High-grade metamorphism flying under the radar of accessory minerals

Uranium-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 maf...

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Bibliographic Details
Published in:Geology (Boulder) 2019-06, Vol.47 (6), p.568-572
Main Authors: Thiessen, Eric J, Gibson, H. Daniel, Regis, Daniele, Pehrsson, Sally J, Cutts, Jamie A, Smit, Matthijs A
Format: Article
Language:English
Subjects:
absolute age
Canada
Canadian Shield
Churchill Province
Cratons
Flight
Garnet
garnet group
Geochronology
Geochronometry
Geology
granulites
Heat flow
Heat transmission
high-grade metamorphism
igneous and metamorphic rocks
Isotopes
Lead
Lu/Hf
Magma
metamorphic rocks
Metamorphism
Metamorphism (geology)
Mineral assemblages
mineral composition
Minerals
Modelling
Monazite
nesosilicates
North America
Nunavut
orthosilicates
P-T conditions
Paleoproterozoic
Petrology
Phase diagrams
polymetamorphism
Precambrian
Proterozoic
Radar
Radiometric dating
Rae Province
Recrystallization
silicates
U/Pb
upper Precambrian
Uranium
Zircon
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Summary:Uranium-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.
ISSN:0091-7613
1943-2682
DOI:10.1130/G45979.1