Zircon formation in mafic and felsic rocks of the Bushveld Complex, South Africa: Constraints from composition, zoning, Th/U ratios, morphology, and modelling

Abstract

Zircon is a potential petrogenetic indicator that can be used to derive physicochemical conditions during magma crystallization. In this study, such conditions are obtained from zircon of both felsic and mafic rocks of the Bushveld Complex (BC), which are characterized by a wide range in bulk-rock Zr contents (4–552 ppm). For that, information from bulk-rock compositions, petrography, zircon trace element data and morphologies are combined with results of thermodynamic modelling using the software packages rhyolite-MELTS and Perple_X. In felsic rocks (Lebowa Granite, Rashoop Granophyre), zircon is formed in rutile-free assemblages together with olivine-clinopyroxene-amphibole-biotite-ilmenite-titanite-apatite after ≥20% fractional crystallization at 761–935 °C (mean: 860 °C), based on Ti-in-zircon thermometry using aTiO2 = 0.3, in agreement with independent geothermometers and modelling results. The resulting zircon populations show {100}- and {101}-dominated morphologies as well as high ƩREE (mean: 651 ppm) and low Ti contents (mean: 9.5 ppm), and only minor zoning in Th/U (0.3–0.8) and Nb/Ta (1.2–4.4). Identical zircon characteristics in gabbros and diorites of the Upper Zone within the Rustenburg Layered Suite (RLS) suggest admixing of felsic melts during ingression of mafic parental magmas. In contrast, zircons in mafic rocks of the lower RLS (Basal Ultramafic Sequence to lower Main Zone) show significantly lower ƩREE (mean: 324 ppm), commonly higher Ti contents (8–60 ppm), as well as large variations in Ti, U, Th contents, Th/U (0.2–24) and Nb/Ta ratios (0.15–18) as well as in zircon morphology. These zircons mostly occur in rutile-bearing intercumulus domains associated with orthopyroxene-biotite-amphibole-plagioclase-quartz-rutile and are formed at 690–962 °C (mean: 835 °C; aTiO2 = 1.0) based on Ti-in-zircon thermometry. These temperatures are in good agreement with zircon morphologies, but mostly higher than those obtained by rhyolite-MELTS modelling, suggesting zircon growth at <810 °C after >75% fractional crystallization of high Mg andesitic (B1) and tholeiitic parental magma (B2). These lower temperatures perhaps result from oversimplified modelling parameters or may reflect variable mixing of parental magma with evolved resident magma. Zircon populations in rutile-bearing mafic rocks of the lower RLS reveal two distinct zoning trends: an early trend at high Ti (>20 ppm), characterized by increasing Th/U (0.5 to 18) at decreasing U (175 to 10 ppm) from core to rim and a reverse trend at lower Ti contents. Both trends require zircon formation in intercumulus melt pockets at high zircon/melt ratios. The high-Ti trend can be explained by Rayleigh-like fractionation due to zircon growth together with rutile, both having highly different partition coefficients for U ≫ Th. The low-Ti trend results from zircon growth after onset of the biotite-in reaction, causing breakdown of previously formed rutile, thereby releasing U but no Th. The absence of pronounced Th/U zoning of zircon in felsic rocks reflects zircon growth in less fractionated melts, resulting in ThU fractionation compensated by coeval crystallization of abundant rock-forming minerals, all being highly incompatible for Th and U.