Exotic rifted passive margin of a back-arc basin off western Pangea: geochemical evidence from the Early Mesozoic Ayú Complex, southern Mexico
Bibliographic record
Abstract
Recent detrital zircon studies of metamorphosed and polydeformed rocks of the early Mesozoic Ayú Complex in southern Mexico suggest an allochthonous origin along the western Pangean margin. Bulk-rock geochemistry of the ca. 170–200 Ma ortho-amphibolites suggests a composition ranging from alkalic and transitional basalts to normalized mid-ocean ridge basalt (N-MORB) tholeiites. Rare earth element (REE) patterns of alkaline basalts (Group I) are characterized by steep negative slopes, whereas transitional basalts (Group II) show moderate light REE (LREE) enrichment. Subalkalic Group III displays slight LREE enrichment and Group IV has relatively flat REE patterns with slight depletion in LREEs. Multiple trace element plots of Group III–IV amphibolites reveal strongly negative Nb–Ta anomalies caused by subduction zone contamination. Initial ϵNd values (t = 190 Ma) of the amphibolites range from +9.01 to –2.16. Alkalic basalts have negative ϵNd values, suggesting derivation from an older subcontinental mantle source (T DM = 877 and 791 Ma). Group II–IV amphibolites have positive ϵNd values ranging from +2.31 to +9.01, indicating a transition from an older to a relatively juvenile mantle source that is typical of a back-arc setting. The geochemistry of the metasedimentary rocks suggests derivation from an acid-arc source. Chondrite-normalized REE patterns are characterized by enriched LREEs, flat HREE, and negative Eu anomalies. Sm–Nd systematics indicate that most samples were derived from cratonic basement and plot within the Oaxacan Complex envelope with ϵNd values (t = 195 Ma) ranging from –5.53 to –7.65. We interpret two samples with higher ϵNd values (–1.42 and +1.06) to reflect the additional influence of a more juvenile component. The amphibolites and metasedimentary rocks of the Ayú Complex document back-arc activity and are inferred to be correlative with various western Mexican Triassic–Jurassic mafic suites and the Potosí fan that formed along the western rifted margin of Pangea.
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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 | 0.000 |
| Scholarly communication | 0.000 | 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".