MELT-RELATED WEAKENING AND PROGRESSIVE DEFORMATION OF LOWER CRUSTAL GRANITOIDS: ANATEXIS OF THE FEHR GRANITE, ATHABASCA GRANULITE TERRANE, NORTHERN SASKATCHEWAN

The ca. 2.6 Ga Fehr granite underwent extensive partial melting at ca. 1.9 Ga. Regional heating associated with emplacement of the Chipman mafic dike swarm was synchronous with ductile shearing and anatexis of the fertile Fehr granite. At least two partial melting reactions have been identified in the Fehr granite: 1) biotite dehydration melting and 2) eutectic style melting, but associated with elevated temperatures and the influx of fluids. Early in the melting process, leucosome (incipient Fehr granite partial melt) was preferentially localized along a subhorizontal S₁ cleavage. Shortening of melt-rich and melt-poor domains led to buckling and the development of a tens-of-centimeter-scale ‘megacrenulation’ fabric with leucosome accumulation along the upright S₂ cleavage. The development of the ‘megacrenulation’ fabric within the Fehr granite migmatite also appears to have acted as a pump to mobilize felsic partial melt away from generation sites in the form of S₂-parallel rhyolitic dikes. Partial melting, melt accumulation, and subsequent melt mobilization in the Fehr granite also controlled the form and geometry of later Chipman mafic dikes. Mafic dikes that intruded non-migmatitic Fehr granite are straight and parallel-sided, but mafic dikes that intruded the migmatitic Fehr granite are anastomosing, with irregular margins and terminations that commonly disaggregate into mafic pillows. Interaction of mafic dikes and the Fehr granite led to a positive feedback loop whereby mafic dikes promote heating and melting of the Fehr granite, which in turn leads to entrapment of later mafic dikes, further heating and additional partial melting of the Fehr granite, and ultimately, weakening of the lower crust. Field evidence, microstructures and microgeochemical features suggest that the degree of partial melting and relative degree of melt expulsion provided a major control on crustal strength during anatexis. These features can be used to track the record of melt-enhanced deformation linked with high-temperature metamorphism.The Fehr granite provides a record of the kinds of interactions between metamorphism,deformation, heating, melting, magma interaction and crustal strength that may characterize the present-day deep crust.