From subduction to collision in the Parautochthon and autochthon of the NW Variscan Iberian Massif

Abstract. A stacking of nappes forms the Parautochthonous Domain located beneath the Allochthonous Complexes of the NW Iberian Massif. Lower Paleozoic metasedimentary and felsic metavolcanic rocks, including some riebeckite gneiss, form the uppermost nappe of the Parautochthon. Micaschists of this nappe may contain albite porphyroblasts with aligned mineral microinclusions defining a relic internal schistosity. Thermobarometric estimates on the relic mineral assemblage suggest an early M1 stage of High-Pressure, Low-Temperature (HP-LT) metamorphism (11–14 kbar; 450–500 °C), probably related to the same continental subduction process that affected the overlying Lower Allochthon nappes or Basal Units. This uppermost nappe was emplaced onto its relative autochthon by a broad, ductile shear zone that shows top-to-the-E/ENE shear-sense and whose exposure has lateral continuity through to the basal thrust of the Lower Allochthons in the Braganca Complex. The HP-LT metamorphism and its hanging wall position relative to the basal thrust of the Allochthonous Complexes, suggest a re-interpretation of this uppermost parautochthonous nappe as another nappe of the Lower Allochthon. In turn, the other parautochthonous nappes comprise both, Lower Paleozoic pre-orogenic rocks, with no M1 relics, and Upper Paleozoic syn-orogenic rocks. Shear-sense criteria related to the current juxtaposition of the Parautochthon Domain onto the Central Iberian autochthon show contrasting top-to-the-S/SE kinematics. The whole orogenic section was affected by an M2 episode of recrystallization under intermediate P/T conditions during the first stages of collision. P-T estimates on kyanite-bearing schists from the upper sections of the autochthon (9.0 kbar; 425–450 °C) are consistent with a geothermal gradient somewhat lower than the classical Barrovian one at the beginning of collision. Thermobarometric calculations on schists from the lower parautochthonous nappes yield peak metamorphic conditions around 7.5 Kbar and 600–700 °C, constraining the original thickness of the allochthonous/parautochthonous pile to about 22.5–27.0 km. Schists sampled from deeper sections of the autochthon yield M2 conditions around 11–12 kbar and 700–725 °C, matching those registered by correlative regions from Central Iberia. Subsequent M3 syn-collisional recrystallization under High-Temperature and Low-Pressure conditions is associated with top-to-the-N/NW and top-to-the-S/SE extensional flow. The early metamorphic evolution of the autochthons, Parautochthon and Allochthonous Complexes of NW Iberia recorded a transition in P/T regimes and appears as a model case of a change from subduction to collision.