GENERATION AND CONTAMINATION OF GRANITIC MAGMA IN THE DEEP CRUST, ATHABASCA GRANULITE TERRANE, SASKATCHEWAN

The 2.6 Ga Fehr granite occurs within the Athabasca granulite terrane, a large and compositionally heterogeneous section of exhumed continental lower crust. The 1.9 Ga Chipman mafic dike swarm intruded the Fehr granite during regional ductile shearing. Heating, related to the Chipman swarm, led to extensive partial melting (20 - >60 volume % leucocratic partial melt) in the Fehr granite and tonalitic partial melting from migmitization of older Chipman dikes. At least two melting reactions are inferred in the Fehr granite: 1) incongruent Bt-dehydration melting, Bt + Kfs + Pl + Qtz + H₂O to Grt + melt and 2) batch melting associated with influx of fluids. Tonalitic partial melts in Chipman dikes were produced through Hbl-dehydration melting to Grt + melt. Fehr granite partial melt has a hypersolvus character, containing perthitic feldspar with globular Ab symplectites. Xenocrystic plagioclase (An₁₈-An₂₂) preserves anti-rapikivi textures. At least three magmas chemically and mechanically interacted: 1) granitic melt of Fehr granite (74-79 wt% SiO₂), 2) tonalitic melt of Chipman dikes (60-68 wt% SiO₂), and 3) basaltic Chipman magma (58 wt% SiO₂). Mechanical and chemical interactions (i.e. felsic schlieren and resorbed Kfs phenocrysts in mafic dikes; mafic pillows and fingers in leucocratic melt) created a range of hybrids with new igneous phases. Fehr granite partial melt was localized along the subhorizontal S₁ cleavage. Shortening of melt-rich/melt-poor domains on S₁ led to a megacrenulation fabric with leucocratic partial melt accumulation along the upright S₂ cleavage. Shortening also induced felsic partial melt extraction and formation of rhyolitic dikes. Partial melt concentration in the Fehr granite provided a first-order control on the form and geometry of later Chipman dikes. Mafic dikes are straight and parallel outside of Fehr granite, but are anastomosing with irregular terminations and mafic pillows within granitic migmatite. This evidence suggests that: 1) mantle-related mafic magma can be contaminated by leucocratic partial melt within the deepest continental crust, 2) fertile granitoids in the deep crust can affect the mechanics of dike passage and emplacement, and 3) coalescence and expulsion of granitic partial melt both control and are controlled by structures within the felsic migmatite.