A prominent metamorphic complex composed mainly of K-feldspar gneiss,
with many quartz-feldspar layers and veins, occurs in the Otter Lake t
errane of the southern Grenville Province of the Canadian Shield. Inte
nse deformation of the complex is indicated by folded and disjointed a
mphibolite dikes; many veins are also folded. The K-feldspar gneiss co
nsists of quartz, plagioclase, K-feldspar, biotite, local garnet, and
rare sillimanite. With few exceptions, the mineral assemblage of each
vein is the same as that of the enclosing gneiss, including garnet and
sillimanite, where present, and minor minerals, magnetite, zircon, ap
atite, and allanite. The volume fraction K-feldspar/(K-feldspar + plag
ioclase) ranges widely (from 0.05 to 0.90), but compared with adjacent
gneiss, K-feldspar is always higher and biotite is lower. The chemica
l composition of veins is similar to that of enclosing gneiss, but K a
nd Ba are higher and Mg, Fe, Mn, Ti, Zr, Rb, and P are lower. The bulk
composition of veined gneiss where veins are numerous and that of adj
oining gneiss where veins are scarce are virtually identical. Vein-gne
iss differences in plagioclase and garnet composition are small or imp
erceptible. These results lead to the conclusion that many of the vein
s were locally derived. The rearrangement of atoms needed to produce a
vein is considered in terms of Orville-Fisher exchange reactions, e.g
., the transport of K, Al, and Si from gneiss to sites of vein growth,
in exchange for Mg, Fe, and H. The Ramberg - Robin model of metamorph
ic differentiation, in which transport occurs by crystal-boundary diff
usion, driven by pressure gradients, is proposed as the principal proc
ess of vein formation.