Formation, Crystallization, and Migration of Melt in the Mid-orogenic Crust: Muskoka Domain Migmatites, Grenville Province, Ontario

Abstract

Migmatitic orthogneisses in the Muskoka domain, southwestern Grenville Province, Ontario, formed during the Ottawan stage (c. 1080–1050Ma) of the Grenvillian orogeny. Stromatic migmatites are volumetrically dominant, comprising granodioritic gneisses with 2–5cm thick granitic leucosomes, locally rimmed by thin melanosomes, that constitute 20–30 vol. %, and locally 40–50 vol. %, of the outcrops. Patch migmatites in dioritic gneisses form large (>10m) pinch-and-swell structures within the stromatic migmatites, and consist of decimetre-scale, irregular patches of granitic leucosome, surrounded by medium-grained hornblende–plagioclase melanosomes interpreted as restite. The patches connect to larger networks of zoned pegmatite dykes. Petrographic and geochemical evidence suggests that the patch leucosomes formed by 20–40% fluid-present, equilibrium melting of the dioritic gneiss, followed by feldspar-dominated crystallization. The dyke networks may have resulted from hydraulic fracturing, probably when the melts reached water saturation during crystallization. Field and geochemical data from the stromatic migmatites suggest a similar petrogenesis to the patch migmatites, but with significant additions of externally derived melts, indicating that they acted as conduits for melts derived from deeper structural levels within the orogen. We hypothesize that the Muskoka domain represents a transfer zone for melts migrating to higher structural levels during Grenvillian deformation.