U-Pb zircon ages of Early Archean gneisses from northern Labrador
Bibliographic record
Abstract
Early Archean crustal records are rare, but contiguous units are\nbest preserved in N. Labrador and the NWT (Canada) and in SW\nGreenland. The Saglek-Hebron area (N. Labrador), located at the W.\nextension of the North Atlantic Craton (NAC), contains wellpreserved\nEo-Paleoarchean suites including pre-3.8 Ga Nanok Ferich\nmonzodioritic gneiss, the Nulliak supracrustal assemblage (ca.\n3.8 Ga), 3.7-3.6 Ga Uivak I TTG gneisses, 3.5-3.4 Ga Uivak II augen\ngneisses and Mesoarchean 3.2 Ga Lister gneiss [1-3]. Saglek dykes\nare present in the Eo and Paleoarchean gneisses, but not in the\nyounger Lister gneisses. Despite confirmation of the antiquity of the\narea [3,4] a comprehensive zircon U-Pb dating with LA-ICPMS\nemploying cathodluminescence (CL) imaging has not been\nundertaken for orthogneisses and supracrustal suites. CL images are\nessential to discuss inherited grains, pristine core and overgrowth.\nWe conducted LA-ICPMS U-Pb geochronological study of\nzircons from TTG Uivak I gneiss from the Saglek-Hebron area. The\nCL images of zircon grains display internal structures of oscillatory\nzoning and homogeneous core with overgrowth rim. Results show\nthat samples collected as Uivak I TTG gneisses can be classified into\nthree groups based on the distribution of zircon ages. The first group\nof TTGs is characterized by both presence of older zircons than 3.8\nGa, with the maximum age of 3914 ± 40 Ma in 207Pb/206Pb age, and\napparent lack of 3.6 to 3.8 Ga zircons. These are obviously members\nof the Nanok gneiss. Based on intrusive relationships observed in the\nfield, the Nanok gneiss is pre-date emplacement of the protoliths of\nthe Uivak I gneisses. The second and third groups have clear peaks at\n3.7-3.6 Ga and ca. 3.3 Ga in their age distribution of zircon cores,\nindicating that TTGs of the second and third groups correspond to\nUivak I gneiss and the Lister gneiss, respectively. Importantly,\novergrowth rims of zircons we analyzed here show ca. 2.7 Ga, which\nreflect zircon growth during late Archean thermal event in the NAC,\npossibly associated with assembly of different terranes within the\ngneiss complex. We show that the combination of in-situ U-Pb\ndating and CL imaging can reveal the tectonothermal history of early\nArchean from the gneisses in N. Labrador.
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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 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.003 | 0.002 |
| Science and technology studies | 0.001 | 0.000 |
| 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 | 0.001 |
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".