Origin and significance of noritic blocks in layered anorthosites in the Bushveld Complex, South Africa

• Published in: Contributions to mineralogy and petrology 2022, Vol.177 (1), Article 11
• Main Authors: Bourdeau, J. E., Hayes, B., Zhang, S. E., Logue, A., Bybee, G. M.
• Format: Article
• Language: English
• Subjects: Analysis; Assimilation; Chambers; Composition; Earth and Environmental Science; Earth Sciences; Flow rates; Flow velocity; Fossils; Geology; Isotopes; Lava; Magma; Magma chambers; Mathematical models; Mineral Resources; Mineralogy; Numerical models; Olivine; Original Paper; Petrogenesis; Petrology; Plagioclase; Slurries; Strontium; Strontium 87; Strontium isotopes
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-021-01872-8

Layered mafic–ultramafic intrusions, such as the Rustenburg Layered Suite (RLS) of the Bushveld Complex (South Africa), are the fossilized remnants of upper crustal magmatic plumbing systems and they can be studied to provide important insights into magma chamber dynamics. In this study, we describe the field and petrological relationships, as well as the chemistry (mineral, bulk-rock majors, traces and Sr–Nd isotopes) of 13 blocks that are hosted in a layered sequence of poikilitic anorthosites in the upper Main Zone of the RLS to constrain their petrogenesis. The blocks (olivine-bearing norite, norite and poikilitic norite) are 0.2 to ~ 6 m in size and are rectangular in shape. The blocks are distinct in atomic Mg# (average Mg# 73.9 mol%) and in Sr–Nd isotopes (average initial 87 Sr/ 86 Sr of 0.707344 and ε Nd of − 5.23 at 2.06 Ga) compared to their host anorthosites (average Mg# 61.2 mol%, initial 87 Sr/ 86 Sr of 0.707291 and ε Nd of − 8.31 at 2.06 Ga). We suggest that the blocks represent noritic material that formed in a conduit or staging chamber system below the RLS and were carried into the RLS by carrier magmas (plagioclase slurries). The blocks have Sr–Nd isotopic compositions that can be explained by the assimilation of upper crust, whereas the Sr–Nd isotopic compositions of the host anorthosites can be explained by the assimilation of lower crust. We also develop a numerical model in which we establish the crystallinity and magma flow rate of the plagioclase slurries that carried these cumulate blocks into the RLS.