Abstract
Abstract. 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 b-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. The microstructural study shows the existence of two phyllosilicate growth events in the chlorite zone, the main one (M1) related to the development of a Devonian foliation S1 and a minor one (M2) associated with a crenulation cleavage (S2) developed in middle–upper Carboniferous times. M1 entered well into epizone (greenschist facies) conditions. M2 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 %–10 %) in muscovite (and low values of K-white mica b-cell dimension; 8.995 Å mean value), indicating a low pressure–temperature gradient, which is unexpected in a subduction-related accretionary prism. Highlights A multidisciplinary approach has been applied to study the metamorphism of the Pulo do Lobo metapelites. Devonian metamorphism entered epizone conditions. Carboniferous metamorphism reached the anchizone–epizone boundary. The inferred low-pressure gradient is incompatible with a subduction-related accretionary prism.
Highlights
IntroductionThe determination of temperature and pressure (P –T ) conditions reached by the low-grade metasedimentary units stacked in the hinterlands of orogens constrains their tectonometamorphic evolution (e.g., Goffé and Velde, 1984; Franceschelli et al, 1986; Ernst, 1988; Gutiérrez-Alonso and Nieto, 1996; Frey and Robinson, 1999; Bousquet et al, 2008; Lanari et al, 2012)
The metamorphism of the Iberian Variscides has been mostly studied on high-grade metamorphosed rocks in order to characterize and obtain the P –T –t paths of suture-related units (e.g., Gil Ibarguchi et al, 1990; Abalos et al, 1991; Escuder Viruete et al, 1994; Barbero, 1995; Arenas et al, 1997; Fonseca et al, 1999; López-Carmona et al, 2013; Martínez Catalán et al, 2014)
Obtaining new results from the low-grade rocks of the Pulo do Lobo domain, a suture-related low-grade unit in SW Iberia, is of prime importance in order to understand its significance and tectonometamorphic evolution, whose interpretations have been the cause of discrepancies, and to reconstruct the overall history of the SW Iberian Variscides
Summary
The determination of temperature and pressure (P –T ) conditions reached by the low-grade metasedimentary units stacked in the hinterlands of orogens constrains their tectonometamorphic evolution (e.g., Goffé and Velde, 1984; Franceschelli et al, 1986; Ernst, 1988; Gutiérrez-Alonso and Nieto, 1996; Frey and Robinson, 1999; Bousquet et al, 2008; Lanari et al, 2012). The metamorphism of the Iberian Variscides has been mostly studied on high-grade metamorphosed rocks in order to characterize and obtain the P –T –t (pressure– temperature–time) paths of suture-related units (e.g., Gil Ibarguchi et al, 1990; Abalos et al, 1991; Escuder Viruete et al, 1994; Barbero, 1995; Arenas et al, 1997; Fonseca et al, 1999; López-Carmona et al, 2013; Martínez Catalán et al, 2014). Obtaining new results from the low-grade rocks of the Pulo do Lobo domain, a suture-related low-grade unit in SW Iberia, is of prime importance in order to understand its significance and tectonometamorphic evolution, whose interpretations have been the cause of discrepancies, and to reconstruct the overall history of the SW Iberian Variscides.
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