Origin and evolution of the Variscan Léon eclogites: insights from zircon and garnet petrochronology
Bibliographic record
Abstract
Eclogites record the deep burial and exhumation of crustal terranes, which are key tectonic markers in the evolution of orogenic belts. Unraveling the evolution of eclogites and eclogite-bearing terranes may nevertheless be challenging, especially in old and poly-tectonic orogens such as the Variscan orogen, where these are scarce and potentially reworked. The eclogite-bearing Léon Domain (Armorican Massif, France) is one such occurrence. The Léon Domain is situated at the crossing of several (micro)continents and has long been recognized as a potential recorder for the assembly of this part of the Variscan orogen. The origin and metamorphic history of these eclogites remains nevertheless largely unconstrained. In this study, samples of the Léon eclogite were subjected to garnet and zircon petrochronology. In situ U-Pb ages from zircon cores constrain the protolith age at 385.7±2.9 Ma (concordia age, mean square weighted deviate (MSWD) = 8.7 and corresponding weighted mean age at 385.7±1.6 Ma, MSWD = 1.14), which is similar to zircon U-Pb ages reported from enclosing orthogneisses. Low δ 18 O values (median at 4.32 ‰) from these cores indicate interaction with surface fluids during magmatic crystallization. Garnet growth in the eclogites, as constrained by Lu-Hf geochronology, occurred at 346.5±0.8 Ma (MSWD = 0.6; Fe-Ti-rich eclogite) and 349.1±1.6 Ma (MSWD = 0.3; kyanite-bearing eclogite). Pseudosection modeling, Zr-in-rutile thermometry, and major- and trace-element maps in garnet were used to constrain the garnet growth history close to the metamorphic peak from 740 °C at 2.15 GPa to 760 °C at 2.5 GPa. Zircon separates from a cross-cutting felsic intrusion provided 321.6±2.7 Ma (MSWD = 0.76), which is taken to constrain the age of deformation and migmatization during exhumation. The new results are inconsistent with the presence of a major suture in the Léon Domain; they instead indicate that the eclogite derives from a small and short-lived back-arc basin, in which large bodies of felsic magma (enclosing orthogneiss) and smaller basaltic intrusions (the eclogite protolith) were emplaced and allowed to interact with meteoric water. The Léon Domain records rapid, cyclic inversion of this basin and thus may serve as an ancient analogue to the evolution of the Central Neotethys system, where short-lived alternating pulses of crustal extension and compression correlate with episodes of trench retreat and advance. The new tectonic interpretation for the Léon Domain supports a strong control of the slab dynamics within the frame of the protracted subduction history of the Rheic paleo-ocean.
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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 | 0.002 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 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".