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TitleEvidence for regional-scale, pluton-driven, high-grade metamorphism in the Archaean Minto Block, northern Superior Province, Canada.
AuthorBédard, J
SourceJournal of Geology 111, 2, 2003 p. 183-205, Open Access logo Open Access
Alt SeriesGeological Survey of Canada, Contribution Series 2001154
PublisherUniversity of Chicago Press
Mediapaper; on-line; digital
File formatpdf
NTS24C; 24D; 24E; 24F; 24K; 24L; 24M; 24N; 25D; 25E; 34; 35A; 35B; 35C; 35F; 35G; 35H
Areanorthern Quebec
Lat/Long WENS -79.0000 -68.0000 62.0000 56.0000
Subjectsigneous and metamorphic petrology; geochemistry; Archean; igneous rocks; tonalites; trondhjemites; granites; granodiorites; structural features; shear zones; deformation; petrography; dykes; metamorphism; Minto Block; Superior Province; enderbites; Precambrian
Illustrationssketch maps; tables; photomicrographs; graphs
AbstractThe Archaean Minto Block of the Superior Province is dominated by foliated tonalite, trondhjemite, granite, granodiorite, enderbite, and clinopyroxene- or orthopyroxene-bearing hornblende-tonalite; most partly retain igneous microtextures. Outcrop-scale structures such as melt-filled shear zones and deformed intraplutonic dikes imply that deformation was synmagmatic. Compositions of epidote (pistachite 25-30), garnet, and muscovite (FeO* 4.5-6 wt%, TiO2 0.5-1.7 wt%) are typically igneous. Epidote commonly has allanite cores, some with igneous growth zoning. Epidote, muscovite, and allanite are embayed where they touch the quartzofeldspathic matrix. These embayments are interpreted as magmatic resorption structures. Biotite and muscovite armor epidote and allanite against resorption. Since epidote and muscovite stabilities are pressure dependent, these resorption structures suggest that magmas ascended as crystal-charged mushes. The Al-in-hornblende geobarometer yields regionally uniform pressures (4-6.4 kbar), with tonalite from the eastern Minto Block yielding pressures about 1 kbar higher than elsewhere. The plagioclase-hornblende thermometer yields similar temperatures for tonalite-trondhjemite (666°-793°C at 5 kbar) and pyroxene-tonalite (723°-783°C) but higher temperatures (801°-948°C) for hornblende-bearing enderbite. Reintegrated preexsolution antiperthite grains plotted on a 5 kbar solvus yield 550°-870°C for tonalite-trondhjemite and 810°-1045°C for enderbite. Comparison of geothermobarometric calculations with published tonalite phase equilibria suggests near-solidus deformation. Thus, petrographic, mineralogic, field, and microstructural data are inconsistent with tectonometamorphic models requiring transposition and complete recrystallization of preexisting fabrics during regional orogenesis. Amphibolite-grade supercrustal rocks are associated with tonalite-trondhjemite or granite-granodiorite plutons, while granulite-grade belts are associated with enderbite and clinopyroxene- or orthopyroxene-bearing tonalite. This dichotomy suggests that metamorphism of supercrustal rocks reflects localized equilibration with adjacent plutons rather than orogenesis.

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