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

Memoria para optar al grado de Doctora.-- Facultad de Ciencias Geológicas, Universidad Complutense de Madrid (20/12/2021).The 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. To understand the tectonic evolution of the orogens and address the associated paleogeographic reconstruction, it is important to unravel the tectonothermal evolution experienced by their high-P metamorphic belts. To do so, this PhD Thesis presents a petrological and geochemical study of key lithologies from each of the main high-P metamorphic belts of the Iberian Massif. The methodology employed includes a previous analysis of the regional context of the units involved, a detailed petrographic study, interpretation of the whole rock composition of the studied lithologies and the investigation of the mineral composition and crystal-chemistry via electron microprobe analysis. With this machine, X-ray maps of selected areas and point microanalysis of the phases were performed in order to understand the composition space and to perform the thermodynamic modelling of the high-P lithologies. The results obtained have been published in different specialized journals in the course of the PhD, all of them of relevance according to their bibliographic metrics, and are presented here as a compilation of publications. The order of exposition follows the chronology of the high-P events recorded in the Variscan Orogeny, from the oldest to the youngest episode. The first publication offers a study of high-P migmatitic paragneisses from the Sobrado Unit (Upper Allochthon from the Órdenes Complex). To do so, the residual composition was modelled and metamorphic peak conditions of ~ 15 kbar and ~ 835 ºC were calculated. Peak conditions were followed by an isothermal exhumation down to 10 kbar and by a subsequent decompression characterized by a strong cooling up to ~ 5 kbar and ~ 550 ºC. Besides, the prograde path of the paragneisses was assessed using a melt-reintegration approach adding a theoretical melt to the residual composition and performing the thermodynamic modelling of the new pre-melt extraction composition. The conclusions of this research were used to constrain the dynamic context of the initial high-P, high-T metamorphism in the Variscan Orogen. The second article focuses on the tectonothermal evolution of a tectonic block of metavolcanites from the Somozas Mélange. This block contains broken pillow breccias with some isolated and close-packed pillow lavas, in addition to minor lava flows with basaltic andesite composition and diabasic dykes. The metahyaloclastitic matrix of the breccias contain a complex mineral assemblage constituted by garnet, kyanite, chlorite, phengite, paragonite, margarite, quartz, epidote, hematite and rutile. The thermodynamic modelling of the samples indicate metamorphic peak conditions of ~ 18 kbar and 550 ºC reached during the deep burial and accretion to a subduction channel ensued by an almost isothermal drastic exhumation. The third and fourth publications analyze high-P rocks that belong to the Cubito-Moura Unit and the Badajoz-Córdoba Unit (or Central Unit) of the Basal Allochthon from SW Iberia, respectively. While the high-P rocks studied from the Cubito-Moura Unit reached intermediate-T during metamorphism, the high-P rocks analyzed from the Badajoz-Córdoba Unit only reached low-T conditions. In the Cubito-Moura Unit, glaucophane-bearing eclogites show a prograde evolution up to the baric peak at ~ 24 kbar and ~ 575 ºC. The thermal peak was reached later at ~ 19 kbar and ~ 630 ºC, and data points to a subsequent isothermal drastic exhumation to 15 kbar. In the Badajoz-Córdoba Unit, the thermodynamic modelling of blueschists-facies metapelites with Mg-rich chloritoid reveals minimum pressures of ~ 20 kbar at ~ 525 ºC and an evolution that implies moderate heating during decompression. The results obtained in this PhD Thesis, along with previous data, were used to explore the correlation of the high-P belts across the Iberian Massif as well as the whole orogen. The correlation between Iberia and the easternmost sectors of the orogen is, overall, possible, since equivalent units are recognized along the different massifs. Among the four high-P events that can be observed in the Variscan Orogen, the fourth and youngest one (c. 340 Ma) was not assessed in this PhD due to its limited extent in the Iberian Massif. Nevertheless, it significance is discussed in the final chapter of the PhD Thesis.Peer reviewe