High-grade metamorphism flying under the radar of accessory minerals

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.