Tonian Arc Magmatism in Central Madagascar: The Petrogenesis of the Imorona-Itsindro Suite
Bibliographic record
Abstract
The East African Orogen is one of the largest orogens that formed during the Ediacaran to Cambrian amalgamation of Gondwana. In the Mozambique Belt, the East African Orogen represents the amalgamation of Neoproterozoic India with the Congo-Tanzania-Bangweulu Block. In the Arabian-Nubian Shield, the orogen consists of a pre-Neoproterozoic continental terrane surrounded by Neoproterozoic juvenile oceanic arc–like terranes. A large ocean named the Mozambique Ocean divided these Neoproterozoic landmasses. Many oceanic sutures tie together the various terranes in the northern orogen, but the location of potential sutures become less obvious moving south. The ca. 100 My duration of subduction of the Mozambique Ocean resulted in voluminous magmatism in Madagascar, named the Imorona-Itsindro Suite, but the location of possible suture zones in Madagascar is controversial. Understanding the petrogenesis of this suite is critical for developing Neoproterozoic paleogeographic plate reconstructions. The Tonian age of the Imorona-Itsindro Suite is well documented, but the geochemistry of the suite provides ambiguous evidence for the tectonic setting. Geochemically, these rocks are predominantly calc-alkaline, with characteristics consistent with emplacement within a volcanic or continental arc. The suite has variable but mostly high 87Sr/86Sr and low 143Nd/144Nd signatures, indicating significant crustal involvement. Changes in subduction-zone dynamics, accompanied by variable crustal anatexis and assimilation, contributed to geochemical ambiguities that have prompted some authors to suggest an active rift as an alternative tectonic model. However, the geochemical signatures are more compatible with an Andean-like arc tectonic setting. We suggest that a prolonged history of subduction (>100 My) provided sufficient time for the continental arc to mature and for the development of shallow mantle source metasomatism.
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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 | 0.002 | 0.002 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 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".