Origin and evolution of the Grenvillian Oaxacan Complex, southern Mexico: Hf isotopic and U-Pb geochronologic constraints
Bibliographic record
Abstract
ABSTRACT The Oaxacan Complex is the largest outcrop of Grenville-age rocks in Mexico, constituting the main crustal fragment in the backbone of Oaxaquia. It is mainly composed of scarce metasediments, intruded by arc, alkalic, and tholeiitic magmas (ca. 1.3 to ca. 1.01 Ga) and later affected (ca. 0.99 Ga) by granulite-facies metamorphism. A detailed study, combining U-Pb geochronology by laser ablation–inductively coupled plasma–mass spectrometry, with in situ Hf isotopes in zircon grains, allowed comparison of the age and isotopic patterns of the Oaxacan Complex granulite rocks with those from other similar outcrops in Mexico (Huiznopala and Novillo Gneisses, Guichicovi Complex) and with the neighboring orogens such as the Grenville Province of the eastern United States and Canada, the Sveconorwegian orogen of SW Baltica, and some of the localities in which Mesoproterozoic rocks border the Amazonian craton of South America (Colombia, Peru, Brazil). Detrital zircon ages show that most metasedimentary rocks are younger than 1.4 Ga (only three samples contained zircon grains between 1.6 and 1.4 Ga), whereas U-Pb dating of igneous rocks (1245–1161 Ma) confirmed previous findings. Hf isotopes of dated zircon grains show that few crystals have negative εHf(t) values, indicating a recycling component from an older crust, but most of them are moderately primitive, with εHf(t) values of up to +12, and linear arrays parallel to the 176Lu/177Hf average crustal evolution model. Those Hf values are indicative of partial assimilation of an older crustal component, with Hf model ages of ca. 1.65–1.50 Ga. Comparison of these data helps to constrain possible Mesoproterozoic conjugate margins of Oaxaquia and propose a paleogeographic model in which Oaxaquia acted as the leading edge of Amazonia, together with the Colombian terranes, and received sedimentary input from different sources such as the southern Sveconorwegian orogen, the U.S.–Canada eastern Grenville Province, and some of the Mesoproterozoic belts bordering the Amazon craton.
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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 | 0.003 | 0.002 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".