EGU25-11959, updated on 15 Mar 2025
https://doi.org/10.5194/egusphere-egu25-11959
EGU General Assembly 2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
Strain localization at eclogite-facies conditions: interplay between fluids, metamorphism and deformation (Mt. Emilius klippe, Western Alps)
Serena Cacciari1, Giorgio Pennacchioni1, Giovanni Toffol2, Marco Scambelluri3, and Enrico Cannaò4
Serena Cacciari et al.
  • 1Università degli Studi di Padova, Department of Geosciences , Padova, Italy (serena.cacciari@studenti.unipd.it)
  • 2School of Earth and Environmental Sciences, Cardiff University, Cardiff, United Kingdom
  • 3Dipartimento di Scienze della Terra, dell'Ambiente e della Vita, Università di Genova, Genoa, Italy
  • 4Dipartimento di Scienze della Terra Ardito Desio, Università di Milano, Milan, Italy

Availability of fluids and induced metamorphic reactions are primary factors controlling the rheological behaviour of rocks. During subduction, fluids enhance the kinetics of eclogitization reactions, playing a fundamental role in promoting strain localization and shear zone nucleation. In particular, reaction-induced grain-size reduction has long been considered one of the most effective strain weakening mechanisms. To investigate the relationship between fluid-rock interaction, metamorphism and deformation, we focus on pre-Alpine ultramafites and mafic granulites of the Austroalpine Mt. Emilius klippe (Western Alps) that underwent eclogite-facies metamorphism during Alpine subduction.

The Mt. Emilius ultramafites consist of enstatite, diopside, olivine and spinel websterites deformed along a hydrated mantle shear zone that developed a fine-grained (10 µm) ultramylonitic assemblage of enstatite, diopside, olivine, anorthite, kaersutite1. During Alpine HP metamorphism, fine-grained (down to 2 µm) aggregates of jadeite, quartz, kyanite, clinozoisite (Czo) completely and statically replaced plagioclase, locally forming spatially continuous layers. Such fine-grained, hydrated aggregates did not promote any ductile eclogite-facies deformation.

The pre-Alpine mafic granulite consisted of assemblages of medium-grained garnet (Grt), diopside, plagioclase and subordinate hornblende that were replaced by Grt, omphacite (Omph), amphibole, phengite, chlorite and Czo during Alpine eclogite-facies metamorphism2. Early Alpine deformation (D1A) developed a pervasive eclogitic foliation (S1A) parallel to the granulitic layering2. This event was promoted by complete transformation and reaction-induced grain-size reduction (down to a few tens of µm) of plagioclase to Czo aggregates, together with replacement of hornblende by fine-grained chlorite-garnet-amphibole-epidote-Phe. A second eclogite-facies deformation event (D1B) is represented by localized ductile deformation closely linked to development of Czo, Omph, tremolite, Grt-filled veins and associated host-rock alteration haloes. Ductile shear is typically localized to the outer boundary of Omph-rich alteration haloes forming paired shear zones. A set of samples ranging from haloes with well-developed flanking shear zones to haloes free of shear localization was collected to investigate the role of fluid-rock interaction on shear zone nucleation and strain localization.

Preliminary data indicate that Omph-rich haloes surrounding Czo-Grt veins induced hardening in the host metagranulite (undeformed and foliated, S1A) associated with extensive replacement of the Czo aggregates (after sites of granulitic plagioclase) by Omph. However, this replacement did not always result in hardening and consequent strain localization at the outer boundary of the halo. In samples lacking shear localization, Omph accommodates deformation homogenously across the halo dominantly by diffusion creep (variable CPO, quasi-random distribution of misorientation angles, weaker SPO), with minor contribution of crystal plasticity (rare subgrains). The predominant contribution of diffusion was likely assisted by availability of fluids.

The processes driving frequent strain localization and formation of paired shear zones at the outer boundary of hardened haloes are still matter of ongoing study. Progressive advancement of the reaction front towards the host rock may form a compositional gradient across the halo, where chemical/mineralogical modifications may play a major role in determining the rheological behaviour.

[1] Benciolini, 1996. Memorie Scienze Geologiche, 48, 73-91.

[2] Pennacchioni, 1996. Journal of Structural Geology, 18, 549-561.

How to cite: Cacciari, S., Pennacchioni, G., Toffol, G., Scambelluri, M., and Cannaò, E.: Strain localization at eclogite-facies conditions: interplay between fluids, metamorphism and deformation (Mt. Emilius klippe, Western Alps), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11959, https://doi.org/10.5194/egusphere-egu25-11959, 2025.