Petrogenesis of syenites at Phalaborwa, Kaapvaal Craton: (isotope) geochemical, modelling and age constraints
Bolhar R, Maghdour-Mashhour R, Milani L, Mondal S, O'Sullivan GJ, Klemd R, Chang Q (2026)
Publication Type: Journal article
Publication year: 2026
Journal
Book Volume: 707
Article Number: 123202
DOI: 10.1016/j.chemgeo.2025.123202
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 element compositions (higher K₂O, Al₂O₃; lower CaO, Na₂O, P₂O₅) are explained by accumulation of K-feldspar (minor biotite and clinopyroxene) and dilution by apatite, plagioclase and Ti-oxides.Apatite displays zoning and high REE–Sr contents indicative of a magmatic origin. Minor and compositionally distinct apatite and unradiogenic εNd (2.06 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 fenitizing 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.
Authors with CRIS profile
Involved external institutions
University of the Witwatersrand (WITS)
South Africa (ZA)
University of Pretoria
South Africa (ZA)
Trinity College Dublin
Ireland (IE)
Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
Japan (JP)
South Africa (ZA)
University of Pretoria
South Africa (ZA)
Trinity College Dublin
Ireland (IE)
Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
Japan (JP)
How to cite
APA:
Bolhar, R., Maghdour-Mashhour, R., Milani, L., Mondal, S., O'Sullivan, G.J., Klemd, R., & Chang, Q. (2026). Petrogenesis of syenites at Phalaborwa, Kaapvaal Craton: (isotope) geochemical, modelling and age constraints. Chemical Geology, 707. https://doi.org/10.1016/j.chemgeo.2025.123202
MLA:
Bolhar, Robert, et al. "Petrogenesis of syenites at Phalaborwa, Kaapvaal Craton: (isotope) geochemical, modelling and age constraints." Chemical Geology 707 (2026).
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