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TitleCrystallization and emplacement of the Lac St-Jean Anorthosite Massif (Quebec, Canada)
AuthorWoussen, G; Dimroth, E; Corriveau, L; Archer, P
SourceContributions To Mineralogy and Petrology vol. 76, 1981 p. 343-350,
Alt SeriesEarth Sciences Sector, Contribution Series 20100140
NTS22D/06; 22D/07; 22D/10; 22D/11
AreaSaguenay; Chicoutimi
Lat/Long WENS-71.5000 -70.5000 47.7500 47.2500
Subjectsigneous and metamorphic petrology; tectonics; metamorphic rocks; igneous rocks; anorthosites; leuconorites; migmatites; gneisses; bedrock geology; basement geology; geological evolution; tectonic evolution; Lac St-Jean anorthosite massif; Gneiss Complex
Illustrationslocation maps; schematic representations
AbstractThe Lac St-Jean anorthosite massif underlies an area of over 20,000 km 2 and has been emplaced into migmatitic gneisses of the central granulite terrain of the Grenville Province of the Canadian shield. Field data and petrography in an area straddling the anorthosite-gneiss contact, close to Chicoutimi (Quebec) permits an outline of its tecto-magmatic evolution. Depositional magmatic textures in the massif reveals that it crystallized from a magma in a relatively calm tectonic environment. The absence of fusion in pelitic gneisses at the contact proves that the crystallization did not take place at the level presently exposed. The parallelism of subvertical foliation in the enveloping gneisses and the anorthosite indicates that both were deformed together. It is suggested that the deformation results from a diapiric ascent of the anorthosite massif after its consolidation at depth. The depth of consolidation of the anorthosite is estimated at ~25 30 km from subsolidus reaction between plagioclase and olivine. The diapiric ascent is further substantiated by the fact that three sets of mafic dykes of different ages, intrusive into the anorthosite, have a mineralogy which indicates successively decreasing P, T conditions of emplacement from granulite facies to amphibolite facies. An evolution of the basement gneisses and the anorthosite is proposed as a working hypothesis; it relies on the fact that metabasite dyke swarms in the basement gneisses represent a period of major crustal
extension and could be used as a stratigraphic subdivision of the Grenville Province.