Timescales of migmatization, melt crystallization, and cooling in a Cordilleran gneiss dome: Valhalla complex, southeastern British Columbia
Bibliographic record
Abstract
Integration of in situ and conventional ion microprobe U‐Th‐Pb zircon and monazite geochronology with 40Ar/39Ar hornblende, biotite, and muscovite thermochronology provides a temporal record of metamorphism, partial melting, fluids, and cooling of exhumed midcrustal to lower crustal rocks. The Valhalla complex, southeastern British Columbia, is one of a series of elongate gneiss domes within the Omineca belt of the Canadian Cordillera. We have performed U‐Th‐Pb and 40Ar/39Ar age measurements on migmatitic gneisses from a range of structural levels and settings within the Valhalla complex in an effort to better document the timing of migmatization and exhumation. Stromatic migmatites and leucosomes crystallized in boudin necks yield concordant U‐Pb zircon ages that cluster near 60 Ma and Th‐Pb monazite ages of circa 60–50 Ma. In situ analyses of monazite in samples that contain <10% leucosome yield a bimodal distribution at 70–65 and 62–57 Ma. The younger ages are similar to migmatite zircon ages. 40Ar/39Ar analyses from the same rocks document cooling at 51–49 Ma. These new data, combined with field, structural, and petrologic data from this and previous studies of the Omineca domes, indicate that a large region (thousands of km2) of orogenic crust may have been partially molten during the Early Tertiary (>60–50 Ma). However, the combined data also suggest that melt crystallization (circa 60 Ma) in the Valhalla complex occurred rapidly and before the other domes within the Omineca belt. Cooling at 49 Ma throughout the Omineca is associated with regional extension and exhumation of migmatites in the domes. The presence of abundant migmatites and the similarity in timing of melt crystallization and cooling in domes along the 400‐km‐long Omineca belt is consistent with localization of exhumation at the boundary between the flowing crust of the hinterland and the rigid, colder crust of the Rocky Mountain foreland.
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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 | 0.002 | 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".