Geochemistry and age constraints on metamorphism and deformation in the Fortymile River area, eastern Yukon-Tanana upland, Alaska
Bibliographic record
Abstract
Determining the age of deposition for many of the major rock types within the Fortymile River area is a major challenge for understanding the geologic evolution of the Yukon-Tanana tectonostratigraphic terrane of east-central Alaska. Intense dynamic metamorphism and associated recrystallization, several periods of ductile deformation, and poor outcrops conspire to conceal the age of the rocks in this important region. Definitive fossil evidence in the marble units has been obliterated; felsic gneissic horizons in the sequence could be either volcanic or intrusive in origin. To help resolve some of these issues, we present new geochemical data to further characterize the bedrock assemblage in the Fortymile River area and to address the origin of the major rock units. In addition, new U-Pb-isotopic data on zircons from the early (pre-D 1 ) Steele Creek Dome Orthogneiss and from a late tectonic (late-D 2 ) epidote-bearing leucogranite intrusion help constrain the youngest possible age for the supracrustal rocks, as well as the date of regional D 2 deformation. The Steele Creek Dome Orthogneiss crystallized at 343′4 Ma, indicating that the supracrustal sequence is partly at least Early Mississippian. The leucogranite crystallized during the waning stages of D 2 deformation at 196′4 Ma. U-Pb data on this Early Jurassic leucogranite also indicate that the inherited zircons have a complex history, with analyses yielding ages of 359′5 and 232′7 Ma. Our data confirm earlier 4 0 Ar/ 3 9 Ar ages that suggest an Early Jurassic date for the intense regional D 2 metamorphism and tectonism. However, although we have determined that the sedimentary and volcanic rocks are at least as old as Early Mississippian, the date of deposition for the major supracrustal rock packages in the Fortymile River area of the Yukon-Tanana terrane still remains a major question.
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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 | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 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".