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

Abstract

Abstract. The Pulo do Lobo belt is one of the units related to the orogenic suture 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 the 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 in the dominant Pulo do Lobo metapelites, related to an intraorogenic 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 belt in order to study their poorly known low-grade metamorphic evolution. X-Ray diffraction (XRD) was used to unravel 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 processing with XmapTools software, and chlorite semi-empirical 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 at 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 at middle/upper Carboniferous time. 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 unconformity that separates the Devonian and Carboniferous formations of the Pulo do Lobo belt. The varied results obtained by the different approaches followed, combined with microstructural analysis, are indicative of different snapshots of the metamorphic history. Thus, RSCM temperatures are higher in comparison with the other methods applied, which is interpreted as reflecting a faster reequilibration during the short-lived thermal Mississippian event. Regarding the metamorphic pressure, the data are very homogeneous (very low celadonite content in muscovite and low values of K-white mica b-cell dimension), indicating a low-pressure gradient, which is unexpected in a subduction-related accretionary prism.

Reply to the interactive comment of referee#2

“…"very low to" and "presence of C/S and" have been deleted according to the referee's correction. Line 395 (413-414): we have specifically clarified the term "low-pressure gradient" according to Guidotti and Sassi (1986). Also clarified at the end of the abstract ("low pressure/temperature gradient"; line 51).…”

“…Line 481 (502): "our" has been substituted by "these". Line 498 (520-521): according to Abad et al (2006), the KI value 0.14 ∆ • 2θ is the limit of high epizone conditions. Line 512 (535-536): temperature ranges have been revised, giving the overall range based on the three approaches.…”