GEOCHRONOLOGICAL CONSTRAINTS OF THE PALEOARCHEAN SHURUGWI GREENSTONE BELT IN THE ZIMBABWE CRATON

Archean greenstone belts are significant to reveal geotectonic history in the early Earth. The Zimbabwe craton in southern Africa is one of the Archean continental block. The craton contains numbers of the Archean greenstone belts which are distinguished into four generation, i.e. the ca. 2.6 Ga Shamvian, ca. 2.7-2.8 Ga Bulawayan, ca. 2.9 Ga Belingwean, and the oldest ca. 3.4 Ga Sebakwian. The Shurugwi Greenstone belt (SGB) is one of the most geologically well described unit among the Sebakwian. Previous studies of structural geology indicated that the SGB thrusted onto the allochthonous granitoid basement, and geological structures of it has similarity to those of collision-type orogen. However, chronological constrain for the belt has been limited except for whole rock age of the Mont d’Or granite which intruded into the SGB and detrital zircon ages from the Wanderer Formation conglomerate in the upper unit of the SGB. This study reports new zircon U-Pb age data from clastic rocks in the SGB, the Mont d’Or granite, and granitoid basement around the belt by using LA-ICPMS. Most of the detrital zircons from sedimentary rocks in both the Wanderer Formation in the upper unit and the Mont d’Or Formation in the lower unit were highly metamicted. Few numbers of less metamicted zircons show concordant age around ca. 3.6 Ga. Zircons from the Mont d’Or granite intruded into the SGB show ca. 2.9 Ga, and those from granitoid beneath the SGB show ca. 3.0 Ga. These results indicate that overthrusting of the SGB occurred between 2.9 and 3.0 Ga. Previous constrain for 3.4 Ga deposition of the SGB was removed by our result, nonetheless, the 3.4 Ga depositional age has no contradiction to our result. Provenance of the 3.6 Ga detrital zircons in the SGB might have been in the extant basement of the Zimbabwe craton, but there are still no report of the zircon U-Pb age of the 3.6 Ga granitoid in the craton. Although the geotectonic history of the SGB has room to discuss, this study confirmed allochthonous origin of the SGB and constrain the timing of the thrusting between ca. 2.9 and 3.0 Ga.