGTH ELEMENTS IN PERALKALINE GRANITIC ROCKS: THE GALIÑEIRO ORTHOGNEISSIC COMPLEX,

The Galiñeiro complex, in the Iberian Hercynian belt, is composed of pre-Hercynian gneissic peralkaline granitic rocks that host hydrothermal mineralization of rare-earth and high-field-strength elements. The REE and HFSE are hosted by a large variety of accessory minerals such as bastnäsite, REE silicates (e.g., allanite, thalenite – yttrialite), REE niobotantalates (aeschynite, fergusonite – formanite, samarskite, pyrochlore – betafite), REE phosphates (monazite, xenotime), Th-rich minerals (thorite, P-rich thorite), and zircon. The original magma was derived from a mantle source and most likely had a volatile-rich alkaline basalt composition that evolved by crystal fractionation to give peralkaline silicic differentiates that crystallized at high levels in the crust 460–480 Ma ago. Magmatic fractionation produced residual fluids extremely enriched in fluorine and other ligands, which complexed the rare earths and high-field-strength elements. The decrease in fluorine activity related to falling temperature and the crystallization of F-bearing major minerals produced the breakdown of complexes, releasing the rare earths and the rest of the high-field-strength elements, which then precipitated HFSE- and REE-rich minerals. The elevated fluorine, CO2 and sulfide contents of the Galiñeiro system, together with the energy inputs related to different phases of the Hercynian metamorphism, caused episodic remobilization, reflected by the zircon U–Pb concordant ages at 370 and 310 Ma, and the exotic geochemical features (Zr/Hf> 100, elevated levels of Be, Th, and HREE) of some samples