Petrology, age, and tectonic setting of the rapakivi-bearing Margaree pluton, Cape Breton Island, Canada: evidence for a Late Devonian posttectonic cryptic silicic-mafic magma chamber
Bibliographic record
Abstract
The Margaree pluton extends for >40 km along the axis of the Ganderian Aspy terrane of northern Cape Breton Island, Nova Scotia. The pluton consists mainly of coarse-grained megacrystic syenogranite, intruded by small bodies of medium-grained equigranular syenogranite and microgranite porphyry, all locally displaying rapakivi texture. The three rock types have similar U–Pb (zircon) ages of 363 ± 1.6, 364.8 ± 1.6, and 365.5 ± 3.3 Ma, respectively, consistent with field and petrological evidence that they are coeval and comagmatic. The rare earth elements display parallel trends characterized by enrichment in the light rare earth elements, flat heavy rare earth elements, moderate negative Eu anomalies, and, in some cases, positive Ce anomalies. The megacrystic and rapakivi textures are attributed to thermal perturbation in the magma chamber caused by the mixing of mafic and felsic magma, even though direct evidence of the mafic magma is mainly lacking at the current level of exposure. Magma evolution was controlled by fractionation of quartz, K-feldspar, and Na-rich plagioclase in molar proportions of 0.75:0.12:0.13. The chemical and isotopic (Sm–Nd) signature of the Margaree pluton is consistent with the melting of preexisting continental crust that was enriched in heat-producing elements, likely assisted by intrusion of mantle-derived mafic magma during Late Devonian regional extension. The proposed model involving magma mixing at shallow crustal levels in a cryptic silicic-mafic magma chamber during post-Acadian extension is consistent with models for other, better exposed occurrences of rapakivi granite in the northern Appalachian orogen.
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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 | 0.002 | 0.002 |
| Science and technology studies | 0.002 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| 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".