Pillow lava basalts with back-arc MORB affinity from the Usagaran Belt, Tanzania: Relics of Orosirian ophiolites


The Paleoproterozoic Usagaran Orogenic Belt in East-Africa preserves relics of Paleoproterozoic volcanic-arc magmas, and subducted and displaced oceanic-crust. This article describes the geochemical characteristics of displaced pillow basalts from the Usagaran Belt (Konse Group). Our data indicate that the Konse pillow basalts have tholeiitic composition and are overprinted by greenschist facies metamorphic conditions but their primary geochemical signatures are preserved by REE and fluid immobile elements. The (La/Sm)N ratios (0.62 - 1.09) and REE patterns point to Normal-MORBs and Transitional-MORBs mantle source. The analysis of high-valency elements and trace element patterns points to a mixed signature of MORB tholeiites and island-arc tholeiites with elevation of Ba, Th, U, Eu and Sr. This composition is similar to that of Phanerozoic back-arc suprasubduction-zone ophiolites. Therefore, Usagaran Belt pillow basalts make analogue of the Tethyan-type suprasubduction-zone ophiolite evolution and emplacement in the Precambrian. The emplacement of the Konse pillow basalts is older than the neighbouring 1920 - 1870 Ma volcanic-arc magmas and is probably coeval with the formation of the 2000 Ma Yalumba eclogites with MORB affinity. Therefore, the Konse Konse pillow basalts might have been displaced from its suprasubduction-zone tectonic setting of igneous construction within the Yalumba ocean basin around 2000 Ma.

Journal of the Geological Society, London, v. 176, no. 5, p. 1007-1021, doi:10.1144/jgs2018-205