Petrogenesis of syenites at Phalaborwa, Kaapvaal Craton: (isotope) geochemical, modelling and age constraints
Bibliographic record
Abstract
Whole rock major and trace element data and apatite compositions (including Nd isotopes) for fifteen syenites from intrusions surrounding the Phalaborwa carbonatite-phoscorite Complex are presented to evaluate magmatic sources, processes, and also petrogenetic relationships across alkaline provinces in the Kaapvaal Craton. Thermodynamic modelling (alphamelts) of fractional crystallization assuming a gabbroic parental melt reproduces syenite compositions, involving early clinopyroxene and apatite removal, dominant K-feldspar crystallization with minor ilmenite and magnetite, and late biotite and quartz fractionation. Discrepancies between observed and modelled major elements (higher K₂O, Al₂O₃; lower CaO, Na₂O, P₂O₅) are explained by accumulation of K-feldspar (minor biotite and clinopyroxene) and dilution of apatite, plagioclase and Ti-oxides. Apatite displays zoning and high REE–Sr contents are indicative of a magmatic origin. Minor and compositionally distinct apatite and unradiogenic εNd₂.₀₆ Ga (−1.4 to −12.0) imply assimilation of granitic/gneissic basement in some syenites. For most syenites, however, mixing models using suitable endmembers do not reproduce whole rock compositions. LA-SF-ICP-MS U Pb ages of zircon, titanite, rutile and monazite range from 2.03 to 1.86 Ga. Titanite and rutile ages (1936 ± 24 Ma, 1860 ± 65 Ma, 2 SE) record post-emplacement hydrothermal alteration. Only one monazite age of 2032 ± 9 (2 SE) Ma likely dates magmatic emplacement, slightly post-dating emplacement of the 2060 Ma carbonatite-phoscorite Complex, but coeval with mafic dyke intrusions. Imprecise zircon upper-intercept ages (2.5–1.9 Ga) and petrographic features in zircon, titanite and rutile are ascribed to hydrothermal alteration by fenitising fluids or thermal overprinting related to dyke emplacement. We propose a model whereby alkaline melts at Phalaborwa were (partly) derived from metasomatically-enriched subcontinental lithospheric mantle (SCLM). Mantle (re)fertilization likely began during a 2.9–2.8 Ga subduction event and continued with a major ~2.0 Ga hydrous metasomatic episode. A shared SCLM signature among several Kaapvaal alkaline complexes therefore registers (plume-induced?) melting of a subduction-metasomatized lithospheric mantle as an important crust formation process at 2 Ga.
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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) | 0.001 | 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) | 0.008 | 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".