Pressure–temperature conditions and age of metamorphism in the Archean Acasta Gneiss Complex: constraints from phase equilibrium modelling and in situ garnet Lu-Hf geochronology

The Acasta Gneiss Complex (AGC) in northwestern Canada is home to the oldest known evolved (felsic) rocks on Earth, dating back to around 4.03 billion years (Ga). These rocks preserve evidence for multiple episodes of magmatism, metamorphism, and deformation, offering insights into the geological processes that shaped the Earth's crust throughout the Archean and late Hadean. However, the metamorphic pressure–temperature (P–T) conditions of this complex remain poorly constrained. In this study, we use phase equilibria modelling and in situ garnet Lu-Hf geochronology to analyse two garnet-bearing tonalitic gneisses in the AGC, providing the first quantitative P–T constraints for a late Paleoarchean tectono-metamorphic event in the AGC. Our results indicate metamorphic peak conditions of approximately 725-780°C and 4.5-6.2 kbar, with limited partial melting (<7 vol.%) of the felsic gneisses at these crustal levels. In situ Lu-Hf garnet geochronology suggests that this metamorphic event occurred between 3.3-3.2 Ga, consistent with previous findings of high-grade metamorphism at that time. Isotopic disturbance of garnet at approximately 1.9 Ga is interpreted to reflect partial resetting of the Lu-Hf systematics in response to fluid-present re-equilibration during the Paleoproterozoic Wopmay orogeny. Our study extends the limited dataset of published P–T data for Mesoarchean and older metamorphic rocks and shows that tonalitic gneisses in the AGC evolved along a high apparent thermal gradient of 125-150°C/kbar.