Constraints from diffusion profiles on the duration of high-strain deformation in thickened crust

Abstract

Diffusion profiles in garnet are used to determine the duration of movement on shear zones. In the Musgrave Block, central Australia, Grenville age granulites were overprinted at depths of ~40 km by high-strain zones during the intracratonic Petermann orogeny (ca. 550 Ma ago). In these zones, quartzofeldspathic mylonites contain garnet from the early granulite facies event that developed compositional gradients during the high-pressure overprint. The diffusion of Ca into garnet is linked with the release of Ca in plagioclase (the only calcic mineral in the rock) during neocrystallization of sodic feldspar during high-strain deformation. Available diffusion data on garnet directly define the duration of high-strain deformation at eclogite facies. Our results indicate that the integrated time scales of movement in individual high-strain zones were short lived (between 0.07 Ma and 1.4 Ma), and that wide shear zones were active longer than narrow ones. In addition, we find that relict K-feldspar in one of the narrow eclogite facies shear zones yields a 40 Ar- 39 Ar age that is partially reset, indicating that the thermal event was also short lived. These short time scales support the hypothesis that shear heating was the dominant source of heat for the eclogite facies overprint.