Barrovian metamorphism in the central Kootenay Arc, British Columbia: petrology and isograd geometry
Bibliographic record
Abstract
A narrow, partly fault-bounded belt of Barrovian amphibolite facies rocks transects the central Kootenay Arc in the internal zone of the southeastern Canadian Cordillera. The following zones of increasing metamorphic grade are recognised in metapelites: chlorite/biotite, garnet, staurolite, kyanite, sillimanite, and sillimanite + K-feldspar. The garnet and higher-grade zones outline two joined domains: a N- to NNE-trending curved belt, >100 km long and 5–20 km wide, that is approximately parallel to the strike of stratigraphic units; and a SSE-trending belt, >70 km long and 10–15 km wide, that transects strike. Isograds in the N–NNE-trending belt outline an elongate bull’s-eye pattern with highest-grade rocks in the centre, coincident with the position of a structural culmination. Isogradic surfaces have an elongate domal form centred around this culmination. Rocks in the kyanite and sillimanite zones were metamorphosed at ~25 km (∼7 kbar (1 kbar = 100 MPa)) and >650 °C during Early Cretaceous crustal thickening; rocks in the garnet zone experienced peak temperature conditions (∼500 °C) at lower pressure, suggesting the piezothermic array for the belt has a positive slope. The western flank of the N- to NNE-trending belt is cut by the west-dipping Gallagher fault zone (GFZ), whereas the eastern boundary of the SSE-trending fork is marked by the Purcell Trench fault (PTF). These Palaeocene–Eocene normal faults truncate Early Cretaceous isogradic surfaces and juxtapose regions with contrasting structural and metamorphic histories. Low-grade rocks in the hanging wall of the GFZ underwent peak regional metamorphism during the Early–Middle Jurassic, prior to intrusion of the 159–173 Ma Nelson batholith at a depth of ∼12–14 km. With the exception of a zone along the southern “tail” of the Nelson batholith, rocks in the hanging wall of the GFZ were not affected by Cretaceous metamorphism or penetrative deformation. Rocks in the amphibolite-facies belt yield Palaeocene–Eocene K–Ar and 40Ar/39Ar mica cooling ages, whereas K–Ar biotite ages in the hanging wall of the GFZ record Jurassic – Early Cretaceous cooling, and those in the hanging wall of the PTF are mid-Cretaceous. Although the GFZ and PTF accommodated differential exhumation during the Palaeogene, significant relief on isogradic surfaces was established prior to normal faulting, during Early Cretaceous metamorphism and deformation.
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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) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.005 | 0.006 |
| Science and technology studies | 0.003 | 0.001 |
| Scholarly communication | 0.002 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.006 | 0.001 |
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".