BULK LU-HF GARNET GEOCHRONOLOGY IN THE SOUTHERN MARGINAL ZONE, LIMPOPO BELT, SOUTH AFRICA
A sample of amphibolite facies metapelite from the SMZ was prepared by crushing, Frantz electromagnetic mineral separation, handpicking, and dissolution to remove excess biotite and accessory minerals. Following LA-ICP-MS trace element analysis, bulk Lu-Hf garnet geochronology was performed on five garnet fractions, a matrix sample, and two whole-rock samples following standard dissolution procedures and column chromatography to separate out Lu and Hf isotopes. These separates were analyzed using a Neptune MC-ICP-MS to determine the isotope ratios, and an isochron using two garnet fractions and a matrix point resulted in a preliminary bulk Lu-Hf age of 2834 +/- 5 Ma (MSWD: 20).
This bulk age prompts further analysis as it indicates an older age than expected for the peak metamorphic event in this portion of the SMZ. Potentially, the granulite facies and retrograde portions of the SMZ docked with the Kaapvaal Craton at different times. The age also indicates that a record of any 2.1 Ga retrograde event, if it occurred here at all, will be difficult to confirm even with zoned garnet analysis. Additional analyses are expected in spring 2022 to determine the impact of zircon inclusions on the age.
References: 1Nicoli et al., 2015, Journal of Metamorphic Geology; 2Madlakana et al., 2020, Precambrian Research