Isotopic Disequilibrium Between Migmatites and Protolith: Insights from a Variscan Anatectic Complex (NW of Iberian Variscan Belt, Portugal)

Isotopic disequilibrium during the formation of high-temperature (HT) metamorphic complexes by anatexis during continental collision is a process that deserves intense discussion since it is fundamental to understand the evolution of continental crust. The axial sector of the Iberian Variscan Belt (IVB) is known by the profusion of synorogenic granites that are sometimes clearly associated with the migmatites composing the HT metamorphic complexes. The Pedregal Migmatitic Complex is located in the autochthonous domain of the IVB and is composed of metatexites and diatexites associated to syntectonic two-mica granites. The anatectic process occurred by dehydration melting of muscovite and biotite with the growth of peritectic minerals such as garnet, K-feldspar, and sillimanite in metatexites; and K-feldspar, sillimanite, and hercynite in diatexites reaching the metamorphic peak at 313.5 ± 0.5 Ma. A process of residuum-melt separation during crustal melting is attested by the Pedregal migmatites, giving origin to metatexites and residual diatexites as indicated by field evidence and their geochemical signature. Zircon oxygen isotopes and inherited zircon ages point to the Douro-Beiras Supergroup metasedimentary sequence (Beiras group) as a possible protolith of the Pedregal diatexites. Conversely, the isotopic composition of the diatexites suggests isotopic disequilibrium caused by residual mineral phases (biotite, monazite and garnet).