Paleoproterozoic A-Type Postcollisional Granitic Magmatism Associated with the Arrowsmith Orogeny during the Global Tectonomagmatic Lull
Bibliographic record
Abstract
The tectonomagmatic history of the early Paleoproterozoic (Siderian) global tectonomagmatic lull (TML; ca. 2.4–2.2 Ga) is poorly known, limiting understanding of lithospheric processes during the Archean to Proterozoic transition. In this study, we identify and describe an unusual ca. 2.4–2.3 Ga postcollisional A-type granite suite associated with orogenic collapse of a continental collision in the middle of the TML. The postcollisional A-type granitoids consist of monzogranite to tonalite intrusions of the North Shore plutonic suite, emplaced during the middle and later stages of the ca. 2.5–2.3 Ga Arrowsmith orogeny, which represents a long-lived accretionary orogen on the northwestern margin of the Archean Rae craton in northern Canada. New laser ablation (LA)-ICP-MS U-Pb zircon age data indicate that the North Shore plutonic suite represents protracted granitic magmatism over ca. 70 My. The new U-Pb ages combined with previous and new LA-MC-ICP-MS Hf isotope and secondary-ion mass spectrometry O isotope zircon data reveal compositional heterogeneities not previously recorded by bulk isotopic methods and require changes to the classification of the granitoids and their role in the Arrowsmith orogeny. Zircon εHf values of the North Shore plutons are consistent with a mixture of juvenile and older crustal components in their sources, but their nonenriched zircon δ18O values are inconsistent with a large supracrustal melt component, precluding an S-type granite parentage. Instead, their trace-element proportions in discrimination diagrams and zircon δ18O values show A-type granite affinities, which are often missed in studies of ancient, poorly defined orogens. Aluminous compositions of the A-type granites are compatible with emplacement in a continent-continent collision, following postcollisional slab break-off and delamination. Postcollisional emplacement of the A-type granites is consistent with other geologic evidence suggesting that subduction beneath the Rae craton margin ceased by 2.4 Ga.
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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 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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".