ON THE ROLE OF GARNET IN CONTINENTAL LOWER CRUST

Garnet-producing prograde and retrograde reactions provide first-order controls on the composition, rheology, density, and overall tectonic behavior of continental lower crust. Three processes are fundamental to the production of garnet in this environment: (1) fluid-absent partial melting of biotite-bearing paragneisses and orthogneisses yielding peritectic garnet + melt, (2) crystallization of mafic to intermediate igneous rocks in the low-pressure granulite field (i.e., Opx + Pl assemblages) followed by metamorphism during cooling into the high-pressure granulite field (i.e., Cpx + Grt + Pl assemblages), and (3) emplacement, differentiation, and (re-)melting of a mafic underplate leading to production of melt and a Grt + Cpx residue. This work focuses on the role of the first two processes in the petrological and rheological evolution of a large exposure of exhumed continental lower crust in the western Canadian Shield. In each example, the role of deformation during garnet growth is evident during syn-kinematic metamorphic reactions that accompanied ductile, melt-enhanced lower crustal flow and concomitant isobaric cooling. Monazite is associated with garnet-rich felsic granulites in each case and serves as an important monitor of the occurrence of melting and garnetiferous restite production in the deep crust.