THE OLDEST AGE LIMIT OF THE LOMAGUNDI-JATULI EVENT: ZIRCON GEOCHRONOLOGY OF THE PALEOPROTEROZOIC MAGONDI SUPERGROUP IN ZIMBABWE

In order to clarify the detailed crust history of the Zimbabwe Craton and to constrain the timing of the largest carbonate carbon isotope fluctuation in the Earth’s history (the Lomagundi-Jatuli event; LJE), we analyzed zircon U-Pb ages of sedimentary rocks (the Deweras and Lomagundi Groups of the Magundi Supergroup) and granitoids in the northwestern Zimbabwe Craton. The 2.2-2.1 Ga Magondi Supergroup on the Zimbabwe Craton is regarded as a passive continental margin deposits. All zircons in the granitoids show igneous ages of ca. 2.65 Ga, which are consistent with the previously reported 2.65 Ga age of the Wedza granitoid suite. One granitoid sample includes 2.9 Ga and 2.7 Ga xenocrysts which were likely derived from older granitoids of this region (e.g. ca. 2.9-2.85 Ga Chingezi and 2.7 Ga Sesombi suites). The youngest zircon from the Deweras Group recorded a ²⁰⁷Pb-²⁰⁶Pb age of 2,216±22 Ma, which is about 50 m.y. younger than previously reported U-Pb age of baddeleyite in a troctolitic dyke in the underlying basement of the Magondi Supergroup. The new ages constrain the onset timing of LJE to be older than 2,216±22 Ma, in accordance with the previous dating result of 2,221±5 Ma from the Peraphoja Belt in Finland. Two sandstones from the Lomagundi Group yielded only Archean zircons of 3.0-2.6 Ga. Archean detrital zircons from the analyzed samples commonly show three age peaks at ca. 2.9-2.85 Ga, ca. 2.7 Ga, and ca. 2.65Ga. These correspond respectively to the ages of the Chingezi, Sesombi, and Wedza suites which are distributed in the NW region of the Craton. One zircon grain of 3.0 Ga is regarded to have derived from the Tokwe gneiss segment which is the oldest unit in the Craton and located about 200 km to the south from the distribution of the Magondi Supergroup. Grains older than 3 Ga were not found in this area, although dominant occurrence of >3 Ga zircons were reported from older Archean sediments in the center part of the craton. Judging from the < 3 Ga ages of the basement, we interpret that Mesoarchean crusts once dominated in Zimbabwe but they have disappeared later in the Neoarchean probably through active crustal reworking/recycling.