Multiple,isotopically heterogeneous plagioclase populations in the Bushveld Complex suggest mush intrusion |
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Institution: | 1. Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 5106403, China;2. Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong, China;3. Geological Survey of Canada, 601 Rue Booth Street, Ottawa, Ontario K1a 0e8, Canada;1. State Key Laboratory of Geological Processes and Mineral Resources, University of Geosciences, Beijing 100083, China;2. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510650, China |
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Abstract: | The Bushveld Complex and other layered intrusions show significant initial isotopic heterogeneity, both between and within co-existing cumulate minerals. Various processes have been proposed to account for this, including (i) intrusion of variably contaminated crystal mushes from deeper staging chambers, (ii) blending of semi-consolidated crystal mushes as a result of subsidence during cooling, (iii) variable infiltration of contaminants into a partially solidified crystal mush, (iv) density-driven mixing of minerals from isotopically distinct magma pulses, (v) contamination of crystals at the roof of the intrusion and mechanical incorporation of such contaminated crystals into the lower crystallisation front as a result of gravitational instability at the upper crystallisation front, and (vi) late-stage metasomatic processes. In order to assess the likely process(es) responsible for initial isotopic heterogeneities within the Bushveld Complex, we analysed core and rim domains of 12 plagioclase crystals from the Main and Upper zones of the Bushveld Complex for their Sr-isotopic compositions. The data show the presence of multiple, isotopically heterogeneous populations of plagioclase occurring within the same rocks. The data presented here are best explained through the intrusion of variably contaminated crystal mushes derived from a sub-compartmentalized, sub-Bushveld staging chamber that underwent different degrees of contamination with crustal rocks of the Kaapvaal craton. |
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Keywords: | Isotopic disequilibrium Crystal mushes Magma chamber dynamics Crustal contamination Bushveld Complex |
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