The Petrogenesis of the Lower Critical Zone and Lower Group Chromitites, Eastern Bushveld Complex: Incremental Development of Igneous Layering and Injection of Chromite Slurries
Bibliographic record
Abstract
Abstract The petrogenesis of the mafic-ultramafic layered rocks of the Bushveld Complex, South Africa, remains uncertain despite more than a century of intensive research. There is little evidence for the existence of a shallow crustal magma chamber, a principle entrenched in the scientific literature for many years, and we prefer an alternative hypothesis in which igneous layering developed from successive intrusive events. Geochemical discontinuities are explained by radical switches in the composition of intruding parental magmas. Here we investigate the origin of the Lower Critical zone, a sequence of five pyroxenitic and two peridotitic units containing the Lower Group (LG) of chromitites, of which LG6 is a major resource for chromium ore in the eastern limb. The pyroxenitic and peridotitic units are not part of magmatic cycles. Two discrete lineages of parental magma are recognized: a pyroxenitic magma sourced from deep crustal staging chambers, and a mantle-derived peridotitic magma. The pyroxenitic units reveal an absence of modal layering (with the exception of the 0.1- to 1-m-thick layers of chromitite) and limited height-related fractionation trends (enstatite: En86–811). Magma was fed as thin laminae. Intrusion rates were insufficient to create a magma reservoir because the supply of magma matched the rate of batch crystallization. There is little evidence of a fractionated residue having been removed. The recognition of minor geochemical trends of Mg- and Fe-enrichment in the pyroxenitic units reflects minor oscillations in the composition of the parental magma. The composition of the orthopyroxene and of the Cr spinel was primarily established at depth. The anomalous concentrations of chromite are explained by the introduction of pyroxenitic magmas containing crystals of Cr spinel, sufficiently abundant in some cases as to have created crystal slurries. The Cr spinel was entrained at depth, in the staging chambers. Each layer of chromitite is related to injection of a crystal slurry. The tiny size of the rounded xenocrysts of Cr spinel facilitated the mobile and dynamic nature of slurries. Coarse-grained mosaics of Cr spinel developed in the compacted layers of chromitite from prolonged periods of annealing. The irregular spacing of the chromitites, and occurrence of variable concentrations of disseminated Cr spinel in the pyroxenites, is explained by fluctuations in the proportion of entrained xenocrysts, rather than changes in the composition of the melt fraction of the magma. The peridotitic units are interpreted as sills, the chromitite layers having been inherited from the earlier formed pyroxenitic stratigraphy. The most primitive layers of dunite and harzburgite (olivine: Fo90-85 in which Fo = forsterite) accumulated in the center of sills, where a high magma flux facilitated growth of pegmatitic oikocrysts.
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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) | 0.001 | 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 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".