AGES OF THE BUSHVELD COMPLEX

Determining the precise age and duration of magmatism of the Bushveld Complex, South Africa, the world’s largest layered intrusion, has been a longstanding problem. Mafic layered intrusions are typically assumed to represent simple stratigraphic successions of cumulates without significant time gaps between units. We have determined high-precision U-Pb zircon ages using the chemical abrasion-ID-TIMS technique, with EARTHTIME tracers and synthetic solutions, for 12 samples covering the entire stratigraphic sequence of the 8 km-thick Bushveld Complex. These dates establish a 5 million year record of magmatism and reveal the presence of distinct stratigraphic time gaps and non-stratigraphic ages. Based on zircon crystallization ages for samples in the Upper Critical Zone, including two different pyroxenites from beneath the PGE-rich UG2 chromitite and the Bastard Reef just below the Main Zone contact, the earliest phases of Bushveld magmatism occurred at ca. 2059-2060 Ma, coeval with the nearby ca. 2060 Ma Phalaborwa carbonatite. A pyroxenite from the Lower Critical Zone and two samples of the PGE-rich Merensky Reef (Western and Eastern Lobes) near the top of the Upper Critical Zone are identical in age at ca. 2057 Ma (age revised from [1]), indicating that the Merensky Reef was emplaced into pre-existing and solidified cumulates of the Upper Critical Zone. The most voluminous magmatic episode in the Bushveld Complex occurred in the interval of 2055-2056 Ma, including a harzburgite from the Lower Zone and a norite from the lowermost Main Zone (“Tennis Ball Marker”) at ca. 2056 Ma, and an Upper Zone gabbro ~50 m below the roof and various felsic roof rocks at ca. 2055 Ma. These dates support the proposal that some of the overlying Rooiberg Group felsites and related granophyres are fractional crystallization products of Bushveld mafic liquids [2]. The U-Pb geochronological results for the Bushveld Complex, combined with on-going results from the Stillwater Complex, indicate that distinct time gaps and non-stratigraphic ages may well be typical of major mafic layered intrusions formed under open-system processes.

[1] Scoates and Friedman (2008) Economic Geology 103:465–471; [2] Mathez, VanTongeren, and Schweitzer (2013) Contributions to Mineralogy and Petrology 166:435–449.