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Solid Earth An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 14 Jun 2019

Research article | 14 Jun 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).

Fracturing and crystal plastic behavior of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)

Friedrich Hawemann1, Neil Mancktelow1, Sebastian Wex1, Giorgio Pennacchioni2, and Alfredo Camacho3 Friedrich Hawemann et al.
  • 1Department of Earth Sciences, ETH Zurich, CH8092 Zurich, Switzerland
  • 2Department of Geosciences, University of Padova, Padova, Italy
  • 3Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada

Abstract. Garnet is a high strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade deformation, except under relatively high temperature conditions. The microstructure of granulite facies garnet in felsic lower-crustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal-plastic deformation. Granulite facies metamorphism at ~ 1200 Ma generally dehydrated the rocks and produced mm-sized garnets in peraluminous gneisses. A later ~ 550 Ma overprint under sub-eclogitic conditions (600–700 °C, 1.1–1.3 GPa) developed shear zones and with abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. The granulitic fractured garnet porphyroclasts in mylonites show high calcium content along rims and fractures. However, in certain cases, these rims are narrower than equivalent rims along original grain boundaries, indicating contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal-plastic deformation, that coincide with areas of enhanced diffusion, usually along zones of crystal lattice distortions and dislocation walls and by subgrain rotation recrystallization. Fracturing of garnet under dry lower crustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte.

Friedrich Hawemann et al.
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Friedrich Hawemann et al.
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