Development of fluid pathways during granulite to amphibolite facies metamorphism in Krossøy, the Bergen Arcs

Lorena H. Filiberto¹*, Håkon Austrheim², Andrew Putnis^1,3

¹Institut für Mineralogie, University of Münster, 48149 Münster, Germany

²The Njord Centre, University of Oslo, 0136 Oslo, Norway

³School of Earth and Planetary Sciences, Curtin University, Perth, 6845, Australia

Fluid pathways play a crucial role in the development of shear zones within the crust of the Earth. The Bergen Arcs have been subject to various hypotheses regarding the localisation of rheologically weaker zones that could lead to shear zone formation (Jamtveit et al. 2019; Incel et al. 2022). These weaker areas might be associated with initial seismic events that produced brittle fracturing, resulting in the development of faults and fractures that may serve as potential fluid pathways to allow mineral re-equilibration and ductile deformation.

This study focuses on Krossøy, an island situated in the northernmost part of the Bergen Arcs, Western Norway where patterns of deformation, textural evolution and metamorphism might differ from other more extensively studied areas in the southern part of the region such as the island of Holsnøy and Radøy (Austrheim 1987; Mukai et al. 2014). Krossøy exposes anorthosites, from the old granulitic basement, that are intruded by a series of subparallel mafic granulitic dykes forming a “dyke swarm”. These have not been previously described elsewhere in the Bergen Arcs. We present our first results on microstructural analysis through Electron Backscattered Diffraction (EBSD) and, mineral and chemical evolution using Electron Microprobe analysis (EMPA). 

As anorthosites are plagioclase-rich rocks, our results show the chemical and textural evolution of feldspars from an early stage granulitic anorthosite formed 930 Ma ago to a Caledonian mylonite (440-420 Ma) formed during shear zone development and under amphibolite facies conditions. By examining the fluid pathways, we seek to determine their relationship with the formation of rheologically weaker areas in the crust and how the dynamic interplay between fluid infiltration and deformation mechanisms may play an important role by changing the metamorphic conditions, textures and mineral assemblages in the rocks.

References:

Austrheim, H. (1987). Earth and Planetary Science Letters, 81(2–3), 221-232.

Incel, S., Labrousse, L., Hilairet, N. et al. (2022). Reaction-induced embrittlement of the lower continental crust. Geology, 47(3).

Jamtveit, B., Petley‐Ragan, A. et al. (2019). Journal of Geophysical Research: Solid Earth, 124(8), 7725-7755.

Mukai, H., Austrheim, H., Putnis, C. V., and Putnis, A. (2014). Journal of Petrology, 55(8), 1457-1477.

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