ARCHEAN HIGH PRESSURE-HIGH TEMPERATURE METAMORPHISM: 700 MA OF LOWER CRUSTAL EVOLUTION

The East Athabasca mylonite triangle, northern Saskatchewan, which is part of the 3000-km-long Snowbird Tectonic Zone of the western Canadian Shield, contains a diverse assemblage of Archean high-pressure rocks. The southern, structurally highest domain, the "Upper Deck" consists of mylonitic garnet-kyanite quartzo-feldspathic gneisses with several meter-thick layers of garnet clinopyroxenite, sapphirine granulite, and mafic garnet-clinopyroxene granulite. Thermobarometric data indicates this entire 400 square km package of rocks underwent hi-P metamorphism at >15-20kb and 1000°C, making this one of the most extensive and best exposures of Archean high-P rocks any where in the world. Mafic granulite layers contained within the felsic gneisses in the Upper Deck record near-isothermal decompression to 10-12 kb pressures. U-Pb data on zircon and monazite from the felsic gneisses and garnet clinopyoxenites indicates that the high-P metamorphism is ca 2.6 Ga. Decompression to 10-12 kb occurred just after peak conditions which is documented by similar pressures across the Upper Deck-Lower Deck boundary. The boundary is an intrusive contact between the Upper Deck gneisses and the 2.6 Ga opx-bearing Godfrey granitic gneiss which records peak PT conditions of 12 kb and 850-950°C. U-Pb data from rutile indicate that this entire package of rocks was exhumed by ca 1.9 Ga. We propose long term storage of these rocks in the lower crust (10-12 kb) from 2.6 to 1.9 Ga followed by rapid exhumation during a major period of regional deformation. Intriguing aspects of the metamorphic history are firstly that it is Archean in age, making it one of the oldest examples of hi-P metamorphism, and secondly, the Snowbird Zone does not appear to coincide with an Archean plate boundary, a common setting for metamorphosing rocks at such extreme conditions. Thus the long-lived history and preservation of hi-P, hi-T metamorphic assemblages in the lower crust provide important constraints on the development of cratonic geotherms in this area.