Thermobarometry and crystal chemistry evolution of low-grade metamorphism in central Pyrenees

Nicolas Bellahsen and Benoit Dubacq
ISTeP, Sorbonne Université, Paris, France

Major Results

  1. pervasive late-Variscan hydrothermal pre-kinematic alteration at at 300-230 Ma, at 300-350°C in Bielsa massif;
  2. syn-kinematic Alpine metamorphism at ~40 Ma, 280-300°C in mylonites and fractured granites of Bielsa massif;
  3. high element mobility in distributed mylonites of Bielsa vs. low element mobility in localized strain zones of Maladeta massif;
  4. partial chlorite replacement even in high strain zone due to disconnected nano defects outside areas of fluid channelization.


In the 305 Ma-old Varsican basement of the Bielsa massif (Axial Zone of the Pyrenees) microstructural and petrographic analysis, low-temperature thermobarometry and in situ U-Th/Pb dating of anatase, titanite and monazite show extensive late-Variscan pre-orogenic (pre-Alpine) alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270-350°C at 230-300 Ma. Compositional maps of chlorite including iron speciation were compared to nanostructures observed by transmission electron microscopy in increasingly-strained samples. They show in the undeformed granitoid local equilibria, pseudomorphic replacement and high compositional heterogeneities due to variable reaction mechanisms and element supply, little interconnected nanoporosity and isolation of fluid in local reservoirs. The late-Variscan event is followed by a second fluid-rock interaction stage marked by new crystallization of phyllosilicates at temperatures of 250-300°C and associated with the formation of mylonitic shear zones and fractures parallel to the shear planes. Alpine chlorite grows in mm-sized fractures from a free channelized fluid, preserving late-Variscan chlorite within the matrix, or more pervasively in mylonites, where fluids percolate in micro and nanocracks allowing extensive but incomplete replacement of chlorite.


U-Pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40-70 Ma. Hence, the Variscan basement was softened at late- to post Variscan time, at least 150-200 Ma before the main Alpine shortening while Alpine-age compression lead to the formation of a dense net of mylonites. The associated deformation, both distributed at the scale of the Bielsa massif and localized at decametric scale in mylonitic corridors, precedes the strain localization along the major thrusts of the Axial zone. At the micro and nanoscale, this study shows that local equilibria and high compositional heterogeneities in phyllosilicates as chlorite are preserved according to (i) matrix-fracture porosity contrasts and (ii) the location and connection of nanoporosity between crystallites of phyllosilicates. In the Maladeta massif, where the late-Variscan alteration is less pervasive, geochemical analyses show a lower mobility of trace element from the undeformed granite to mylonites than in Bielsa. Here strain is mainly accommodated by quartz and localized in km-spaced, discrete mylonitic zones instead of in a net of distributed phyllosilicate-bearing shear zones as in Bielsa.
Field structures around Lake Urdiceto Mylonitic corridor. Photomicrographs and backscattered electron images (BSE) and chemical composition of Chlorite (Colored) of granitic and granodioritic basement with increasing deformation.
Age distribution (this study and biblio results). Analysis and dating mineral (Zircon, Anatase, Titanite and Monazite). Chlorite thermometry shows a pre-kinematic (300-230 Ma) alteration at 300-350°C. Then a 40Ma mylonitic deformation at 280-300°C

Pre‐orogenic upper crustal softening by lower greenschist‐facies metamorphic reactions in granites of the central Pyrenees

Pre‐orogenic upper crustal softening by lower greenschist‐facies metamorphic reactions in granites of the central Pyrenees

Airaghi, L., Bellahsen, N., Dubacq, B., Chew, D., Rosenberg, C., Janots, E., Waldner, M. and Magnin, V. (2020). Pre-orogenic upper crustal softening by lower greenschist facies metamorphic reactions in granites of the central Pyrenees. Journal of Metamorphic Geology, 38(2), 183-204.

keywords: Axial Zone, Bielsa, chlorite–white mica thermobarometry, mylonites, U–Th/Pb anatase–titanite–monazite dating


Shortening of the axial zone, pyrenees: Shortening sequence, upper crustal mylonites and crustal strength 

Shortening of the axial zone, pyrenees: Shortening sequence, upper crustal mylonites and crustal strength

Bellahsen, N., Bayet, L., Denele, Y., Waldner, M., Airaghi, L., Rosenberg, C.,… & Vacherat, A. (2019). Shortening of the axial zone, Pyrenees: Shortening sequence, upper crustal mylonites and crustal strength. Tectonophysics, 766, 433–452.

keywords:  collision, rheology, Pyrenees, basement, greenschist

DOI: https://doi. org/10.1016/j.tecto.2019.06.002