Granulite microfabrics and deformation mechanisms in southern Madagascar

Optical microstructures and crystallographic preferred orientations were studied in naturally deformed granulite- to high-amphibolite facies quartzo-feldspathic rocks in southern Madagascar. The microstructures of coarse-grained granulite suggest that feldspar and quartz accommodated deformation by both dislocation and diffusion creep in the absence of melt. The extreme ductility of feldspar in dynamically recrystallized granulite is explained by activity of dislocation creep, in conjunction with stress-controlled intracrystalline diffusion. In the studied rocks, the considerable volume of quartz is not interconnected even at high strain. The lack of its interconnectivity in coarse-grained granulites and in amphibolite facies granoblastic platy quartz rocks is explained by an extreme stability of the load-bearing framework structure at high-temperatures. In dynamically recrystallized granulite, the feldspar viscosity decreases so that quartz becomes enveloped by a weak feldspar matrix which inhibits its coalescence and interconnectivity. We predict an important decrease in strength of quartzo-feldspathic granulites due to activity of diffusional creep and convergence of viscosity of recrystallized feldspar and quartz. (C) 1999 Elsevier Science Ltd. All rights reserved.