TRACE ELEMENT VARIATIONS IN ZIRCON FROM THE BUSHVELD COMPLEX, SOUTH AFRICA

Primary igneous zircon occurs in ultramafic-mafic cumulates of the Paleoproterozoic Bushveld Complex, South Africa, the world’s largest layered intrusion. Zircon is mostly anhedral, typical of interstitial morphologies, with simple to complex sector zoning as revealed by cathodoluminescence imaging. Trace element concentrations were determined by laser ablation (LA)-ICP-MS. Chondrite-normalized rare earth element (REE) patterns show typical depletion of light REE and enrichment of heavy REE with positive Ce (Ce/Ce* = 10-55) and negative Eu anomalies (Eu/Eu* = 0.19-0.43). Progressive zircon crystallization from fractionated interstitial melt in each sample is recorded by systematically increasing Hf concentrations and decreasing temperature constrained from Ti-in-zircon thermometry with all samples containing coexisting rutile + quartz. Zircon analyzed from both a Lower Zone harzburgite and from a norite at the base of the Main Zone has remarkably similar chemistry with sub-parallel REE patterns, Hf = 6290-9200 ppm, Ti = 8-38 ppm (Ti-in-zircon temperatures of 725-888°C), and similar ranges of Th and U (Th = 84-860 ppm, U = 79-704 ppm) with typical magmatic Th/U =0.79-1.59. Zircon from a gabbro near the top of the Upper Zone (UZ) and an overlying granophyre (Stavoren), part of the Rashoop Granophyre Suite, also reveal strong geochemical similarities, suggesting that they are related, with a narrow range of Hf = 6920-7370 ppm, low Ti =4-12 ppm (and correspondingly low temperatures from 658-759°C), and low Th/U = 0.36-0.63. Four pyroxenite samples from the Critical Zone, including the UG2 chromitite, Merensky Reef, and Bastard Reef, have zircon with more variable chemistry, higher and larger ranges of crystallization temperatures, and highly anomalous Th/U (up to 40) compared to the other Bushveld samples. In these pyroxenites, Hf = 6390-12,360 ppm, Ti = 8-50 ppm, corresponding to temperatures of 724-921°C, and typical Th (39-818 ppm) concentrations are coupled with low U (3-153 ppm). These results provide a detailed record of the geochemical evolution of fractionated interstitial melt in the Bushveld Complex over a wide temperature interval (~900°C down to below 700°C) that can be used to reconstruct the different magma compositions involved in its petrogenesis.