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TitleHigh-grade metamorphism flying under the radar of accessory minerals
AuthorThiessen, E J; Gibson, H D; Regis, D; Pehrsson, S J; Cutts, J A; Smit, M A
SourceGeology vol. 47, no. 6, 2019 p. 568-572, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20190048
PublisherGeological Society of America
Mediapaper; on-line; digital
File formatpdf (Adobe® Reader®); html
ProvinceNorthwest Territories
AreaWholdaia Lake
Lat/Long WENS-104.0333 -104.0167 60.7333 60.7167
Lat/Long WENS-104.0167 -104.0167 60.7333 60.7167
Lat/Long WENS-104.0167 -104.0167 60.7333 60.7167
Subjectstectonics; igneous and metamorphic petrology; geochronology; tectonic history; crustal evolution; crustal thickness; metamorphism; polymetamorphism; foliation; heat flow; radiometric dating; uranium lead dating; zircon dates; recrystallization; bedrock geology; lithology; metamorphic rocks; mafic rocks; granulites; phase equilibria; modelling; granulite facies; mineral assemblages; Archean; Rae Craton; Wholdaia Lake Shear Zone; Precambrian
Illustrationslocation maps; geoscientific sketch maps; plots; histograms; photomicrographs; profiles; phase diagrams
ProgramGEM2: Geo-mapping for Energy and Minerals South Rae Province Bedrock/Surficial geology
Released2019 04 16
AbstractUranium-lead (U-Pb) accessory mineral petrochronology has been increasingly used to constrain the timing of tectonometamorphic events. However, because mafic rocks commonly lack minerals with a high U/Pb ratio, they may be underrepresented in the chronologic record. This study on polymetamorphic mafic granulites from the Archean Rae craton (northern Canada) provides a striking example of a metamorphic cycle that has been entirely overlooked. We utilized Lu-Hf garnet geochronology and equilibrium phase diagram modeling to characterize two high-pressure granulite-facies mineral assemblages that affected the 2.6 Ga protolith. Zircon and garnet recrystallization occurred at 1.87 Ga within a gneissic foliation, while a coarse-grained garnet precursor nucleated 230 m.y. earlier during a stage of high heat flow within thickened lower crust, the latter of which is nearly absent in the zircon and monazite age record except for rare igneous occurrences. Combined garnet geochronology and petrological modeling reinforce a ca. 1.9 Ga age for high-grade overprinting in the southern Rae craton and clearly show within the same sample that U-Pb accessory minerals did not grow during a newly identified 2.11 Ga granulite-facies event.

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