Palaeoproterozoic Kate granitoid suite provides evidence for slab break-off during the assembly of the Tanzania Craton and Bangweulu Block

Abstract

The Archean Tanzania Craton and the Bangweulu Block are separated by the Ubendian-Usagaran Belt, which developed during the Palaeoproterozoic Eburnean orogeny that produced the Congo Craton during the Columbia Supercontinent Cycle. Voluminous 1.80–1.90 Ga metagranitoids and volcanics were intruded both on the edge of the Bangweulu Block and in the Ubendian-Usagaran Belt in response to this geodynamic process. In recent years, research revealed that the extensive 1.90–1.80 Ga plutonic-volcanic suites exposed in the Ubendian- Usagaran Belt (on the Tanzania Craton side) resulted from widespread crustal anatexis caused by a regional thermal anomaly. However, the heating mechanism remains unclear. On the other (Bangweulu Block) side, high-precision chronological and geochemical data for the coeval Kate-Kipili felsic suite which crops out on the boundary of the Western Ubendian Corridor and the Bangweulu Block are scarce. The origin and tectonic setting of this suite are controversial, but crucial to understanding the crustal evolution during the Paleoproterozoic Ubendian orogeny. To address this issue, this paper is focused on the petrogenesis and tectonic setting of the Kate granitoids. New LA-ICPMS zircon U-Pb ages of the Kate granitoids revealed magmatic episodes from 1.87 to 1.90 Ga. Geochemically, the Kate granitoids belong to the magnesian alkalic-calcic rock series. They exhibit distinctive features including high Ga/Al and K₂O/Na₂O; high concentrations of Nb, Ta, Zr, and Y; and low Eu, Sr, Cr, Co, and Ni. The Kate granitoids could be classified into at least two groups in terms of geochemical characteristics, namely A2-type granites and slab failure plutons, respectively. The Kate-Kipili complex is considered to be related to the process of slab break-off, and the main difference between slab failure magmatism and A-type magmatism is probably the depth of the magma source. Slab break-off is the most likely mechanism for the magmatic intrusions and volcanism that occurred at 1.90–1.80 Ga in SW Tanzania and the Bangweulu Block.