Graphite deformation in marble and mylonitic marble, Grenville Province, Canadian Shield

Abstract

In the southern Grenville Province of the Canadian Shield (Otter Lake area), high‐grade marble, gneiss and amphibolite have been folded about north‐ to north‐east‐trending axes; mylonite zones, parallel to layering and 0.1–10 cm wide, are locally present in marble.In nonmylonitic marble, graphite occurs as c. 1–mm hexagonal prisms, which are commonly 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 surrounding calcite (e.g. <0.30 compared with 1.8–2.7 wt% MgO, and 0.02–0.12 compared with 0.13–0.18 wt% FeO); the composition of fracture‐filled dolomite is similar to that of the surrounding dolomite. In semimylonite, graphite forms elongate streaks of fragmented crystals and, in mylonite, further fragmentation has occurred to produce extremely small particles. The fragmentation has not destroyed the atomic structure (hexagonal 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 relics. Amphibole and pyroxene crystals remained undeformed but are locally replaced by calcite.The numerous microprocesses that have evidently occurred in marble and mylonitic marble of the study area are: coarsening (calcite, graphite), twinning (calcite, dolomite), slip (calcite, 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.