Inception of a Devonian subduction zone along the southwestern Gondwana margin: <sup>40</sup>Ar–<sup>39</sup>Ar dating of eclogite–amphibolite assemblages in blueschist boulders from the Coastal Range of Chile (41°S)
Bibliographic record
Abstract
Coarsely crystalline residual blueschist boulders from the Coastal Range of south-central Chile (41°S) contain relict eclogite–amphibolite assemblages that provide evidence of pre-Carboniferous high pressure relative to temperature (high P/T) metamorphism along the southwestern continental margin of Gondwana. Early assemblages in the exotic boulders include omphacite, garnet, and hornblende that indicate eclogite-facies conditions of T = 553 ± 30 °C and P > 1.32 ± 0.04 GPa during metamorphism, corresponding to a low geothermal gradient of <12.5 °C/km. These phases are replaced to varying degrees by sodic amphibole + epidote assemblages. Relict hornblende from an early garnet amphibolite assemblage within blueschist yields an 40Ar–39Ar plateau age of 361 ± 1.7 Ma, providing a minimum age for early high P/T metamorphism. Coarse white micas that partially replace the hornblende and are in textural equilibrium with glaucophane yield plateau and near-plateau ages of 325 ± 1.1 Ma and ∼320 Ma, respectively. We interpret these data to indicate that late Paleozoic high P/T metamorphism related to subduction of oceanic lithosphere along the southwestern paleo-Pacific margin of Gondwana began prior to 361 Ma (Late Devonian). Subsequent retrograde metamorphism involving fluid infiltration and decreasing thermal gradients resulted in conversion of coarse eclogite–amphibolite to blueschist by ∼325 Ma in the dated samples.
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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 | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 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".