Local isobaric heating above an extensional detachment in the middle crust of a Variscan allochthonous terrane (Órdenes complex, NW Spain)

In one of the most studied Variscan exposures, the órdenes allochthonous complex (NW Spain), the transition between medium-pressure (MP) and high-pressure (HP) units in the SW of the complex has been identified as an extensional shear zone: the Fornás detachment. Migmatitic paragneisses crop out discontinuously along that boundary, at the base of the MP ensemble (O Pino unit). The metamorphic reaction sequence, mass balance calculations, and phase diagram modeling investigated in these paragneisses are interpreted in terms of an approximately isobaric heating path (8 ± 0.8 kbar), from ~650 °C to 740 °C, crossing into the melt- and K-feldspar-bearing stability fields. These anatectic conditions are evidenced by the presence of leucosomes through progressive muscovite and biotite melting reactions. Our results indicate that the heating path evidenced by the migmatitic paragneisses is directly related to the subtractive nature of the Fornás detachment, with heat transferred from the footwall to the hanging-wall unit.