U-Pb zircon ages of Early Archean gneisses from northern Labrador
Bibliographic record
Abstract
Early Archean crustal records are rare, but contiguous units are best preserved in N. Labrador and the NWT (Canada) and in SW Greenland. The Saglek-Hebron area (N. Labrador), located at the W. extension of the North Atlantic Craton (NAC), contains wellpreserved Eo-Paleoarchean suites including pre-3.8 Ga Nanok Ferich monzodioritic gneiss, the Nulliak supracrustal assemblage (ca. 3.8 Ga), 3.7-3.6 Ga Uivak I TTG gneisses, 3.5-3.4 Ga Uivak II augen gneisses and Mesoarchean 3.2 Ga Lister gneiss [1-3]. Saglek dykes are present in the Eo and Paleoarchean gneisses, but not in the younger Lister gneisses. Despite confirmation of the antiquity of the area [3,4] a comprehensive zircon U-Pb dating with LA-ICPMS employing cathodluminescence (CL) imaging has not been undertaken for orthogneisses and supracrustal suites. CL images are essential to discuss inherited grains, pristine core and overgrowth. We conducted LA-ICPMS U-Pb geochronological study of zircons from TTG Uivak I gneiss from the Saglek-Hebron area. The CL images of zircon grains display internal structures of oscillatory zoning and homogeneous core with overgrowth rim. Results show that samples collected as Uivak I TTG gneisses can be classified into three groups based on the distribution of zircon ages. The first group of TTGs is characterized by both presence of older zircons than 3.8 Ga, with the maximum age of 3914 ± 40 Ma in 207Pb/206Pb age, and apparent lack of 3.6 to 3.8 Ga zircons. These are obviously members of the Nanok gneiss. Based on intrusive relationships observed in the field, the Nanok gneiss is pre-date emplacement of the protoliths of the Uivak I gneisses. The second and third groups have clear peaks at 3.7-3.6 Ga and ca. 3.3 Ga in their age distribution of zircon cores, indicating that TTGs of the second and third groups correspond to Uivak I gneiss and the Lister gneiss, respectively. Importantly, overgrowth rims of zircons we analyzed here show ca. 2.7 Ga, which reflect zircon growth during late Archean thermal event in the NAC, possibly associated with assembly of different terranes within the gneiss complex. We show that the combination of in-situ U-Pb dating and CL imaging can reveal the tectonothermal history of early Archean from the gneisses in N. Labrador.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
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 | 0.003 |
| 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) | 0.001 | 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".