Zircons of magmatic rocks can show enormous variations in Th/U ratios (0.2 to 100) and extreme Th/U zoning. We present data from felsic and mafic rocks of the Bushveld Complex in South Africa. Zircon grains in mafic cumulate rocks reveal Th/U ratios up to 70, those in felsic rocks barely exceed 1.0. In mafic rocks zircon mostly occur together with Rt-Bt-Kfs-Qtz in intercumulus domains, and crystallized during final magma cooling between 900 and 700°C, after >75% of fractional crystallization. The resulting zircons reveal very distinct Th/U zoning trends. Group (1) zircons show systematic increase in Th/U from core to rim (from 0.5 to 20), accompanied by a systematic decrease in U content (from >170 to 10 ppm), group (2) zircons the opposite trend, and group (3) zircons nearly no zoning. Modelling result reveal that all three zoning trends can be explained by minor differences in Bt-Rt-Zrc crystallization history. Trend (1) results from Rayleigh-like fractionation due to zircon growth (±Rt), having different partition coefficients for U ≫ Th. Trend (2) results from zircon growth after onset of biotite-in reaction, causing breakdown of previously formed rutile, thereby releasing U≫Th. Trend (3) results from mass balance constrains, causing mutual compensation of fractionation effects. The absence of pronounced Th/U zoning of zircons in felsic Bushveld rocks also results from compensation of zircon fractionation due to coeval crystallization of abundant rock-forming minerals (Opx-Cpx-Hbl-Pl-Kfs-Qtz) at an early stage of fractional crystallization (10-20%), all being highly incompatible for Th and U .  Gudelius et al. (2020). Chemical Geology 546, 119647.
Armin Zeh1, Dominik Gudelius2, Allan H Wilson3
1KIT, Germany; 2KIT, Germany; 3WiTs, Johannesburg, South Africa