Origin and evolution of the Variscan Léon eclogites: insights from zircon and garnet petrochronology
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Bibliographic record
Abstract
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 δ18O 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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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 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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