Eoarchean to Neoarchean evolution of the Nuvvuagittuq Supracrustal belt: New insights from U-Pb zircon geochronology
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Bibliographic record
Abstract
The Nuvvuagittuq Supracrustal belt in northern Québec is a rare ∼9 km^2^ fragment of the Earth9s early crust. The belt contains a metamorphosed volcano-sedimentary sequence that is at least Eoarchean in age, including amphibolites of the Ujaraaluk Unit that may have protoliths as ancient as 4400 million years old (Ma; O9Neil and others, 2012). Upper amphibolite facies metamorphism and high-strain deformation have obscured many primary field relationships and disturbed whole-rock isotope systematics, leading to debate over the interpretation of previous geochronological data. We report new SHRIMP U-Pb isotopic analyses of zircons from key meta-igneous and newly identified metasedimentary units of the belt. The analyzed samples fall into four categories: (a) felsic gneisses, including sheets interlayered with the supracrustal assemblage and a tonalitic orthogneiss at the margin of the belt; (b) a newly identified metasedimentary unit; (c) fuchsite-bearing quartz-rich layers; (d) mafic gneisses, including a garnet-biotite amphibolite from the Ujaraaluk Unit and a meta-gabbro. The felsic lithologies place constraints on the minimum age of the supracrustal assemblage; oscillatory-zoned zircons separated from a felsic orthogneiss sheet form a discordant array with an upper intercept age of 3774 ± 32 Ma, and the tonalitic orthogneiss yielded oscillatory zoned zircons with an upper intercept age of 3781 ± 11 Ma. A metasedimentary unit, dominated by quartz, albite and clinozoisite, has been identified within a lower strain domain, interfolded with amphibolites of the Ujaraaluk Unit. A sample of this unit yielded zircons with highly variable CL structures and near concordant ages of between 3390 and 3780 Ma. Zircons from a fuchsite-bearing quartz layer have oscillatory zonation similar to the orthogneiss grains, and define an upper intercept age of 3794 ± 16 Ma. Petrologic observations and geochemical data suggest that this lithology is a metasomatically altered felsic orthogneiss band, rather than a meta-sedimentary rock belonging to the supracrustal package. Electron microscopy of polished thin sections was used to locate zircons within 6 samples of the Ujaraaluk Unit amphibolites and two samples of meta-gabbro. Large (≤500 μm) zircons separated from a garnet-biotite amphibolite, together with grains from a meta-gabbro, are sector zoned and yield ages of 2700 to 2640 Ma, recording the timing of pervasive high-temperature metamorphism in the region. The new zircon U-Pb data add important new detail to the geological history of the Nuvvuagittuq belt. The volcano-sedimentary sequence and mafic-ultramafic intrusive rocks were formed prior to 3800 million years ago. Subsequent geological evolution included extensive and protracted early Archean TTG magmatism and crustal re-melting, Paleo- to Mesoarchean uplift and sedimentation and Neoarchean high-grade metamorphism and regional magmatism during consolidation of the Superior craton.
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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 | 0.002 |
| Science and technology studies | 0.000 | 0.003 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 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 it