Testing a back-arc ‘aulacogen’ model for the Central Metasedimentary Belt of the Grenville Province
Bibliographic record
Abstract
Abstract Nearly 70 new Nd isotope analyses are presented for plutonic orthogneisses from the Grenvillian Central Metasedimentary Belt (CMB) in order to test a back-arc aulacogen model for its origin. Nd isotope signatures of metaplutonic rocks are used as probes of the formation age of the crust at depth, revealing sharp boundaries between old crustal blocks and juvenile (1.2–1.35 Ga) Elzevirian-age crust. Firstly, a hidden block of old crustal basement is revealed between areas of juvenile crust south of Douglas, Ontario. Secondly, TDM ages refine the boundary between juvenile crust and old basement (1.35–1.55 Ga) within the Weslemkoon batholith, showing this pluton to be a polygenetic stitching pluton that straddles a hidden crustal boundary. Finally, the CMB boundary zone is shown to form a sharp age boundary between juvenile and old crustal domains, and is interpreted as a reactivated rift-bounding normal fault. When the distribution of rift-related alkaline rocks is compared with these crustal boundaries, the Bancroft nepheline syenite suite is centrally located in a juvenile ensimatic zone between blocks of old basement. Such a location, near the axis of a juvenile crustal segment, implies emplacement late in the rifting process. Similarly, the Blue Mountain nepheline syenite appears to post-date an earlier rifting event to the southeast. Hence, a multi-stage model is proposed for the evolution of a back-arc aulacogen, which is consistent with the distribution of marble and volcanic/plutonic units in the CMB. The model places the Bancroft nepheline syenites in a precise plate tectonic context for the first time.
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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 | 0.001 | 0.004 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.001 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.002 | 0.001 |
| Open science | 0.002 | 0.001 |
| Research integrity | 0.001 | 0.000 |
| Insufficient payload (model declined to judge) | 0.004 | 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".