<i>In Situ</i> Lu–Hf Garnet Dating of Archean Deep Crust Granulites from the Polymetamorphic Grenville Front Tectonic Zone
Bibliographic record
Abstract
Abstract Recent advances in geochronological techniques now make it possible to efficiently decipher the timing and duration of geological processes in complex high-grade polymetamorphosed orogenic terranes. This is the case of the Grenville Front Tectonic Zone, which truncates the Superior Craton to the southeast. The zone exposes parautochthonous Archean rocks that underwent mid- to high-pressure granulite facies metamorphism of uncertain age. The metamorphic assemblages have been either interpreted as Archean and associated with the final stages of the Superior Craton assembly or as the result of Mesoproterozoic Grenvillian metamorphism, based on cross-cutting relationships and traditional geochronology methods such as U–Pb zircon and 40Ar–39Ar mica dating. Herein, we revisit the extent of the Grenvillian metamorphic overprint in the parautochthonous domain and provide new age constraints for granulite-facies metamorphic assemblages through in situ garnet dating within migmatitic paragneiss, migmatitic orthogneiss, and mafic granulites, combined with in situ trace element mapping. Six samples, which show bell-shaped and occasionally sharp and oscillatory lutetium growth zoning in garnet, yield garnet Lu–Hf isochrons with identical Archean dates of c. 2.6 Ga. Sparse analyses of the material trend towards Grenvillian ages (c. 1 Ga) in one sample from which garnet shows lutetium zoning consistent with post-growth fluid-assisted disturbance. Overall, our results indicate that the widespread granulite-facies metamorphism within the Grenville Front Tectonic Zone is dominantly late Neoarchean in age, unveiling a rare exposure of Archean lower crust in the southeastern Superior Craton. Our results also point towards a limited Grenvillian metamorphic overprint, though the spatial extent and precise thermal conditions of this metamorphism are still unknown. The results presented herein demonstrate the potential of in situ isotopic geochronology on rock-forming minerals like garnet in polymetamorphic terranes.
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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 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 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.
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