Abstract
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.
Highlights
Uranium-bearing accessory minerals are common in crustal rocks and widely used to constrain the timing and duration of igneous, metamorphic, and deformation events
The southeastern Rae craton exemplifies a polymetamorphic amphibolite- to granulite-facies region (Berman et al, 2007) where U-bearing accessory minerals have been used to constrain the timing of numerous tectonometamorphic events that occurred between initial crust formation at 2.7– 2.6 Ga to stabilization after 1.82 Ga (Fig. 1B; e.g., Berman et al, 2013; Bethune et al, 2013; Dumond et al, 2015)
Using Lu-Hf garnet geochronology with complementary petrological modeling, garnet dates presented correspond to a HP stage that has remained largely undetected in the U-Pb mineral record of the southeastern Rae craton
Summary
Uranium-bearing accessory minerals are common in crustal rocks and widely used to constrain the timing and duration of igneous, metamorphic, and deformation events. The southeastern Rae craton (northern Canada; Fig. 1A) exemplifies a polymetamorphic amphibolite- to granulite-facies region (Berman et al, 2007) where U-bearing accessory minerals have been used to constrain the timing of numerous tectonometamorphic events that occurred between initial crust formation at 2.7– 2.6 Ga to stabilization after 1.82 Ga (Fig. 1B; e.g., Berman et al, 2013; Bethune et al, 2013; Dumond et al, 2015). From 2.28 to 2.00 Ga, regional extension is manifested by sedimentation, anorthosite magmatism, and formation of mafic dikes (Rainbird et al, 2010; Card et al, 2014; Regan et al, 2017) These processes did not favor zircon growth (Fig. 1B), and, their records are dwarfed by those of other events. CITATION: Thiessen, E.J., et al, 2019, High-grade metamorphism flying under the radar of accessory minerals: Geology, v. 47, p. 568–572, https://doi.org/10 .1130/G45979.1
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