PRESSURE-TEMPERATURE-TIME CONSTRAINTS ON THE METAMORPHISM OF MAFIC GRANULITES AND META-IRONSTONES OF THE PIKWITONEI GRANULITE DOMAIN, MANITOBA

Archean cratons are tectonically stable areas preserved in the Earth’s crust that hold potential clues to understand the tectono-metamorphic processes that operated on early Earth. However, interpreting these clues presents potential challenges due to the extensive modification of cratonic rocks over billions of years of post-orogenic activity. The Pikwitonei Granulite Domain (PGD), located in Manitoba (northwest Superior Province) is a seemingly intact high- to ultrahigh-temperature (HT-UHT) Archean metamorphic terrane showing limited retrogression, providing an optimal locality to decipher these processes. In this work, we have selected two mafic granulites and two meta-ironstones from outcrops spanning the northwestern to central PGD. Petrographic studies reveal that the samples preserve peak assemblages equilibrated both at hand specimen as well as microscopic scales. Our primary objective is to constrain whether certain pressure-temperature-time pathways are unique to specific tectonic settings and hence, may help us infer the tectonic mechanism that led to the stabilization of this domain. To achieve this, we have applied thermodynamic models along with major (Al-in-Orthopyroxene) and trace element (Ti-in-Zircon) thermobarometry to constrain the peak metamorphic conditions and garnet (Lu-Hf) geochronology to understand the timing of the prograde metamorphism. Minimal presence of recrystallized melt is observed in the mafic granulites, indicating some changes to the bulk composition through melt removal. To account for the melt loss, we apply a stepwise down-temperature melt-reintegration approach at constant pressure to model the bulk composition before melt loss occurred. We then use that bulk composition to model the prograde to peak temperature and pressure of granulite-facies metamorphism. The modeling results imply a peak temperature of >950°C at a pressure of ~0.8 GPa. Bulk Lu-Hf garnet ages for the four lithologies range from 2.6-2.8 Ga, indicating a domain-wide prolonged timespan of peak metamorphic conditions. Ongoing work seeks to elucidate the mechanisms responsible for UHT metamorphism, with possible factors including higher mean mantle temperature, smaller crustal thicknesses, and/or high radiogenic heat production in the crust.