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Formation of transgressive anorthosite seams in the Bushveld Complex via tectonically induced mobilisation of plagioclase-rich crystal mushes

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Geoscience Frontiers 2016年第6期7卷 875-889页

作者： Wolfgang D.Maier bartosz T.karykowski Sheng-Hong Yang School of Earth and Ocean Sciences Cardiff UniversityUK Oulu Mining School University of OuluFinland

The formation of anorthosites in layered intrusions has remained one of petrology＇s most enduring enigmas. We have studied a sequence of layered chromitite, pyroxenite, norite and anorthosite overlying the UG2 chromitite in the Upper Critical Zone of the eastern Bushveld Complex at the Smokey Hills platinum mine. Layers show very strong medium to large scale lateral continuity, but abundant small scale irregularities and transgressive relationships. Particularly notable are irregular masses and seams of anorthosite that have intrusive relationships to their host rocks. An anorthosite layer locally transgresses several 10 s of metres into its footwall, forming what is referred to as a ＂pothole＂ in the Bushveld Complex. It is proposed that the anorthosites formed from plagioclase-rich crystal mushes that originally accumulated at or near the top of the cumulate pile. The slurries were mobilised during tectonism induced by chamber subsidence, a model that bears some similarity to that generally proposed for oceanic mass flows. The anorthosite slurries locally collapsed into pull-apart structures and injected their host rocks. The final step was down-dip drainage of Fe-rich intercumulus liquid, leaving behind anorthosite adcumulates.

关键词： Anorthosite Layered intrusion Bushveld Complex South Africa Chromitite

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Element mapping the Merensky Reef of the Bushveld Complex

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Geoscience Frontiers 2021年第3期12卷 259-278页

作者： William Smith Wolfgang Maier Stephen Barnes Gareth Moorhead David Reid bartosz karykowski School of Earth and Ocean Sciences Cardiff UniversityCardiff CF240DEUnited Kingdom CSIRO Mineral Resources Perth 6160Australia CSIRO ClaytonVictoriaAustralia University of Cape Town South Africa Fugro Germany Land GmbH 12555 BerlinGermany

The Merensky Reef hosts one of the largest PGE resources *** has been exploited for nearly 100 years,yet its origin remains *** the present study,we characterised eight samples of the reef at four localities in the western Bushveld Complex using micro-X-ray fluorescence and field emission scanning electron *** results indicate that the Merensky Reef formed through a range of diverse *** exhibited by chromite grains at the base of the reef are consistent with supercooling and in situ *** local thickening of the Merensky chromitite layers within troughs in the floor rocks is most readily explained by granular *** and deformation textures in pyroxenes of the Merensky pegmatoid bear testament to recrystallisation and *** footwall rocks to the reef contain disseminations of PGE rich sulphides as well as olivine grains with peritectic reaction rims along their upper margins suggesting reactive downward flow of silicate and sulphide ***-hosted melt inclusions containing Cl-rich apatite,sodic plagioclase,and phlogopite suggest the presence of highly evolved,volatile-rich *** reverse zonation of cumulus plagioclase in the footwall of the reef indicates dissolution or partial melting of plagioclase,possibly triggered by flux of heat,acidic fluids,or hydrous ***,these data suggest that the reef formed through a combination of magmatic,hydrodynamic and hydromagmatic processes.

关键词： Merensky Reef Bushveld Complex Magmatic sulphide Platinum-group elements PGE Anorthosite

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