PETROLOGY OF VEINED GNEISSES OF THE OTTER COMPLEX, SOUTHERN GRENVILLEPROVINCE

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.