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

Abstract

International audienceGarnet (10 vol.%; pyrope contents 34-44 mol.%) hosted in quartzofeldspathic rocks within a large vertical shear zone of south Madagascar shows a strong grain-size reduction (from a few cm to 300 lm). Electron back-scattered diffraction, transmission electron microscopy and scanning electron microscope imaging coupled with quantitative analysis of digitized images (PolyLX software) have been used in order to understand the deformation mechanisms associated with this grain-size evolution. The garnet grain-size reduction trend has been summarized in a typological evolution (from Type I to Type IV). Type I, the original porphyroblasts, form cm-sized elongated grains that crystallized upon multiple nucleation and coalescence following biotite breakdown: biotite + sillimanite + quartz = garnet + alkali feldspar + rutile + melt. These large garnet grains contain quartz ribbons and sillimanite inclusions. Type I garnet is sheared along preferential planes (sillimanite layers, quartz ribbons and ⁄ or suitably oriented garnet crystallographic planes) producing highly elongated Type II garnet grains marked by a single crystallographic orientation. Further deformation leads to the development of a crystallographic misorientation, subgrains and new grains resulting in Type III garnet. Associated grainsize reduction occurs via subgrain rotation recrystallization accompanied by fast diffusion-assisted dislocation glide. This plastic deformation of garnet is associated with efficient recovery as shown by the very low dislocation densities (1010 m)3 or lower). The rounded Type III garnet experiences rigid body rotation in fine-grained matrix. In the highly deformed samples, the deformation mechanisms in garnet are grain-size- and shape-dependent: dislocation creep is dominant for the few large grains left (>1 mm; Type II garnet), rigid body rotation is typical for the smaller rounded grains (300 lm or less; Type III garnet) whereas diffusion creep may affect more elliptic garnet (Type IV garnet). The P-T conditions of garnet plasticity in the continental crust (‡950 C; 11 kbar) have been identified using two-feldspar thermometry and GASP conventional barometry. The garnet microstructural and deformation mechanisms evolution, coupled with grain-size decrease in a fine-grained steady-state microstructure of quartz, alkali feldspar and plagioclase, suggests a separate mechanical evolution of garnet with respect to felsic minerals within the shear zone. Key words: deformation mechanisms; garnet plasticity; perthite; shear zone

Ultrahigh‐temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long‐lived high‐T orogen

“…Martelat et al. () determined the P–T conditions of equilibrium of a garnet leucogneiss within the Zazafotsy shear zone near Ihosy to be 800–950°C/0.9–1.1 GPa using two‐feldspar thermometry (Benisek, Dachs, & Kroll, ; Benisek, Kroll, & Cemič, ; Fuhrman & Lindsley, ) and garnet–sillimanite–quartz–plagioclase barometry (TWEEQ). Both pressure and temperature may have been overestimated; recalculation of their GASP pressures using the thermocalc solution models and end‐member mineral data (for direct comparison with Boger et al., ) and their feldspar temperatures using the solution model of Benisek et al.…”

“…The Zr-in-rutile temperatures come from the Beraketa shear zone (Figure 1a), which defines the western margin of the domain; the interior of the domain might be expected to be hotter. Martelat et al (2012) determined the P-T conditions of equilibrium of a garnet leucogneiss within the Zazafotsy shear zone near Ihosy to be 800-950°C/0.9-1.1 GPa using two-feldspar thermometry (Benisek, Dachs, & Kroll, 2010;Benisek, Kroll, & Cemi c, 2004;Fuhrman & Lindsley, 1988) and garnet-sillimanite-quartz-plagioclase barometry (TWEEQ). Both pressure and temperature may have been overestimated; recalculation of their GASP pressures using the THERMOCALC solution models and end-member mineral data (for direct comparison with Boger et al, 2012) and their feldspar temperatures using the solution model of Benisek et al (2010) (correcting for K-Na diffusion using the method of Kroll, Evangelakakis and Voll (1993) for direct comparison with our results below) returns P-T conditions of 0.6-0.7 GPa and 750-850°C; these values are comparable to those suggested for the area near and north of Ihosy by Boger et al (2012) and Raith, Rakotondrazafy, and Sengupta (2008).…”

The strain‐dependent spatial evolution of garnet in a high‐P ductile shear zone from the Western Gneiss Region (Norway): a synchrotron X‐ray microtomography study

“…() and Martelat et al . (). Garnet in mylonitic eclogites from SW Norway was shown to have been deformed by grain‐boundary diffusion creep and by pressure solution (Smit et al ., ).…”

“…Garnet also readily partakes in mylonitic deformation: crystal plastic deformation of garnet at lower crustal conditions was documented by, for example, Ji & Martignole (1994, Prior et al (2002), Storey & Prior (2005), Bestmann et al (2008), Massey et al(2011) and Martelat et al (2012). Garnet in mylonitic eclogites from SW Norway was shown to have been deformed by grain-boundary diffusion creep and by pressure solution (Smit et al, 2011).…”

Localization of submicron inclusion re-equilibration at healed fractures in host garnet

“…Following TEM observations in deformed garnets (Ji and Martignole 1994;Voegelé et al 1998a;Voegelé et al 1998b), evidence for crystal plastic deformation in garnet has grown steadily (Prior et al 2002;Storey and Prior 2005;Vollbrecht et al 2006;Massey et al 2011;Martelat et al 2012), but has so far only been linked to inclusion re-equilibration in one case (Bestmann et al 2008).…”