Geochronology and geochemistry of rare-element pegmatites from the Superior Province of Canada
Bibliographic record
Abstract
The crystallisation age and thermochronology of late- to post-orogenic, rare-lithophile element (Li, Cs, Ta) pegmatites that are chemically associated with peraluminous granites have been determined from U-Pb TIMS and LA-MC-ICP-MS analyses of columbite-tantalite, and UV -laser <sup>40</sup>Ar/<sup>39</sup>Ar analyses of mica. <br><br>In the northwest Superior Province, U-Pb ages reveal that the Pakeagama Lake pegmatite crystallised at 2673 ±8Ma. Rare-element pegmatites in the western Superior Province yield crystallisation ages of 2649 ±4Ma and 2644 ±8Ma for the Separation Rapids group, and 2665 ±lOMa for the Mavis Lake group, showing that they are temporally associated with adjacent peraluminous granites. Replacement zones of columbite-tantalite grains from the pegmatites show disturbance to the U-Pb system, probably caused by interaction with residual fluids and sub solidus metasomatic processes that lasted for -30Ma after crystallisation. <br><br>Mica from rare-element pegmatites is characterised by distinct compositional zoning with muscovite cores and F-, Li- and Rb-rich zinnwalditellepidolite rims. The zinnwalditellepidolite rims that replace the original muscovite were most likely precipitated from a residual melt or fluid phase during the late-magmatic to hydrothermal period of pegmatite formation. <br><br>Within these micas, <sup>40</sup>Ar/<sup>39</sup>Ar apparent ages young towards the grain boundary and also towards the core - rim boundary, which suggests that the compositional zones form separate argon diffusion domains. However, similar <sup>40</sup>Ar/<sup>39</sup>Ar apparent ages are found within F-rich zinnwaldite/lepidolite and coexisting muscovite indicating that diffusion rates may be comparable. <br><br>Mica from rare-element pegmatites of the Superior Province yield intra-grain <sup>40</sup>Ar/<sup>39</sup>Ar apparent age variations of up to 800Ma within single crystals. Theoretical slow cooling models do not match the measured <sup>40</sup>Ar/<sup>39</sup>Ar profiles and cannot account for the large apparent age variations. Thermal history models that produce the best fit to apparent age profiles include two re-heating events; a minimum 350°C re-heating at -2450Ma and <i>circa</i> 300°C re-heating at 1850Ma, both for a period of at least 10Ma. These apparent ages are concordant with two major Proterozoic events affecting the Superior Province; intrusion of the Matachewan - Hearst dyke swarms and the Trans Hudson Orogeny, respectively.
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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 | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.003 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.016 | 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".