SM-ND, U-PB, AND RB-SR GEOCHRONOLOGY AND LITHOSTRUCTURAL RELATIONSHIPS IN THE SOUTHWESTERN RAE PROVINCE - CONSTRAINTS ON CRUSTAL ASSEMBLY IN THE WESTERN CANADIAN SHIELD

The Rae province is an area of Archean crust bounded by the Early Proterozoic Thelon orogen to the northwest and mylonites of the Snowbird Tectonic Zone to the southeast. The southern part of the province in Saskatchewan is a high-grade complex of ultramafic, mafic and felsic orthogneisses with interlayered supracrustal units. The oldest rocks in the area were derived from layered mafic plutonic complexes and supracrustal units. The earliest deformation (D1) resulted in development of a layer-parallel foliation preserved in small lensoid pods of anorthosite, gabbro, and pyroxenite. A pyroxenite yields a Sm-Nd T(DM) age of approximately 3.5 Ga and is isotopically similar to a mafic granulite (metabasalt?) associated with early supracrustal units. These rocks were intruded by a sequence of mafic and tonalitic orthogneisses that yield a Sm-Nd isochron age of 3.32 +/- 2.87 and T(DM) ages of approximately 3.2-3.1 Ga. The mafic-tonalitic gneisses were subsequently migmatized producing anatectic net veins of tonalitic to granitic pegmatite. These were deformed during D2 deformation. The terrane was then intruded by minor granodioritic to tonalitic plutonic sheets and a swarm of mafic dikes that were deformed by the last major period of inhomogeneous deformation (D3) which ended at approximately 2.0 Ga. The Clearwater Complex, a large 24 x 8 km long anorthosite massif, intruded the gneiss complex following D3. and yields a Sm-Nd mineral-whole-rock isochron age of 1.85 Ga. Sm-Nd, Rb-Sr, and U-Pb SHRIMP data record multiple thermotectonic events and provide constraints on crustal evolution of the Rae province. U-Pb SHRIMP data for metamorphic zircons separated from an early anorthosite associated with units that give pre-3.2 Ga Nd model ages show considerable variation in Th/U ratio. Zircons yield virtually concordant data that record Archean (2.9 Ga) and several Early Proterozoic (between 2.3 and 1.85) thermotectonic events. Sm-Nd data are generally poorly correlated possibly as a result either of slight remobilization of REE's during granulite facies metamorphism or that rocks with distinct origins were compared. Rb-Sr whole-rock and mineral data show substantial re-equilibration in mafic-tonalitic gneisses at approximately 2.8 Ga. and in supracrustal units at approximately 2.3 Ga. Also, a discordant granitic dike and biotite separated from a metapelite yield Rb-Sr T(DM) ages of approximately 1.9 Ga. These metamorphic ages are interpreted to reflect thermotectonism associated with formation and assembly of juvenile Early Proterozoic crust in the Talston-Thelon plutonic zone to the west, and with terminal collision in the Trans-Hudson orogen to the east.