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 is an exemplary site of preserved relics of Paleoproterozoic volcanic-arc magmas, and subducted and displaced oceanic crust. This paper 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 mid-ocean ridge basalt (N-MORB) and transitional MORB (T-MORB) 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 are an 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. Thus, the Konse pillow basalts might have been displaced from their suprasubduction-zone tectonic setting of igneous construction within the Yalumba ocean basin around 2000 Ma.