ULTRAHIGH TEMPERATURE METAMORPHISM OF THE ARCHEAN PIKWITONEI GRANULITE DOMAIN

Granulites are the dense, strong metamorphic rocks that record high crustal heat flow and comprise much of Earth’s stable Archean cratonic roots. Therefore, detailed petrologic studies of granulite terranes can reveal the mechanisms for extreme heating and stabilization of early continental crust. The Pikwitonei granulite domain (PGD) comprises the NW edge of the Archean Superior Province and is the largest preserved granulite terrane in the craton, encompassing >15000 km². Constraining the timing, temperatures, depths, and duration of metamorphism in the PGD is crucial for understanding the construction and stabilization of the Superior Province.

Zr in rutile thermometry and thermodynamic modelling of rocks from the central part of the PGD indicate peak P-T conditions of >890 °C at 7-8 kbar. Such ultrahigh temperature (UHT) conditions agree with previous studies. Pseudosections and mineral replacement microstructures suggest a decompressional P-T path at near peak T. U-Pb petrochronology has identified multiple, distinct monazite (mnz) populations. The oldest population in each sample at ~2690-2700 Ma with shallow negative Eu anomalies and steep HREE slopes likely grew during prograde metamorphism in the presence of garnet and plagioclase breakdown. Zircon populations from each sample indicate zircon growth synchronous with this early mnz. HREE depleted mnz populations with pronounced negative Eu anomalies give ages of ~2680 and ~2650 Ma and indicate mnz growth in the presence of garnet and plagioclase. HREE-enriched mnz with pronounced negative Eu anomalies record ages of ~2620-2630 Ma, consistent with mnz growth during garnet breakdown and plagioclase stability. These results suggest that high T conditions may have been sustained over timescales >10¹ Ma, or, alternatively, the PGD experienced multiple heating/cooling cycles over ~70 Ma. Field relationships, in concert with thermometric/geochronologic results from key microstructural settings, suggest that contractional deformation responsible for the dominant SW-striking penetrative fabric in the PGD preceded, was synchronous with, and out-lived peak UHT metamorphic conditions. More precise constraints on the timescales of metamorphism and P-T path are necessary to elucidate the mechanism for extreme crustal heating in the PGD.