THE MESOPROTEROZOIC MUCAJAI ANORTHOSITE - MANGERITE - RAPAKIVI GRANITE COMPLEX, AMAZONIAN CRATON, BRAZIL
Bibliographic record
Abstract
The Mesoproterozoic Mucajai anorthosite – mangerite – rapakivi granite (AMG) complex, in northern Brazil, comprises rapakivi granites, fayalite–pyroxene quartz mangerite and syenite, and a massif-type anorthosite forming an asymmetrically zoned igneous complex without counterpart in the Amazonian craton. The rapakivi granites consist of biotite–hornblende granite, including pyterlites and subordinate wiborgites, and porphyritic biotite granite, and exhibit strong similarities with the classic rapakivi granites of Finland. The presence of a fayalite + ilmenite + quartz assemblage in the mangerites indicates low f (O 2 ) conditions during crystallization. The mangerites and rapakivi granites exhibit geochemical characteristics of A-type granites, including high contents of HFS elements (especially Zr), high Ga/Al, Rb/Ba and Rb/Sr values, high K 2 O contents and high FeO t /(FeO t + MgO), typical of reduced rapakivi granites. The biotite granite and the biotite–hornblende granite are related to each other by fractional crystallization, but the mangeritic rocks probably have an independent origin. A 1538 ± 5 Ma Pb–Pb evaporation age has been obtained for a fayalite–pyroxene mangerite. This age overlaps within analytical errors with the 1527 ± 7 Ma (U–Pb SHRIMP) and 1544 ± 42 Ma (U–Pb conventional method) ages previously reported for the anorthosite and rapakivi granites, respectively. The Nd data, including T DM model ages, in the range of 2.07–2.01 Ga, and e Nd (t) values between −2.4 and −1.3 suggest a juvenile crustal source for the granitic rocks of the complex. The e Nd (t) value calculated for the anorthosite (−2.9) probably reflects crustal contamination. Prominent bodies of rapakivi granite coeval with the Mucajai AMG complex are distributed along a NW–SE belt in the western part of the Guyana shield. The Mucajai AMG complex intruded 1.94 Ga country rocks at least ca . 400 my after the stabilization of this part of the Guyana shield, indicating an intraplate setting, in a geological scenario similar to those of the classical rapakivi complexes of the Fennoscandian shield.
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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 | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| 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".