Progress in resolving the duration of magmatism in the Paleoproterozoic Bushveld Complex

Determining precise crystallization ages and assessing the duration of magmatism associated with the Paleoproterozoic Bushveld Complex, the world's largest layered intrusion located in the northern Kaapvaal craton of South Africa, is critical for establishing the genetic relations among its different rock units (Rustenburg Layered Suite, overlying Rooiberg Group felsic volcanic rocks, intrusive Rashoop Granophyres). We report chemical abrasion ID-TIMS U-Pb zircon results (weighted ²⁰⁷Pb/²⁰⁶Pb ages) for samples spanning the uppermost ~3/4 of the igneous stratigraphy of the layered mafic rocks and the roof. The temporal relationship between the Upper Zone and the roof from the Eastern Limb is constrained by the age of a diorite ~52 m below the roof (2056.52 ± 0.81 Ma; n = 6 single grains) and a granodiorite mixed with hornfels (40:60) and interpreted as partially melted roof material (2054.83 ± 0.86 Ma; n = 7). The slightly younger age of the granodiorite may indicate that the overlying melt sheet was kept molten due to continued influx of the latent heat of crystallization from the underlying 8 km-thick stack of cooling cumulates. The PGE-rich Merensky Reef and the UG2 chromitite occur in the Upper Critical Zone above the ultramafic rocks that comprise the base of the complex. Two samples from the Merensky Reef, separated by ~300 km, have overlapping ages: a sample from Farm Driekop in the Eastern Limb yields an age of 2055.30 ± 0.61 Ma (n = 10) and a sample from the Rustenburg mining section in the Western Limb an age of 2056.13 ± 0.70 Ma (n = 8; age revised from [1]). These ages are indistinguishable from the Upper Zone diorite indicating rapid construction of the upper 2/3 of the Bushveld Complex at ca. 2056 Ma. Preliminary results for zircon from a feldspathic orthopyroxenite immediately beneath the UG2 chromitite (Middelpunt mine, Eastern Limb), ~380 m below the Merensky Reef, reveal unusual U-Pb systematics with low U concentrations (2-27 ppm), exceptionally high Th/U (7.3-28.1) and a range of ²⁰⁷Pb/²⁰⁶Pb ages (2057.8 to 2075.0 Ma; n = 6, all results concordant). These results may reflect a combination of the compositional effects of co-existing U-bearing accessory phases (apatite, rutile) and formation by metasomatism and partial melting of the pyroxenite protolith by rising hydrous interstitial melt that accumulated beneath the impermeable chromitite layer [2]. The ages for additional samples in progress will also be presented to further test the proposal that 15-25% of the original magma volume in the intrusion was expelled to form the Upper Rooiberg Group lavas or Rashoop granophyres [3]. [1] Scoates & Friedman (2008) Economic Geology 103, 465-471; [2] Mathez & Mey (2005) Economic Geology 100, 1616-1630; [3] VanTongeren et al. (2010) Journal of Petrology 51, 1891-1912.