Latest Archean continental growth in a post-collisional setting: Evidence from 2.5 Ga high-K calc-alkaline granitoids in the Rae craton, Northwest Territories, Canada
Bibliographic record
Abstract
Abstract At least 30% of preserved continental crust was generated in the late Archean (3.0–2.5 Ga). Constraints on how late Archean crust formed are therefore important to understanding continental growth. We present whole-rock elemental and Nd isotope data and zircon U-Pb-Hf isotope data for late Archean plutonic (mostly granitoid) rocks in an understudied portion of the southwestern Rae craton. Three plutonic suites with crystallization ages of ca. 2.71–2.69 Ga, 2.58–2.57 Ga, and 2.52–2.49 Ga are documented. The 2.5 Ga suite is characterized in detail because the tectonic setting of 2.5 Ga magmatism in the Rae craton is debated and magmatic rocks of this age are globally understudied. Data from the older suites provide constraints on the isotopic and chemical composition of the local basement. The 2.5 Ga rocks range from mafic to felsic (47–74 wt% SiO2), are high-K calc-alkaline and light rare earth element enriched, exhibit negative Nb anomalies, and yield restricted ranges of initial εNd (−0.5 to +0.6) and weighted mean initial εHf (−1.1 to +1.5) values. Mafic 2.5 Ga magmas derived from a subduction-enriched mantle source that had roughly chondritic isotopic compositions at the time of magma generation. The unradiogenic composition of the mantle source can be explained by: (1) metasomatism of depleted mantle by low Sm/Nd and Lu/Hf fluids or melts well before 2.5 Ga, such the source evolved from a suprachondritic to chondritic isotopic composition by 2.5 Ga, or (2) metasomatism of depleted mantle by melts of unradiogenic (εNd2.5 Ga and εHf2.5 Ga ≤ 0) subducted sediments shortly before or during the 2.5 Ga magmatism. More evolved 2.5 Ga magmas were produced through some combination of fractional crystallization of mantle-derived magmas, assimilation of preexisting crust, and partial melting of juvenile 2.5 Ga crust. Rare ca. 2.57–2.56 Ga zircon xenocrysts in the 2.5 Ga suite are evidence of minor crustal contamination. Neodymium isotope modeling permits, but does not require, assimilation of the local 2.7–2.6 Ga crust. However, the data do require that all components of the suite are, at least in part, juvenile crustal additions. The 2.5 Ga rocks are geochemically similar to post-collisional I-type plutonic rocks from the Phanerozoic Caledonian and Variscan orogens, and their emplacement in a post-collisional setting is consistent with existing constraints on the tectonic evolution of the Rae craton. This study exemplifies how the timing of some significant crustal growth events may not be accurately recorded in depleted mantle model ages, and how integrating isotopic and geochemical data with local geological context helps to identify such events. The results also suggest, along with other studies, that substantial late Archean continental growth may have occurred in post-collisional settings.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.003 | 0.003 |
| Science and technology studies | 0.002 | 0.001 |
| Scholarly communication | 0.002 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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 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".