Origin of Chromitite-Anorthosite Interlayering in the Bushveld Complex
Bibliographic record
Abstract
ABSTRACT An idealized stratigraphic section of the five main chromitite-bearing cyclic units of the Upper Critical Zone of the Bushveld Complex typically consists of basal chromitite overlain by pyroxenite, norite, and anorthosite. Such units can be explained by classical fractional crystallization from a Mg-rich basalt followed by injection of new magma. However, in some cases both the underlying and overlying rocks of the chromitite are anorthosite. Furthermore, most of the interstitial material in the chromitites is not pyroxene, which would be the mineral expected to crystallize with chromite, but rather it is oikocrystic plagioclase. Finally, recent age dating suggests that the cyclic units may not have been emplaced in stratigraphic order, with the upper group chromitites being older than the underlying middle group chromitites. In order to reconcile some of these data we compiled field and drill core observations from the Upper Critical Zone. Of particular interest is the MG2–MG3 interval, showing pronounced layering, abundant autoliths of various shapes and compositions, as well as flame structures and boudinage. Many of these features are typical of sedimentary mass flows, suggesting that the MG2–MG3 interval formed via granular flow of intruding chromite-orthopyroxene crystal slurries that deposited chromitite and orthopyroxenite. The heat of the new magma influx caused partial melting of the host leuconorite to produce restitic anorthosite and buoyant noritic melt. The latter infiltrated the slurry, resulting in a disequilibrium assemblage of cumulus chromite and oikocrysts of plagioclase.
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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 | 0.001 | 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 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 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".