Superposed Neoproterozoic and Silurian magmatic arcs in central Cape Breton Island, Canada: geochemical and geochronological constraints
Bibliographic record
Abstract
Isotopic and geochemical data indicate that intrusions in the eastern Creignish Hills of central Cape Breton Island, Canada represent the roots of arcs active at ∼ 540–585 Ma and ∼ 440 Ma. Times of intrusion are closely dated by (1) a nearly concordant U–Pb zircon age of 553±2 Ma in diorites of the Creignish Hills pluton; (2) a lower intercept U–Pb zircon age of 540±3 Ma that is within analytical error of 40Ar/39 Ar hornblende plateau isotope-correlation ages of 545 and 550±7 Ma in the River Denys diorite; and (3) an upper intercept U–Pb zircon age of 586±2 Ma in the Melford granitic stock. On the other hand, ∼ 441–455 Ma 40Ar/39 Ar muscovite plateau ages in the host rock adjacent to the Skye Mountain granite provide the best estimate of the time of intrusion, and are consistent with the presence of granitic dykes cutting the Skye Mountain gabbro–diorite previously dated at 438±2 Ma. All the intrusions are calc-alkaline; the Skye Mountain granite is peraluminous. Trace element abundances and Nb and Ti depletions of the intrusive rocks are characteristic of subduction-related rocks. The ∼ 540–585 Ma intrusions form part of an extensive belt running across central Cape Breton Island, and represent the youngest Neoproterozoic arc magmas in this part of Avalonia. Nearby, they are overlain by Middle Cambrian units containing rift-related volcanic rocks, which bracket the transition from convergence to extension between ∼ 540 and 505/520 Ma. This transition varies along the Avalon arc: 590 Ma in southern New England, 560–538 Ma in southern New Brunswick, and 570 Ma in eastern Newfoundland. The bi-directional diachronism in this transition is attributed to northwestward subduction of two mid-ocean ridges bordering an oceanic plate, and the migration of two ridge–trench–transform triple points. Following complete subduction of the ridges, remnant mantle upwelling along the subducted ridges produced uplift, gravitational collapse and the high-temperature/low-pressure metamorphism in the arc in both southern New Brunswick and central Cape Breton Island. The ∼ 440 Ma arc magmatism in the Creignish Hills extends through the Cape Breton Highlands and into southern Newfoundland, and has recently been attributed to northwesterly subduction along the northern margin of the Rheic Ocean.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.003 | 0.003 |
| Science and technology studies | 0.002 | 0.001 |
| Scholarly communication | 0.002 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.005 | 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".