HFSE (High Field Strength Elements)-transport and U–Pb–Hf isotope homogenization mediated by Ca-bearing aqueous fluids at 2.04 Ga: Constraints from zircon, monazite, and garnet of the Venetia Klippe, Limpopo Belt, South Africa

Abstract

Results from laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) and isotope dilution (ID) analyses of minerals and rocks from a single outcrop of the Venetia Klippe of the Limpopo Belt indicate that the U–Pb and Hf isotope system homogenized on the decimetre scale under amphibolite-facies conditions of ⩽645±25°C and ⩽7.0±1.1kbar, i.e. in the presence of an aqueous fluid phase. For a metabasite sample, homogenization is supported by isotope analyses of metamorphic zircon, garnet, and whole rock, which yield a six-point Lu–Hf isochron age of 2039.7±3.4Ma, with initial 176Hf/177Hf of 0.28126±0.00001, and a U–Pb zircon age of 2042±10Ma. The occurrence of a few inherited magmatic zircon cores with ages up to 2705Ma, and with significantly lower initial 176Hf/177Hf of 0.28112, however, indicate that homogenization was incomplete. For a chlorite–biotite–garnet schist isotope homogenization is reflected by within error identical zircon and monazite U–Pb ages of 2045±10Ma and 2041±8Ma, respectively, and by a zircon-garnet-whole rock Lu–Hf isochron age of 2083±63Ma, with an initial 176Hf/177Hf of 0.28140±0.00003. Contemporaneous formation of metamorphic zircon, monazite and garnet in the chlorite schist is not only supported by the isotope data, but also by chlorite inclusions in all three minerals, and by inclusions of metamorphic zircon in garnet. The inclusion textures and the identical initial 176Hf/177Hf support the conclusion that metamorphic zircon grains precipitated from an aqueous fluid phase, after dissolution of zirconium-bearing phases elsewhere, followed by a major HFSE transport, and Hf isotope homogenization. This fluid perhaps was Ca-bearing, as is suggested by the fact that garnet in the schist sample is the only Ca-bearing phase, and that metamorphic monazite, dating the metamorphic peak, is partially replacement by apatite. The fact that the metamorphic zircon rims in the metabasite sample have significantly lower initial 176Hf/177Hf than expected from the Lu–Hf isotope analyses of relic zircon cores and whole rock additionally hint that Ca-HFSE-bearing fluids infiltrated from surrounding quartzo-feldspathic gneisses, which had much lower 176Hf/177Hf at the time of metamorphism.