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