Hafnium Isotopic Composition of the Bushveld Complex Requires Mantle Melt–Upper Crust Mixing: New Evidence from Zirconology of Mafic, Felsic and Metasedimentary Rocks

The origin of magmas that formed the Bushveld Complex remains highly debated in spite of many decades of intense research. Previous geochemical–petrological studies have shown a strong mantle derivation resulting ultimately in highly economic ore bodies of platinum group elements and chromium. However, geochemistry also points to the contribution of a significant crustal component, which may have been derived singly or in combination from a number of different sources. These include subcontinental lithospheric mantle that was enriched prior to Bushveld magma formation, possibly by subduction, assimilation of lower and upper crust during magma ascent, and contamination during magma chamber accretion within sedimentary rocks of the enclosing Transvaal Supergroup. In this study, the contributions of these various reservoirs will be evaluated by employing Hf isotopic data of well-characterized zircon grains in mafic, felsic and metasedimentary rocks, together with Zr–Hf bulk-rock compositions. The results reveal that magmatic zircon grains in mafic cumulate rocks from the floor to the roof of the c. 9 km thick Rustenburg Layered Suite (RLS) show essentially the same variations in εHf2·055 Ga from −7·5 to −10·2 as those of metamorphic zircon grains and overgrowths in the immediate surrounding quartzite and metapelitic rocks, as well as in granitic melt batches, granophyres, and the upper Rooiberg volcanics. The same values are also obtained by estimating the average Hf isotopic compositions of detrital zircon grains in many quartzite and metapelitic rocks from the surrounding Magaliesberg (εHf2·055 Ga = −6·2 to −10·8, six samples, maximum deposition age at 2080 Ma) and Houtenbeck formations (εHf2·055 Ga = −7·1 to −8·9, three samples, maximum deposition age at 2070 Ma), and by a six-point isochron of a garnet-schist from the Silverton Formation (εHft = −6·6 ± 0·7; age = 2059·4 ± 2·7 Ma). Zircon morphologies, zoning patterns, Hf isotopic data and petrological constraints furthermore reveal that metamorphic zircon was precipitated from aqueous fluids and/or felsic melts at temperatures between 550 and 900 °C, and that the Hf isotopic composition became homogenized during fluid transport in the contact aureole. However, results of numerical modelling indicate that fluid infiltration had only a minor effect on the Zr–Hf budget and Hf isotopic composition of the RLS, and that these parameters were mainly controlled by the mixing of melts derived from three major sources: (1) the asthenospheric mantle (>20 %); (2) enriched subcontinental lithospheric mantle (<80 %); (3) assimilation of significant amounts of crust (up to 40 %). The modelling furthermore suggests that assimilation of lower Kaapvaal Craton crust was minor (<15 %) during B1 (high-Mg andesite) magma formation, but up to 40 % during B3 (tholeiite) magma formation. The minor variation in εHft of zircon throughout the entire stratigraphy of the RLS resulted from the interplay of three dominant contributing factors: (1) intrusion of hot (>1200 °C) mantle-derived magmas with relatively low Zr–Hf concentrations having a similar εHf2·055 Ga of −8·5 ± 1·9 to that of upper crust rocks surrounding the RLS; (2) significant assimilation of volcanic and metasedimentary rocks with high Zr–Hf concentration; (3) mingling, mixing and/or diffusive exchange of Zr and Hf between crust and mantle-derived melts and aqueous fluids prior to late-magmatic crystallization of zircon at temperatures between 700 and 900 °C. This study shows that the combination of Zr–Hf bulk-rock data with Hf isotopic data of well-characterized zircon grains provides a powerful tool to quantify various mantle and crustal reservoirs of mafic layered intrusions, and allows new insights into magma chamber and related contact metamorphic processes.