R. Kretz, GRAPHITE DEFORMATION IN MARBLE AND MYLONITIC MARBLE, GRENVILLE PROVINCE, CANADIAN SHIELD, Journal of metamorphic geology, 14(4), 1996, pp. 399-412
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.