Note on the mineral texture and chemistry of the eastern Bushveld Complex

• Main Authors: Sotaro Baba, Tomokazu Hokada, Tetsu Saitoh, Yoshikuni Hiroi, Kazuyuki Shiraishi, Makoto Arima, Robin E. Harmer
• Format: Report
• Language: English
• Published: Department of Crustal Studies, National Institute of Polar Research/Department of Crustal Studies, National Institute of Polar Research/Geological Institute, Yokohama National University/Department of Geology, Faculty of Science, Chiba University/Department of Crustal Studies, National Institute of Polar Research/Geological Institute, Yokohama National University/Council for Geoscience 2001
• Subjects: Rustenburg Layered Suite; Bushveld Complex; mineral texture; mineral chemistry; magma mixing; Memoirs of National Institute of Polar Research; Polar Research
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=2418; http://id.nii.ac.jp/1291/00002418/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=2418&item_no=1&attribute_id=18&file_no=1

Preliminary petrological study was carried out for the Rustenburg Layered Suite in the eastern Bushveld Complex. The Rustenburg Layered Suite consists of the Marginal Zone, Lower Zone, Critical Zone, Main Zone and Upper Zone. The mineral texture and chemical compositional variations were examined from the Lower to the Upper Zone. Although the number of sample is small, the crystallization sequence and presence of two types of magma can be recognized from the following mineral textures and compositions : (1) Plagioclase occurs as a post-cumulus phase in rocks from the Lower Zone to lower subzone in the Critical Zone, whereas it occurs as a cumulus phase in rocks from the upper subzone in the Critical Zone, Main Zone and Upper Zone. (2) A distinctive compositional difference of plagioclase and orthopyroxene can be seen between the lower subzone and upper subzone in the Critical Zone. In the lower subzone in the Critical Zone, post-cumulus plagioclase has low An within the range of 74-50,whereas the upper subzone has An=83-63. This result is consistent with previous work. In addition, Norite in the upper subzone in the Critical Zone has orthopyroxene-developed (Opx-rich) and clinopyroxene-developed (Cpx-rich) domains. In the Opx-rich domain, high-X_<Mg> orthopyroxene and low-An plagioclase inclusions can be seen. On the other hand, both low-X_<Mg> orthopyroxene inclusions and high-An plagioclase inclusions are observed in the Cpx-rich domain. These textural and compositional relationships suggest possible magma mixing on the thin section scale.