Neoproterozoic eclogites in the Paleoproterozoic Ubendian Belt of Tanzania: Evidence for a Pan-African suture between the Bangweulu Block and the Tanzania Craton

Abstract

Geochronological, petrographic and geochemical data from eclogites of the Ufipa Terrane in the Ubendian Belt demonstrate that a Pan-African suture zone dated at 593±20, 548±39 and 524±12Ma (zircon U–Pb SHRIMP) separates the Tanzania Craton from the Bangweulu Block along the Ubendian Belt. These new and surprising data indicate that during the amalgamation of the Gondwana Supercontinent there was a collision between the Archean Cratons of Tanzania and Bangweulu. A clockwise P–T path that climaxed at pressures of 15–20kbar and temperatures of 610–790°C were estimated for these eclogites. This indicates a relatively warm subduction with a geothermal gradient of about 10–11°C/km.Magmatic precursor rocks of kyanite-free eclogites crystallized in the back-arc (group I eclogite) and island-arc (group II eclogite) tectonic settings. The light rare earth elements (LREEs) of group I eclogites range between 10 and 30 times chondritic values suggesting a depleted mantle source similar to that of mid oceanic ridge basalts (MORB). Group II eclogites display characteristic depletions of high-field-strength elements (Na, Ta, Zr and Hf) relative to LREEs that is typical for island-arc basalts.The U–Pb zircon ages at 593±20 and 524±12Ma from the kyanite-free eclogites have a difference of about 70Ma. The time interval of this much long is not likely to represent a single subduction event. Therefore, it is more likely that successive accretions of volcanic-arc rocks occurred at 593±20, 548±39 and 524±12Ma.