Thermodynamic modelling of high-P mineral assemblages from the Iberian Massif

Résumé

he Variscan Orogen formed as a consequence of the collision of two large landmasses, Gondwana to the South and Laurussia to the North, during the final assembly of Pangea that took place in Late Paleozoic times. Nowadays this orogen can be tracked from the Bohemian Massif to the East, through the French Massif Central and the Armorican Massif, to the westernmost Iberian Massif. The most internal zone of the orogen crops out in several allochthonous complexes and it is characterized by several suture zones, featured by high-P metamorphic units and ophiolites. In the Iberian Massif, units that belong to four high-P metamorphic belts are identified, three of which formed during the initial stages of the collision. This PhD Thesis presents the results obtained for the three oldest high-P belts. To the top of the allochthonous complexes, the base of the Upper Allochthon (or Upper Units) is part of a first high-P, high-T metamorphic belt formed during Early Devonian times (c. 400-390 Ma). At the base of the complexes, the Basal Allochthon (or Basal Units) represents a section of another high-P metamorphic belt, in this case characterized by low- to intermediate-T, dated at Late Devonian (c. 377-370 Ma). Finally, the Middle Allochthon contains ophiolitic units of different nature and age. The lower and easternmost section of the ophiolites contains the Somozas Mélange, a serpentinite-matrix mélange formed in a subduction channel that includes abundant tectonic blocks of diverse nature and size. One of the blocks is composed by submarine metavolcanites that underwent high-P metamorphism. The age of the high-P metamorphism is uncertain, although an intermediate age between the high-P metamorphism of the Upper and Basal Allochthons is the most likely...