Reconstruction of the prograde PT history of high‐P migmatitic paragneisses via melt‐reintegration approach and thermodynamic modelling (Allochthonous Complexes, NW Iberian Massif)

Abstract

The Upper Units of the allochthonous complexes of the NW Iberian Massif constitute a terrane with continental affinity. They represent the vestiges of a Cambrian magmatic arc developed in the periphery of Gondwana (West African Craton) which was involved in the Devonian Variscan collision, undergoing high-pressure, hightemperature metamorphism. This includes ultramafic rocks, high-pressure mafic rocks (eclogites and granulites) and high-pressure migmatitic paragneisses. The latter rocks show an extensive migmatization with the leucosomes oriented parallel to the regional foliation. The migmatitic paragneisses are composed of garnet, kyanite, biotite, quartz, plagioclase, K-feldspar, rutile and Ti-hematite. Thermodynamic modelling using the measured bulk composition in the NCKFMASTHO system indicates metamorphic peak conditions of ~15 kbar and ~800-835ºC, followed by a significant cooling. The prograde evolution is assessed by means of a melt reintegration approach, using the composition of the garnet and its inclusions. An appropriate composition of liquid is added to the measured bulk composition to emulate the pre-melting bulk composition.Modelling of this melt-reintegrated composition allows to identify a colder high-pressure episode below ~500ºC. Zircon crystals extracted from the leucosomes show overgrowths crystallized from the partial melt at c. 389 Ma (U-Pb system). The P-T-t path proposed reveals a subduction of the peri-Gondwanan arc-derived section down to mantle depths. An isobaric heating stage occurred as a result of residence at great depths and/or inception of a transient oceanic basin at c. 395 Ma. The ensuing nearisothermal exhumation occurred due to the extension related to the inception of the basin, reaching the thermal peak shortly before c. 389 Ma. Subsequent cooling is related to the underthrusting of colder oceanic and transitional crust below the HP-HT Upper Units.