Conditions and timing of metamorphism in the southern Abitibi greenstone belt, Quebec

Powell, W.G., Carmichael, D.M., and Hodgson, C.J.

1995, Canadian Journal of Earth Sciences, v. 32, pp. 787-805

Regional metamorphism, ranging in grade from the subgreenschist-facies to the greenschist-amphibolite transition, affects all Archean supracrustal rocks (>2677 Ma) in the Rouyn-Noranda area. Contact metamorphic minerals associated with the post-tectonic Preissac-Lacorne batholith (2643 Ma) show no evidence of a regional retrograde event. Accordingly, the age of metamorphism can be bracketed between 2677 and 2643 Ma. Three reaction isograds were mapped in subgreenschist-facies metabasites, dividing low-grade rocks into three metamorphic zones: the pumpellyite-actinolite zone, the prehnite-pumpellyite zone, and the prehnite-epidote zone. In addition, the pumpellyite-actinolite-epidote-quartz bathograd, corresponding to a pressure of approximately 200 MP, occurs on both sides of the Destor-Porcupine fault. Low-pressure regional metamorphism is also indicated both by the occurrence of an actinolite-oligoclase zone, and the persistence of pre-regional metamorphic andalusite. The coincidence of andalusite and the actinolite-oligoclase zone indicates that pressure was <330 MPa at the greenschist-amphibolite transition. The geothermal gradient during metamorphism was approximately 30C/km. Regionally, isograds dip shallowly to the north and trend subparallel to lithological and structural trends. Metamorphic minerals in metabasites define tectonic fabrics only near major fault zones, and near zones of CO2 metasomatism. In biotite zone metasedimentary rocks the schistosity is defined my mica and amphibole.These textures indicate that metamorphism and fabric development were coeval. However, the actinolite-epidote isograd cuts the Porcupine-Destor fault, indicating that regional metamorphism postdates movement along this fault. The strong fabrics associated with the Porcupine-Destor and Larder Lake-Cadillac faults must have developed through a process dominated by flattening strain.