GEOCHRONOLOGIA U-Pb DE GNAISSES E GRANITÓ1DES DO QUADRILÁTERO FERRÍFERO {CRATON DO SÃO FRANCISCO MERIDIONAL): IDADE DO MAGMATISMO E METAMORFISMO ARQUEANO E PALEOPROTEROZÔICO
Bibliographic record
Abstract
U-Pb analyses were carried out on gneisses and granitoids from the Quadrilatero Ferrifero area, in the southern Sao Francisco craton. The Belo Horizonte complex represents a segment of Archaean crust principally composed of migmatized banded TTG-type gneisses. Migmatization event took place at 2860+14/-10 Ma. Granitic plutons intruded in the complex are crust-derived and one of them yielded a zircon age of 2712+5/-4 Ma. The emplacement of a small granitic pluton at 2612+37-2 Ma defines the youngest Archaean magmatic event in the Quadrilatero Ferrifero. This set of geochronological determinations together with published data allows the establishment of a fairly accurate framework for the evolution of the Archaean crust in the Quadrilatero Ferrifero. The main period of crust generation at 3.0 to 2.9 Ga is indicated by Nd isotopic signature of TTG gneisses. The evolutionary trend followed the progressive amalgamation of continental blocks, associated with intensive reworking of the primitive crust, renewed events of granitoid emplacement and greenstone belt deposition. An extensive cratonic area consolidated at ca. 2.6 Ga and was subsequently covered by stable-shelf sediments. The Alto Maranhao pluton was emplaced at 2124±1 Ma. This tonalitic intrusion represents an isotopically juvenile body related to the Transamazonian Orogeny. Titanite of the TTG-gneiss and leucosome align in a discordia with a lower intercept at 2041 ±5 Ma defining the metamorphic peak of this event.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.002 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 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".