Neoproterozoic reworking of the Ubendian Belt crust: Implication for an orogenic cycle between the Tanzania Craton and Bangweulu Block during the assembly of Gondwana

Abstract

The Paleoproterozoic pelitic gneisses from the Ubendian Belt host Neoproterozoic eclogites (590–520 Ma) that formed in a subduction zone. The Neoproterozoic subduction event in the Ubendian Belt was followed by a collision between the Tanzania Craton and the Bangweulu Block and tectono-metamorphic reworking of the Ubendian Belt rocks. This study focuses to characterize the P-T-t conditions of the Neoproterozoic collisional event in the Ubendian Belt and its regional geodynamic significance.Despite hosting Neoproterozoic eclogite, the Ubendian Belt pelitic gneisses have zircons that yield concordant metamorphic ages between 1949 ± 16 and 1901 ± 37 Ma (U-Pb zircon, 2σ errors) having neither clear younger metamorphic ages nor Neoproterozoic rim overgrowth. However, monazite crystals from the same rocks yield clear younger rims with ages between 566 ± 8 and 556 ± 5 Ma overgrowing old cores.Monazite inclusions in garnet yields an age of 566 ± 8 Ma, which implies garnet growth during the Neoproterozoic tectonometamorphic event. Garnet and the associated minerals yield high-pressure metamorphic P-T conditions (P = 9–12 kbar; T = 760–820°C) which indicates a low geothermal gradient of 17–22 °C/km. A clockwise P-T-t path (kyanite to sillimanite reactions) with nearly isothermal decompression (growth of symplectites) after the peak of metamorphism was deduced.The subduction of an oceanic basin between the Tanzania Craton and the Bangweulu Block started at about 590 Ma and resulted in a major collisional event between the Tanzania Craton and Bangweulu Block between 570 and 550 Ma. The collision ended at about 500 Ma during the final stages of Gondwana Supercontinent amalgamation. For this collisional event we therefore propose the term Ufipa orogeny.During the collision, the Ubendian Belt gneisses and the margins of the Bangweulu Block formed part of a passive continental margins that subducted to attain high-pressure metamorphic conditions. The absence of zircons with Neoproterozoic metamorphic ages from older reworked crusts, the lack of pre-collisional volcanic arc, and low geothermal gradient support the metacratonitization process of the Ubendian Belt and the Bangweulu Block.