FluidNET Science Conference Abstract - Exhumation history of the Agly Massif: from suprasolidus restite-melt back-reactions to fluid-induced retrogression
Creators: name: Sander Hoogendoorn
name: Clare J. Warren
name: Leo Kriegsman
name: A. M. Halton
name: Jan R. Wijbrans
Publisher: name: Zenodo
Year: 2024

description: Exhumation history of the Agly Massif: from suprasolidus restite-melt back-reactions to fluid-induced retrogression. 

 

Hoogendoorn, S.B.*,1, Warren, C.J., L. M. Kriegsman2, Halton, A.M.1 and Wijbrans J.R.3

1School of Environment, Earth and Ecosystem Sciences, The Open University, Milton Keynes MK7 6AA, UK, sander.hoogendoorn@open.ac.uk 

2 Department of Research & Education, Naturalis Biodiversity Center, Darwinweg 2, 2333 CR Leiden, the Netherlands

3Department of Petrology, VU University Amsterdam, Amsterdam, 1081 HV

*sander.hoogendoorn@open.ac.uk

 

Numerous migmatite terranes exhibit clockwise pressure-temperature-time (PTt) histories, marked by decompression and cooling following the thermal peak of metamorphism. The retrogression path generates microtextures and associated geochemical signatures that serve as indicators of processes such as fluid infiltration, deformation, melting, and mineral alteration. Accurately interpreting when these microtextures and geochemical signatures formed during the metamorphic evolution can be challenging and require petrographic observations to be integrated with high-resolution geochronology, conventional thermobarometry, and thermodynamic modelling. The Agly Massif in southern France is an exemplary high-grade Variscan (~330-300 Ma) basement terrane that experienced a complex and multi-stage exhumation history, as evidenced by the presence of numerous high-grade (early) and low-grade (later) shear zones that dissect its core. A long-standing debate has revolved around constraining the metamorphic conditions, timing, and geological context of episodes of exhumation, and disentangling their effect on the complexly preserved chemical and textural record.  

 

We have explored the sequence and impact of retrogression on mineral equilibria across various time- and length scales for three migmatitic and one granulitic metapelite sample from the Agly Massif using single grain fusion and in-situ 40Ar/39Ar dating, conventional thermobarometry, thermodynamic modelling, and extensive petrographical investigations. Our findings reveal a multifaceted metamorphic reaction sequence that included biotite dehydration melting, restite-melt back reactions during decompression and early cooling under suprasolidus conditions, and later subsolidus retrograde rehydration. Of particular significance is the decrease in equilibration volume associated with lower temperature reaction; thin-section scale equilibrium was largely maintained during suprasolidus restite-melt-back reactions, whereas subsolidus muscovitization and chloritization only locally overprinted the higher temperature mineralogical and geochemical records. Textures and geochemistry related to subsolidus retrogression required a localized influx of marine or basinal fluids, with the 40Ar/39Ar data revealing that this occurred around ~115-95 Ma. 

 

These findings provide clear insights into the episodic exhumation of the Agly Massif. Specifically, the combination of thin-section scale phase maps and thermodynamic modelling demonstrates that the gneissic basement was only partially exhumed during the late Variscan orogeny (~300-290 Ma), resulting in a near-complete overprint of textures and geochemical signatures related to peak-temperature metamorphism. Further exhumation must have occurred during regional rifting in the Cretaceous (~115-95 Ma) and resulted in the reactivation of high-grade shear zones and fluid infiltration.  These findings carry significant implications for similar polymetamorphic terranes since a careful evaluation of the petrography, geochemistry and geochronology can be used to disentangle complex exhumation histories. 

 

 

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 956127