U-Th-Pb Geochronology and Lu-Hf Isotope Geochemistry of Detrital Zircons in Metasedimentary Rocks of the Southern Coast Mountains Batholith
Bibliographic record
Abstract
Abstract Most metasedimentary rocks in the southern Coast Mountains batholith are of uncertain tectonic affinity because they occur in discontinuous pendants surrounded by large intrusive bodies, and many protolith features are obscured by regional deformation and metamorphism. This study uses U-Th-Pb ages and Lu-Hf isotope signatures of detrital zircons in metasedimentary rocks in Bute, Loughborough, and Knight Inlets in an effort to test possible correlations with the adjacent Wrangellia, Alexander, Taku, Yukon-Tanana, and Stikine terranes. Detrital zircons from metasedimentary samples yield ages that belong to age groups of 590-528 Ma (peak age of 560 Ma), 485-432 Ma (peak age of 452 Ma), 356-286 Ma (peak age of 307 Ma), and 228-185 Ma (peak ages of 215 and 198 Ma). A small number of ~1.1-1.9 Ga grains are also present. εHft values of the 590-185 Ma grains yield a progression from intermediate (0 to +5) values to more juvenile (mostly +4 to +15) values from Neoproterozoic through early Mesozoic time. The Comparison of these results with similar data sets from adjacent terranes demonstrates that primary connections with the Yukon-Tanana and Taku terranes are unlikely but are consistent with primary connections with the Wrangellia, Stikine, and/or Alexander terranes. Unfortunately, the available constraints are not sufficient to eliminate any of these options or the possibility that the pendants are a unique tectonic fragment. Zircons from the metasedimentary samples also yield U-Th-Pb ages of 165-128 Ma (peak age of 152 Ma) and 114-88 Ma (peak age of 102 Ma). εHft values of these zircon domains are mostly juvenile (+7 to +13). Comparison of U concentrations, U/Th values, and CL textures of zircons from the metasedimentary samples, leucocratic sills that intrude the pendants, and surrounding plutonic bodies suggests that most of the young grains, as well as widespread younger rims on older grains, grew during metamorphism associated with emplacement of the adjacent plutonic bodies. Some young grains were derived from thin felsic sills or veins that were unintentionally included in the sampled material.
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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 | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 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.
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".