Deciphering the metamorphic evolution of the Pulo do Lobo metasedimentary domain (SW Iberian Variscides)

• Main Authors: I. Pérez-Cáceres, D. J. Martínez Poyatos, O. Vidal, O. Beyssac, F. Nieto, J. F. Simancas, A. Azor, F. Bourdelle
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-04-01
• Series: Solid Earth
• Online Access: https://www.solid-earth.net/11/469/2020/se-11-469-2020.pdf

<p>The Pulo do Lobo domain is one of the units exposed within the orogenic suture zone between the Ossa-Morena and the South Portuguese zones in the SW Iberian Variscides. This metasedimentary unit has been classically interpreted as a Rheic subduction-related accretionary prism formed during pre-Carboniferous convergence and eventual collision between the South Portuguese Zone (part of Avalonia) and the Ossa-Morena Zone (peri-Gondwanan terrane). Discrete mafic intrusions also occur within the dominant Pulo do Lobo metapelites, related to an intra-orogenic Mississippian transtensional and magmatic event that had a significant thermal input. Three different approaches have been applied to the Devonian–Carboniferous phyllites and slates of the Pulo do Lobo domain in order to study their poorly known low-grade metamorphic evolution. X-ray diffraction (XRD) was used to identify the mineralogy and measure crystallographic parameters (illite “crystallinity” and K-white mica <span class="inline-formula"><i>b</i></span>-cell dimension). Compositional maps of selected samples were obtained from electron probe microanalysis, which allowed for processing with XMapTools software, and chlorite semiempirical and thermodynamic geothermometry was performed. Thermometry based on Raman spectroscopy of carbonaceous material (RSCM) was used to obtain peak temperatures.</p> <p>The microstructural study shows the existence of two phyllosilicate growth events in the chlorite zone, the main one (<span class="inline-formula"><i>M</i>₁</span>) related to the development of a Devonian foliation <span class="inline-formula"><i>S</i>₁</span> and a minor one (<span class="inline-formula"><i>M</i>₂</span>) associated with a crenulation cleavage (<span class="inline-formula"><i>S</i>₂</span>) developed in middle–upper Carboniferous times. <span class="inline-formula"><i>M</i>₁</span> entered well into epizone (greenschist facies) conditions. <span class="inline-formula"><i>M</i>₂</span> conditions were at lower temperature, reaching the anchizone–epizone boundary. These data accord well with the angular unconformity that separates the Devonian and Carboniferous formations of the Pulo do Lobo domain. The varied results obtained by the different approaches followed, combined with microstructural analysis, provide different snapshots of the metamorphic history. Thus, RSCM temperatures are higher in comparison with the other methods applied, which is interpreted to reflect a faster re-equilibration during the short-lived thermal Mississippian event. Regarding the metamorphic pressure, the data are very homogeneous: very low celadonite content (0&thinsp;%–10&thinsp;%) in muscovite (and low values of K-white mica <span class="inline-formula"><i>b</i></span>-cell dimension; 8.995&thinsp;Å mean value), indicating a low pressure–temperature gradient, which is unexpected in a subduction-related accretionary prism. <br/><br/> <strong>Highlights</strong> </p><ul><li> <p id="d1e286">A multidisciplinary approach has been applied to study the metamorphism of the Pulo do Lobo metapelites.</p></li><li> <p id="d1e290">Devonian metamorphism entered epizone conditions.</p></li><li> <p id="d1e294">Carboniferous metamorphism reached the anchizone–epizone boundary.</p></li><li> <p id="d1e298">The inferred low-pressure gradient is incompatible with a subduction-related accretionary prism.</p></li></ul>