Paper No. 36-8
Presentation Time: 8:00 AM-12:00 PM
BULK LU-HF GARNET GEOCHRONOLOGY IN THE SOUTHERN MARGINAL ZONE, LIMPOPO BELT, SOUTH AFRICA
MADSEN, Lauren, Department of Earth and Environmental Science, Wheaton College, 501 College Ave, Wheaton, IL 60187, MANEIRO, Kathryn, Department of Earth and Environmental Science, Wheaton College, 501 College Ave., Wheaton, IL 60187, BROWN, Allison, School of the Earth, Ocean, and Environment, University of South Carolina, 701 Sumter Street, EWS 617, Columbia, SC 29208, DRAGOVIC, Besim, School of Earth, Ocean, and Environment, University of South Carolina, 701 Sumter Street, EWS 617, Columbia, SC 29208 and STEVENS, Gary, Department of Earth Science, Stellenbosch University, Private Bag X1, Matieland, 7602, South Africa
The Limpopo Belt represents a complex of at least three metamorphic belts between the Archean Kaapvaal and Zimbabwe cratons, the southern most of which, the Southern Marginal Zone (SMZ), is interpreted to have accreted to the Kaapvaal Craton at ca. 2.7 Ga1. Peak constraints for the 2.7 Ga age come mostly from zircon and monazite extracted from metapelitic granulites that experienced partial melting, while U-Pb rutile cooling ages suggest retrogression at approximately 2.1 Ga2. In addition, two older U-Pb garnet analyses in the SMZ provide conflicting ages of approx. 3.15 Ga and 2.69 Ga. Textural evidence indicating multiple generations of garnet growth, differences in mineralogy, and the complexity of existing ages lead to speculation over whether the SMZ might consist of sub-belts with different tectonometamorphic histories. This preliminary study provides a bulk Lu-Hf garnet age and a trace element analysis in order to begin resolving questions regarding the complexity of the tectonometamorphic events in the SMZ.
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
![[Visit Client Website]](/img/gsa/banner.jpg)