Baltican crustal provenance for Cambrian–Ordovician sandstones of the Alexander terrane, North American Cordillera: evidence from detrital zircon U–Pb geochronology and Hf isotope geochemistry
Bibliographic record
Abstract
Detrital zircon U–Pb geochronology and Hf isotope geochemistry allow us to decipher the crustal provenance of Cambrian–Ordovician backarc basin strata of the Alexander terrane, North American Cordillera, and evaluate models for its origin and displacement history relative to Baltica, Gondwana, Siberia, and Laurentia. Quartzose shallow-marine sandstones of the Alexander terrane contain a range of Neoproterozoic to Neoarchaean detrital zircons with the most dominant age groupings c . 565–760, 1000–1250, 1450, and 1650 Ma. Subordinate volcaniclastic sandstones yield Cambrian and Ordovician detrital zircons with a prominent age peak at 477 Ma. The detrital zircon age signatures resemble coeval strata in the Eurasian high Arctic, and in combination with faunal and palaeomagnetic constraints suggest provenance from local magmatic rocks and the Timanide orogenic belt and Fennoscandian Shield of NE Baltica. The Hf isotopic compositions of Palaeozoic to Neoarchaean detrital zircons strongly favour Baltican crustal sources instead of similar-aged domains of Gondwana. The Alexander terrane formed part of an arc system that fringed the Uralian passive margin, and its position in the Uralian Seaway allowed faunal exchange between the Siberian and Baltican platforms. The available evidence suggests that the Alexander terrane originated in the Northern Hemisphere and migrated to the palaeo-Pacific Ocean by travelling around northern Laurentia. Supplementary material: U–Pb and Lu–Hf data tables are available at www.geolsoc.org.uk/SUP18557 .
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| 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 teacher head, 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".