Paleoarchean metamorphism in the Acasta Gneiss Complex: Constraints from phase equilibrium modelling and in situ garnet Lu–Hf geochronology
Bibliographic record
Abstract
Abstract The oldest known evolved (felsic) rocks on Earth (c. 4.03 Ga) are found in the Acasta Gneiss Complex (AGC) in north‐western Canada and represent a fundamental keystone in unravelling the geological processes governing crustal growth and differentiation during the Hadean and early Archean. Although the timing of multiple episodes of magmatism, metamorphism and deformation in these tonalitic gneisses has been investigated extensively, the metamorphic pressure–temperature (P–T) conditions recorded by the rocks are poorly constrained. Here, we use phase equilibrium modelling coupled with in situ garnet Lu–Hf geochronology and trace element analysis for two garnet‐bearing tonalitic gneisses to decipher the metamorphic history of the AGC. The observed mineral assemblages are consistent with peak metamorphic conditions of T = 725–780°C and P = 4.5–6.2 kbar and the generation of a small amount of melt (<7 vol.%). Garnet geochronology constrains the age of metamorphism to 3.3–3.2 Ga, consistent with previous evidence for a late Paleoarchean tectono‐metamorphic event in the AGC. Subsequent isotopic disturbance of garnet at c. 1.9 Ga is interpreted to correspond to a modification of the primary Lu–Hf systematics in response to garnet resorption/recrystallization during the Paleoproterozoic Wopmay orogeny, resulting in significant scatter between these two age components. Our study adds to the small number of published P–T data for metamorphic rocks older than 2.8 Ga and shows that tonalitic gneisses in the AGC record a high apparent thermal gradient of ~140°C/kbar in the late Paleoarchean. This thermal gradient is the highest among the limited dataset, but is broadly similar to data from other Paleoarchean‐Mesoarchean crustal rocks in recording high T/P ratios (>77.5°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 | 0.001 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 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".