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
Bibliographic record
Abstract
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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".