Abstract: Monazite U-Th-Pb dating by electron microprobe: a powerful new tool for constraining tectonothermal models
Bibliographic record
Abstract
s – GAC Newfoundland Section, 2007 Spring Technical Meeting Atlantic Geology, 2007, Volume 43, Number 1 Copyright © 2015 Atlantic Geology atlantic geology . volume 43 . 2007 83 Copyright © Atlantic Geology, 2007 tion of structural and stratigraphic observations. We emphasize that these new dates are mainly of use in interpreting ambiguous field data and regional correlations. In central Massachusetts and adjacent areas (from the Bronson Hill arc eastward to the high-grade zones of the Sturbridge area), the conventional view has been of a principal Acadian tectonothermal event at ca. 400 Ma (Robinson’s “classic Acadian”) with a successor event at ca. 360 Ma (his “neo-Acadian”). Targeted monazite dating indicates highestgrade (near-granulite) metamorphism at ca. 435 Ma, with additional events at 400 Ma and 360 Ma. Abrupt age breaks across strike suggest major regional east-west shortening with cryptic major faults. To the west, approaching the Bronson Hill arc, a major belt of late recrystallization at 330 Ma indicates early onset of Alleghanian effects, and post-300 age resetting is seen as main-phase Alleghanian. In western Connecticut (west of the Mesozoic basin), the conventional model is of a Taconic fore-arc allochthonous on Laurentian margin (Grenville) with in situ post-Taconic sedimentary basins, all metamorphosed to kyanite-sillimanite grade in the Acadian, and with minor Alleghanian overprint to the south near Long Island Sound. Our dating shows a complex pattern of intermixed Taconic ages (ca. 520, 470–480 and 445–450 Ma) with multi-stage Acadian ages (> 420, 385–390 and 355–360 Ma), but some locations in older rocks within the supposed Acadian belt show no evidence of preceding Taconic metamorphism. Cameron’s Line, a notable tectonic break highlighted by Rodgers on his 1985 Connecticut geologic map, has been interpreted as the western limit of imbricated Taconic thrust slices containing deep-water rocks. Our dating shows that numerous locations in assumed Cambro–Ordovician rocks east of Cameron’s line did not experience Taconic metamorphism at all, and thus these rocks may have originated east of the Bronson Hill arc prior to the Acadian (broadly defined). This would basically require that Cameron’s line marks the western limit of a major detachment zone rather than the western limit of far-travelled Taconic thrusts. Abstracts – GAC Newfoundland Section, 2007 Spring Technical Meetings – GAC Newfoundland Section, 2007 Spring Technical Meeting
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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 | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.009 | 0.003 |
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".