U-Pb and 40Ar/39Ar Constraints on the Tectonothermal History of the Levack Gneiss Complex, Southern Superior Province, Canada: Implications for the Exhumation of
Deep-Crustal Rocks

N. Wodicka Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A 0E8, Canada


K. D. Card Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A 0E8, Canada

The Levack gneiss complex, which occurs only in the immediate footwall along the northern edge of the Sudbury Igneous Complex (SIC), is unique to the Archaean geology of the Sudbury area, southern Superior Province. The complex consists of intrusive and supracrustal rocks that were deformed and metamorphosed in the mid- to lower crust during the Neoarchaean. One of the most intriguing problems with the geology of the area is the timing of exhumation of these high-grade gneisses. Some researchers believe that the exhumation was a Neoarchaean or Palaeoproterozoic tectonic phenomenon, whereas others argue that unroofing of the gneisses resulted from a meteorite impact. In an effort to constrain the tectonothermal history of the Levack gneiss complex, U-Pb and 40Ar/39Ar analyses were performed on various accessory minerals from the Levack gneisses.

U-Pb zircon ages suggest a major period of granodiorite magmatism in the Levack gneiss complex at about 2670 Ma. This was followed by an early phase of migmatization and emplacement of a monzodiorite pluton between about 2665 and 2655 Ma. Major amphibolite to granulite facies metamorphism, regional deformation, and migmatization culminated at about 2648 Ma. Monazite, titanite, and hornblende analyses suggest a late episode of metasomatism at about 2580 Ma, coeval with a regional phase of hydrothermal activity in the neighbouring Abitibi belt and fault reactivation in the Kapuskasing zone. U-Pb allanite and titanite, and 40Ar/39Ar hornblende cooling ages of ca. 2640 to 2625 Ma suggest that the Levack gneisses were exhumed, at least in part, during the Neoarchaean. Owing to the lack of a strong structural control, the mechanism responsible for this early stage of exhumation remains unclear. However, the temporal relationship between exhumation of the Levack gneisses and emplacement of a late-stage granitic batholith may suggest that exhumation was controlled by late-orogenic extension. The 1850 Ma Sudbury impact event caused partial Pb loss from Neoarchaean monazites and induced new growth of titanites and allanites in some gneisses. However, the present data are insufficient to determine whether the impact event was responsible for the final stages of exhumation of the Levack gneisses, but the spatial relationship between the Levack gneiss complex and the Sudbury structure is consistent with this hypothesis.

The Neoarchaean evolution of the Levack gneiss complex strongly resembles that of the deepest structural level rocks from the Kapuskasing structural zone, where voluminous granitoid emplacement occurred between 2690 and 2660 Ma, granulite facies metamorphism took place at about 2660 and 2645 Ma, and major ductile deformation around 2630 Ma.