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

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.

Detrital zircon similarities and dissimilarities between the Iberian Pyrite Belt, Ossa-Morena Zone and Meguma

“…The northern domain of SPZ, the PLD (Fig. 2), mainly composed of quartzites and greywackes (Braid et al, 2011;Pereira et al, 2018;Pérez-Cáceres et al, 2017), has been interpreted as an accretionary prism related to subduction under the OMZ (Braid et al, 2010), a Laurussian continental platform unrelated to subduction (Pérez-Cáceres et al, 2020), a Laurussia-derived terrane overlain by a Gondwana-derived accretionary prism related to subduction under the OMZ (Braid et al, 2018) and a back-arc basin related to subduction under the SPZ (Rubio Pascual et al, 2013). Nonetheless, Devonian/ Carboniferous?…”

Low-Temperature Chlorite Geothermometry and Related Recent Analytical Advances: A Review

“…Therefore, and contrary to empirical equations, these two geothermometers predict that one chlorite composition corresponds to one temperature of formation, but that at any fixed temperature, a range of chlorite compositions exist and can be theoretically calculated, in agreement with observations of natural chlorites. These two geothermometers are easy to use, circumvent bulk rock composition effects, and perform well in diagenetic to low-grade metamorphic contexts (e.g., [55,[57][58][59][60][61][62]), particularly in the T range where Si-rich chlorites are observed. They were later derived in a graphical way by [23] to estimate T only from Si and R 2+ contents, making it as easy to use as empirical thermometers.…”

“…The effect of the nonconsideration of Fe 3+ has a subordinate role because temperature estimates are, in the first place, controlled by TK and DT substitutions [23,55], in particular at the Fe total content and XFe 3+ ranges usually observed for LT chlorites. Studies that compare T estimates from the thermometer of Bourdelle et al [55] with those of Inoue et al [28], Vidal et al [49,50], or Lanari et al [51], which require prior quantification of Fe 3+ content, conclude at the convergence of the two approaches [23,59,60,62,77], especially when T < 250 • C.…”

The enigmatic curvature of Central Iberia and its puzzling kinematics

“…(1) The Cantabrian Zone represents a Gondwanan thin-skinned foreland fold-and-thrust belt. It has overall low-grade internal deformation and metamorphism and represents the shortening that occurred during Mississippian times (e.g., Marcos and Pulgar, 1982;Pérez Estaún et al, 1988;Gutiérrez-Alonso, 1996;Alonso et al, 2009;Pastor-Galán et al, 2009, 2013b. (2) The West Asturian-Leonese Zone represents a metamorphic fold-and-thrust belt with Barrovian metamorphism that collapsed coevally with thrust emplacement onto the Cantabrian Zone (e.g., Martínez Catalán et al, 1992, 2014Alcock et al, 2009).…”

“…At the crustal-scale, the Cantabrian Orocline represents a first-order vertical-axis buckle fold in plan view that refolds preexisting Variscan structures (e.g., Julivert and Marcos, 1973;Weil et al, 2001). The inner arc of the orocline, or the Cantabrian Zone, is characterized by tectonic transport towards the core of the orocline; i.e., the orocline has a contractional core, where low finite strain values and locally developed cleavages occur (Pérez-Estaún et al, 1988;Gutiérrez-Alonso, 1996;Pastor-Galán et al, 2009). Within the inner core, a variety of structures record non-coaxial strain, which produced complex interference folds and rotated thrust sheets (e.g., Julivert and Marcos, 1973;Julivert and Arboleya, 1984;Pérez-Estaún et al, 1988;Aller and Gallastegui, 1995;Weil, 2006;Weil et al, 2013;Pastor-Galán et al, 2012b;Shaw et al, 2015Del Greco et al, 2016).…”