Origin and mechanical significance of honeycomb garnet in high‐pressure metasedimentary rocks from the Tauern Window, Eastern Alps

Abstract

High-pressure schists (2-2.5 GPa) from the Eclogite Zone in the Tauern Window contain honeycomb garnet in which fine webs of garnet surround strain-free quartz ± carbonate grains. High-resolution X-ray computed tomography shows that the garnet webs form a cellular structure that coats all surfaces of the inclusions. Electron backscatter diffraction analysis shows that the garnet cells are crystallographically continuous with more massive garnet regions, and that the quartz ± carbonate inclusions have random orientations; in contrast, matrix quartz exhibits a prominent crystallographic preferred orientation (CPO). High-resolution transmission electron microscopy shows few dislocations in either the garnet or the inclusion quartz. Most honeycomb garnet is chemically homogeneous, but some displays asymmetric core-rim zoning. Taken together, these observations are most consistent with formation of the garnet sheets via precipitation from a wetting fluid along quartz-quartz grain boundaries, or possibly via wholesale precipitation of garnet + quartz ± carbonate from a fluid. In either case, a silicate-rich aqueous fluid must have been present. The likelihood that a fully wetting fluid existed at high pressure has important implications for rheology during subduction of metasedimentary rocks: strain may be accommodated by grain rotation and sliding in an aqueous silicate slurry, rather than via dislocation creep mechanisms at high pressures. The absence of a CPO in early quartz may thus point to involvement of a pervasive grain-boundary fluid rather than requiring low differential stresses during subduction.

Coeval high‐pressure metamorphism, thrusting, strike‐slip, and extensional shearing in the Tauern Window, Eastern Alps

“…In the Dorfertal, mylonite with S‐plunging downdip stretching lineations is only a few meters away from mylonite with gently E‐plunging stretching lineations that are associated with sinistral S‐side‐down kinematic indicators. Occasionally, gently W‐plunging lineations associated with dextral S‐side‐down kinematics have been reported by Hawkins et al [2007]. No consistent overprinting relationships between downdip and the lateral kinematic indicators occur and our dating (see below) indeed shows that both sets of kinematic indicators formed essentially at the same time.…”

Garnet crystal plasticity in the continental crust, new example from south Madagascar

“…In these dry rocks, fluid dissolution mechanisms (Hawkins et al. , 2007) are not expected to have affected garnet and solely melt‐assisted dissolution and reprecipitation can be envisaged (Álvarez‐Valero et al.…”

Simultaneous operation of opposing reaction mechanisms: The influence of matrix heterogeneity on post‐kinematic garnet crystallization in an inverted metamorphic sequence

“…The irregular microstructures in SIM samples might not be out of place in high-P and/or fluid-deficient rocks (e.g. Hawkins et al, 2007;Spry, 1969;Stöckhert et al, 1997), but are generally not common in low-medium grade metapelites. They are consistent with strongly heterogeneous garnet crystallization, such that combined volume, grain boundary and interphase diffusion was not efficient enough to allow for textural or chemical equilibration across matrix layers, and even along the surfaces of some garnet grains.…”

“…Miyazaki, 2015). Garnet porphyroblasts commonly exhibit dodecahedral-trapezohedral crystal habits, but a wide variety of morphologies which deviate from crystallographically controlled morphologies such as honeycomb, tabular, atoll and spiral shaped have been documented (Biermeier & Stüwe, 2003;Giuntoli, Lanari, & Engi, 2018;Hawkins et al, 2007;Homam, 2003;Robyr et al, 2014;Smellie, 1974;Spry, 1969;Stöckhert et al, 1997), particularly in high pressure or anhydrous lithologies. Polycrystalline textures have also been observed (Whitney, Goergen, Ketcham, & Kunze, 2008;Whitney & Seaton, 2010).…”