U-Pb and Hf isotopic evidence for an Arctic origin of terranes in northwestern Washington
Bibliographic record
Abstract
New field, U-Pb, and Lu-Hf zircon data constrain the geologic history, age, and origin of the Yellow Aster Complex (YAC) in northwestern Washington, providing insight into the tectonic history of this and related Paleozoic arc terranes of the western North American Cordillera. Mapping shows that the oldest YAC rocks consist of quartzofeldspathic paragneiss (meta-arkose) and quartzose calc-silicate paragneiss (metacalcareous siltstone) in gradational contact. Paragneisses are cut by syn-tectonic and post-tectonic intrusions and faulted against granitic orthogneiss. U-Pb zircon results show that (1) maximum depositional ages of paragneisses are Silurian to Early Devonian (432– 390 Ma); (2) detrital zircons from quartzose calc-silicate paragneisses show a broad age peak from 1900 to 1000 Ma, while quartzofeldspathic paragneisses contain several distinct Precambrian age peaks, including at 2.0–1.8 Ga and 2.5–2.4 Ga; (3) paragneisses contain early Paleozoic grains with peaks ca. 420–400 and ca. 460–440 Ma; (4) pre-tectonic orthogneiss and syn-tectonic and post-tectonic dikes range from ca. 410–406 Ma; and (5) intrusive rocks contain apparently xenocrystic ca. 480–440 Ma grains. Lu-Hf isotope data show that nearly all Paleozoic zircons have negative εHf(t) values, and zircons in the meta-arkose samples are more negative than those in the calc-silicate. Zircons in several meta-arkose samples yield εHf(t) values of –40 to –57, rare in the North American Cordillera, and requires the involvement of Mesoarchean to Eoarchean crustal components. The most likely source region with crust as old as Eoarchean and early Paleozoic magmatism is the Greenland Caledonides, which implies derivation from the Arctic margin of northeastern Laurentia or Baltica. The chemistry and petrology of the igneous rocks suggest that the terrane was in a continental arc setting before, during, and after deposition of the sedimentary rocks. The data constrain deformation, metamorphism, and magmatism in the YAC to a brief period in the Early Devonian, from ca. 410 to 400 Ma. Age and Hf patterns of the YAC are similar to elements of the Yukon-Tanana and Alexander terranes. Our study shows that the complex history of metamorphosed terranes requires analysis of multiple isotopic and petrologic proxies, and U-Pb analysis of both igneous (n = 50) and detrital (n = 400) zircons to confirm or refute terrane and provenance correlations.
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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 | 0.000 |
| 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".