Geological, petrologic, isotopic, and geochemical constraints of geodynamic models simulating formation of the archean tonalite-trondhjemite-granodiorite associations in ancient cratons
Bibliographic record
Abstract
The geological setting, geochemistry, and Nd isotopic systematics of tonalite-trondhjemite-granodiortite (TTG) series in ancient cratons are considered. It is shown that the TTG series were formed from ∼4.2 to 2.6 Ga ago in the oldest continental cores; many TTG series do not reveal chronological links to greenstone belts. This follows from the evolution of the Slave Craton in the Canadian Shield, the Vodlozero Craton in the Baltic Shield, and the Pilbara and Yilgarn cratons in the Australian Shield, where greenstone associations postdated TTG series. As has been established at the Baltic Shield, the primary melts of the Mesoarchean TTG associations were formed at a shallower depth (P < 15 kbar) compared to the Neoarchean TTG, likely, due to the increasing thickness of the continental crust beneath the Baltic Shield over time.The Nd isotopic composition of worldwide TTG associations indicates that most of them are characterized by a substantial time interval (>150 Ma) that separates the formation of the TTG melts from the age of the source involved in melting. Taking into account the calculated rate of cooling of the lithospheric plates, these data indicate that most Archean TTG series likely were not formed in the convergent subduction-related and accretionary geodynamic settings. The isotopic and geochemical data constrain compositions of the sources that produced Archean TTG series. Petrologic modeling of the formation conditions and Nd isotopic composition of the metabasalts in greenstone belts show that these rocks could not have been the source of TTG series. The most plausible isotopic and geochemical analogue of this source are the Archean amphibolites (ENd mafic rocks), which differ from the metabasalts of greenstone belts by their lower Sm/Nd ratio and enrichment in some lithophile elements. The available data suggest that the primary TTG melts were generated as products of melting of amphibolites and granulites forming the lower crust.
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How this classification was reachedexpand
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 | 0.001 |
| 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 itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, 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".