Age constraints on the Paleoproterozoic tectonometamorphic history of the Committee Bay region, western Churchill Province, Canada: evidence from zircon and in situ monazite SHRIMP geochronology
Bibliographic record
Abstract
In situ UPb sensitive high-resolution ion microprobe (SHRIMP) analyses of monazite from upper amphibolite-facies paragneiss of the Committee Bay supracrustal belt, central Rae domain, Canada, reveal three age populations: ca. 2350, 1850, and 1780 Ma. The ca. 1850 Ma age also corresponds to growth of low Th/U zircon as indicated by UPb SHRIMP analyses of zircon separates from melanosome and leucosome. The contextual advantage of the in situ monazite analysis allows evaluation of the geochronological data in terms of the regional structural and metamorphic evolution. The region is dominated by a northeast-striking S2 (±S1) fabric, axial planar to tight, northwest-vergent F2 folds. Early garnet is enveloped by this biotitesillimanite ± cordierite S2 fabric. GarnetI hosts ca. 1850 Ma monazite inclusions (with ca. 2350 Ma cores), placing a maximum age on garnetI growth and S2 development. D2 metamorphic conditions progressed through ~3.5 kbar (1 kbar = 100 MPa) and 600 °C to near-peak conditions of ~5 kbar and 675 °C. A minimum age for S2 is provided by unstrained ca. 1820 Ma monzogranite that locally, and regionally, truncates S2. Dominantly ca. 1780 Ma matrix monazite is interpreted to date post-S2 garnetII and cordierite, which record ~5 kbar and 675 °C. These data indicate that the Committee Bay region experienced penetrative D2 tectonometamorphism at ca. 18501820 Ma, with a subsequent static overprint. The absence of a ca. 1.85 Ga plutonic suite in the region suggests that low-pressure metamorphism was a response to thick-skinned crustal thickening initiated at ca. 1870 Ma. The new data highlight the importance of Paleoproterozoic reworking of the central Rae domain in the hinterland of the Trans-Hudson orogen.
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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 | 0.002 | 0.002 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".