Age and petrology of the Machias Seal Island quartz monzodiorite, the southernmost rocks in New Brunswick, Canada
Bibliographic record
Abstract
Machias Seal Island in the northern Gulf of Maine consists of fine- to medium-grained weakly foliated quartz monzodiorite. Although previously inferred to be as young as Devonian, the quartz monzodiorite yielded an Ediacaran–Early Cambrian U-Pb (zircon) age of 542.0 ± 0.9 Ma. Typical Machias Seal Island quartz monzodiorite contains strongly zoned plagioclase (50%), about 30% mafic minerals (biotite and amphibole with relict cores of both orthoand clinopyroxene, and 20% interstitial quartz and orthoclase. The average chemical composition (5 samples) has 60.6% SiO2, with relatively high Al2O3 (over 16%) and low K2O (2.8%). Overall, the chemical characteristics, including low Rb, Y, and Nb, are consistent with emplacement in a continental margin subduction zone. The quartz monzodiorite contains abundant ovoid metadioritic enclaves, likely of cognate origin. Age and compositional similarities strongly suggest correlation of Machias Seal Island quartz monzodiorite with the abundant ca. 550–525 Ma gabbroic to granitic plutons of the Brookville terrane on the mainland of southern New Brunswick. Its age is similar also to the ages of volcanic and some plutonic rocks on Grand Manan Island and in the New River terrane in mainland southern New Brunswick, strengthening the interpretation that all of these areas are part of Ganderia. Hence the Fundy Fault southeast of Machias Seal Island likely marks the boundary between Ganderia and Avalonia in the Gulf of Maine.
 
 RÉSUMÉ
 
 L’île Machias Seal, dans la partie nord du golfe du Maine, se compose de monzodiorite quartzique à grains fins à moyens faiblement feuilletée. Même si on déjà présumé que son origine remonterait au Dévonien, une datation de la monzodiorite quartzique par la méthode U-Pb sur zircon a indiqué un âge d’Ediacara et du début du Cambrien, soit de 542,0 ± 0,9 Ma. Les minéralisations caractéristiques de monzodiorite quartzique de l’île Machias Seal renferment un plagioclase fortement zoné (50 p. 100), environ 30 p. 100 de minéraux mafiques (biotite et amphibole accompagnées de noyaux résiduels d’orthopyroxène et de clinopyroxène), ainsi que 20 p. 100 de quartz et d’orthoclase interstitiels. La composition chimique moyenne (5 échantillons) est de 60,6 p. 100 de SiO2, une teneur relativement élevée de Al2O3 (plus de 16 p. 100) et une faible teneur en K2O (2,8 p. 100). Dans l’ensemble, les caractéristiques chimiques de ces minéralisations, y compris la faible teneur en Rb, Y et Nb, sont généralement associées à un emplacement dans une zone de subduction de la marge continentale. La monzodiorite quartzique contient d’abondantes quantités d’enclaves de métadiorite ovoïdes, vraisemblablement de la même origine. L’âge et les similitudes de composition de ces roches portent à croire qu’une corrélation pourrait s’établir entre la monzodiorite quartzique de l’île Machias Seal et la grande quantité de plutons gabbroïque à granitique de 550–525 Ma observée dans le socle de Brookville, dans la partie continentale du Sud du Nouveau-Brunswick. Par ailleurs, ces roches ont le même âge que celui des roches volcaniques et de certains plutons de l’île Grand Manan et du socle de New River dans la partie continentale du Sud de la province, ce qui viendrait confirmer l’interprétation voulant que toutes ces zones fassent partie de Ganderia. La faille de Fundy au sud-est de l’île Machias Seal marque donc la limite entre Ganderia et Avalon dans le golfe du Maine.[Traduit par la redaction]
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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 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 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 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".