Abstract
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 crystal plastic 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 millimetre-sized garnets in peraluminous gneisses. A later ∼550 Ma overprint under sub-eclogitic conditions (600–700 ∘C, 1.1–1.3 GPa) developed mylonitic shear zones and abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. In the mylonites, granulite facies garnet porphyroclasts are enriched in calcium along rims and fractures. However, these rims are locally narrower than otherwise comparable rims along original grain boundaries, indicating the contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal plastic deformation, which coincides with areas of enhanced diffusion, usually along zones of crystal lattice distortion and dislocation walls associated with subgrain rotation recrystallization. The 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. Highlights. Garnet is deformed by fracturing and crystal plasticity under dry lower-crustal conditions. Ca diffusion profiles indicate multiple generations of fracturing. Diffusion is promoted along zones of higher dislocation density. Fracturing indicates transient high-stress (seismic) events in the lower continental crust.
Highlights
A fundamental problem in geology is the limited preservation of processes in the rock record
In contrast to more recent studies in the Sesia Zone, which propose that precursor fracturing was crucial for dissolution–precipitation and diffusion processes in garnet (Engi et al, 2018; Giuntoli et al, 2018), they considered that there were no signs of crystal plastic deformation in their garnet samples and concluded that a diffusion-induced dislocation migration and/or diffusion-induced recrystallization process was responsible for the development of the observed subgrain texture
The microscope used for transmission electron microscopy (TEM) is a Tecnai F30 with an FEG source operated at 300 kV and equipped with a Gatan 794 MultiScan charge-coupled device (CCD) (ScopeM, ETH Zurich)
Summary
A fundamental problem in geology is the limited preservation of processes in the rock record. In contrast to more recent studies in the Sesia Zone, which propose that precursor fracturing was crucial for dissolution–precipitation and diffusion processes in garnet (Engi et al, 2018; Giuntoli et al, 2018), they considered that there were no signs of crystal plastic deformation in their garnet samples and concluded that a diffusion-induced dislocation migration and/or diffusion-induced recrystallization process was responsible for the development of the observed subgrain texture Garnets can retain their microstructure and chemical composition during retrograde deformation and metamorphism and can preserve indicators for seismic events, which are otherwise possibly erased from the rock record. 1. illustrates the close association between brittle and crystal plastic deformation of garnet under well-established pressure–temperature conditions; 2. infers deformation mechanisms from the observed microstructure; 3. explores the close link between deformation and diffusion in garnet; 4. complements other independent observations indicating transient high stresses in the lower crust
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