Tectonic significance of Late Ordovician silicic magmatism, Avalon terrane, northern Antigonish Highlands, Nova Scotia <sup>1</sup> This article is one of a series of papers published in <i>CJES Special Issue: In honour of Ward Neale</i> on the theme of Appalachian and Grenvillian geology.<sup>2</sup> Contribution to International Geological Correlation Programme (IGCP) Project 497.
Bibliographic record
Abstract
Avalonia was a microcontinent during most of the Ordovician, separating the Iapetus Ocean to the north from the Rheic Ocean to the south. In the northern Antigonish Highlands, Nova Scotia, volcanic rocks (Dunn Point and McGillivray Brook formations) underlie Early Silurian – Early Devonian strata (Arisaig Group) and were thought to represent extension that heralded the development of the basin into which Arisaig Group strata were deposited. However, recent U–Pb (zircon, thermal ionization mass spectrometry) data from rhyolite in the Dunn Point Formation (DPF) yielded an age of 460.0 ± 3.4 Ma, and, here, we report a concordant age of 454.5 ± 0.7 Ma for an ignimbrite in the overlying McGillivray Brook Formation (MBF). These data confirm a ca. 10 million year gap between volcanism and onset of Arisaig Group deposition, which occurred after accretion of Avalonia to Baltica. The DPF and MBF both resemble A-type SiO2-rich magmas. The MBF has very high concentrations of Zr (745–1965 ppm), Y (65–213 ppm), Nb (57 to 185 ppm), and high Ga/Al. Several MBF samples exhibit strong LREE depletion, consistent with fractionation of LREE-bearing accessory phases. εNdt values for MBF (t = 455 Ma) range from +1.5 to +3.9, and overlap with DPF rhyolites that range from +2.9 to +3.7. Depleted mantle model ages for MBF and DPF samples unaffected by accessory phase fractionation are between 0.9 and 1.2 Ga and are similar to TDM values in older (Neoproterozoic, Cambrian) crustally derived felsic rocks, suggesting derivation from the lower crustal basement beneath the Antigonish Highlands. DPF and MBF rocks were probably erupted in a local extensional environment within an ensialic arc, perhaps analogous to the modern Taupo Volcanic Zone in northern New Zealand.
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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 | 0.000 |
| 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 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".