WHAT DO BULK ROCK AND ZIRCON TRACE ELEMENT SIGNATURES AND TEXTURES TELL US ABOUT THE PETROGENESIS OF FELSIC ROCKS IN THE EASTERN BUSHVELD COMPLEX?

The Bushveld Complex, known for the mafic-ultramafic Rustenburg Layered Suite, has many granitic rocks that are commonly attributed to the Lebowa Granite Suite. In the Steelpoort region of the eastern Bushveld Complex several occurrences of these granites may be found. They are attributed to the Nebo Granite, the most voluminous sub-unit of the Lebowa Granite Suite. The Nebo Granite is a medium to coarse grained rock of Qt + Pl + Hbl with perthite; up-section there is less Hbl and plagioclase and an increase in Qt, Bt and perthite. In the Steelpoort region the Nebo Granite occurs as discrete intrusions with ambiguous relationships to their host rocks. Three occurrences of the granite around Steelpoort are differentiated from one another by structural-stratigraphic height and position relative to the Steelpoort Fault. Models for their petrogenesis include fractional crystallization, assimilation or crustal melting. However, the geologic and petrologic connection between these bodies and their relationship to the Rustenburg Suite remain uncertain. A study of accessory mineral assemblages from the Steelpoort region granites and their isotopic, trace element and textural properties is presented. Accessory mineral assemblages from granites at Magnet Heights and Tigershoek were separated into populations by grain size and textures and analyzed for trace elements by LAICP-MS. Many zircon populations have xenocrystic patchy- to mottled-zoned cores with concentrically zoned rims. Inclusions include ilmenite, apatite, magnetite, quartz, monazite, hornblende, and biotite. Other accessory minerals include magnetite and thorite. Absolute (U + Th) content in Magnet Heights zircon is greater than that of Tigershoek zircons. Most populations have gentle positive chondrite normalized REE patterns with small positive to no Ce anomalies and negative Eu anomalies. One Tigershoek population has much stronger Ce and Eu anomalies and steeper REE_CN patterns. Bivariate plots show positive correlation between Y vs Th, and weaker positive correlations between Hf vs (Th + U). The occurrence of inherited, xenocrystic cores supports mass contributions to these granites from partial melting and/or assimilation of the host-rocks to the Bushveld Complex. There is little evidence of a petrogenetic relationship between the Rustenburg Layered Suite and the studied granites.