The Genesis of Late Hercynian Granitoids from Galicia (Northwestern Spain): Inferences from Ree Studies

Two-mica granites and biotite-rich monzogranites and granodiorites constitute the main types of late- to post-kinematic Hercynian intrusions in Galicia (northwestern Iberian massif). Both rock types exhibit analogous total REE content, although the two-mica granites have generally more fractionated normalized spectra. The Hercynian granites studied can be modeled as partial melts, which in some cases experienced feldspar-dominated fractional crystallization. Model source materials may be compared to the rocks outcropping in the area studied. These granites are not accurately described by the I- and S-type classification. Those with S-type characteristics (two-mica granites) are derived mainly from an orthogneiss-rich model-protolith (i.e., I-type source), whereas those with I-type affinities (biotite-rich granitoids) may be modeled from mixtures of orthogneiss and graywacke (eastern Galicia) or from orthogneiss-rich protoliths containing only a small proportion of metabasite (western Galicia). The proposed models indicate that strongly peraluminous, two-mica granites and mildly peraluminous, biotite-rich granitoids can form in a late- to post-collisional setting, unrelated to either subduction or the influx of mantle-derived melts. Recycling of the pre-existent, tectonically thickened continental crust through post-collision thermal recovery was the main mechanism behind the origin of late- and post-kinematic granitoids in this sector of the Hercynian fold belt.