Isobaric heating and cooling path of the lower crust of a Variscan exotic unit: evidences from P -T estimates in NW Iberian metapelitic granulites

Allochthonous complexes (AC) in NW Iberia consist of a pile of exotic units characterized by distinct tectonothermal evolution and lithological association, and separated from each other by tectonic contacts, either thrust or extensional detachments. In the Órdenes AC, three groups of units are recognized from bottom to top in the pile: Basal, Ophiolitic and Upper units. Upper units comprise an ensemble of arc-related rocks with a lower section that underwent a high-P and high-T (HP-HT) evolution, and an upper section with an intermediate - pressure (IP) evolution. Extensional detachments have been commonly identifying at the boundary between HP-HT and IP Upper units, like the Fornás and Corredoiras detachments. Pelitic granulites from one IP upper unit, the O Pino unit, have been investigated. Petrologic studies reveal an isobaric tectono-metamorphic crustal evolution throughout a multidisciplinary integration of: (i) detailed microstructural analysis; (ii) EMP mineral chemistry; (iii) mass-balance of the key and representative chemical reactions observed in the microstructures and subsequent interpretation of the reaction sequence; (iv) P-T estimates and paths from phase diagram modeling. Results in the NCKFMASHT system describe an isobaric (c. 7 kbar) continuous heating (and later cooling) evolution ranging from c. 620 to 680 C crossing into the melt-bearing stability fields. These achieved anatectic conditions are evidenced by the presence of both leucosomes (quartz, plagioclase and muscovite) and glass inclusions, mainly in local garnet cores through the dehydration/melting reactions of muscovite first and biotite later. Chemically distinct generations of garnet and plagioclase are evident and stoichiometrically balance each other in continuous reactions. They are consistent with the observations of both a garnet overgrowth and large patches of plagioclase which host euhedral, smaller garnets. In the cooling episode, the newly-grown garnet reacted with melt and formed a retrograde biotite + aluminosilicate + muscovite generation wrapping around the leucosomes or within the matrix. This implicates the existence of a localized thermal event in the base of the O Pino unit. The structural context of these rocks may represent the origin of the isobaric path as an important extensional shear zone (Fornás detachment) has been localized below the pelitic granulites. The Fornás detachment put in contact the O Pino unit from the HP-HT rocks of the Fornás unit (> 750C,15kbar). The thermal contrast between both units may generate a localized heating at the base of the O Pino unit which is a recognized effect in other extensional detachments. This fact is in accordance with the description of syn-kinematic melts intruded into the Fornás detachment in other NW Iberian areas. We may also speculate with a different scenario where the evolution of the pelitic granulites represent a relic of primitive thermal event barely recognized across the IP Upper units for which more geohronological constraint is still needed.