GRAPHITE DEFORMATION IN MARBLE AND MYLONITIC MARBLE, GRENVILLE PROVINCE, CANADIAN SHIELD

In the southern Grenville Province of the Canadian Shield (Otter Lake area), high-grade marble, gneiss and amphibolite have been folded abou t north- to north-cast-trending axes; mylonite zones, parallel to laye ring and 0.1-10 cm wide, are locally present in marble. In nonmyloniti c marble, graphite occurs as c. 1-mm hexagonal pl isms, which are comm only accompanied by a relatively few crystals that have been deformed, resulting in cleavage separation and the formation of folds and kink bands. Fracture-filled calcite contains less Mg and Fe than surroundin g calcite (e.g. < 0.30 compared with 1.8-2.7 wt% MgO, and 0.02-0.12 co mpared with 0.13-0.18 wt% FeO); the composition of fracture-filled dol omite is similar to that of the surrounding dolomite. III semimylonite , graphite forms elongate streaks of fragmented crystals and, in mylon ite, further fragmentation has occurred to produce extremely small par ticles. The fragmentation has not destroyed the atomic structure (hexa gonal modification) of graphite. The behaviour of biotite was similar to that of graphite, but extreme fragmentation did not occur. Dolomite was more rigid than calcite, and in mylonite it occurs more commonly as relies. Amphibole and pyroxene crystals remained undeformed but are locally replaced by calcite. The numerous microprocesses that have ev idently occurred in marble and mylonitic marble of the study area are. coarsening (calcite, graphite), twinning (calcite, dolomite), slip (c alcite, dolomite, graphite, biotite), strain-induced recrystallization (calcite), microfolding and kink-band formation (graphite, biotite), fragmentation (graphite) and the pressure-induced transport of calcite and dolomite to voids in graphite and biotite.