Granulite microfabrics and deformation mechanisms in southern Madagascar

Optical microstructures and crystallographic preferred orientations were st udied in naturally deformed granulite- to high-amphibolite facies quartzo-f eldspathic rocks in southern Madagascar. The microstructures of coarse-grai ned granulite suggest that feldspar and quartz accommodated deformation by both dislocation and diffusion creep in the absence of melt. The extreme du ctility of feldspar in dynamically recrystallized granulite is explained by activity of dislocation creep, in conjunction with stress-controlled intra crystalline diffusion. In the studied rocks, the considerable volume of quartz is not interconnect ed even at high strain. The lack of its interconnectivity in coarse-grained granulites and in amphibolite facies granoblastic platy quartz rocks is ex plained by an extreme stability of the load-bearing framework structure at high-temperatures. In dynamically recrystallized granulite, the feldspar vi scosity decreases so that quartz becomes enveloped by a weak feldspar matri x which inhibits its coalescence and interconnectivity. We predict an important decrease in strength of quartzo-feldspathic granuli tes due to activity of diffusional creep and convergence of viscosity of re crystallized feldspar and quartz. (C) 1999 Elsevier Science Ltd. All rights reserved.