A relic of the Mozambique Ocean in south-east Tanzania

Abstract

The Neoproterozoic geology of East Africa records the events leading up to the formation of Gondwana and subsequently Pangea, and hence understanding the geodynamic history of this area offers a unique insight into supercontinent development. Eighteen new U-Pb zircon and two U-Pb monazite ages were collected in conjunction with 110 whole-rock lithogeochemistry analyses from a region of south-east Tanzania we designate the ‘Ntaka Terrane’. These new data suggest this area of the East African Orogen constitutes an exotic nappe terrane that was part of the Mozambique Ocean in the Neoproterozoic. The lithological sequence in the region consists of a 770–720 Ma ‘basement’ of intermediate-felsic paragneiss possibly derived from a volcano-sedimentary sequence and recording arc-like geochemical signatures. These are intruded by 700–650 Ma amphibolites (±garnet) with mafic-intermediate magmatic protoliths that also have arc-like geochemistry. This sequence is then intruded by a set of ca. 670–650 Ma ultramafic, dominantly pyroxenitic sills collectively termed the Ntaka Ultramafic Complex, which have a much more mantle-like, non-arc origin, possibly formed as the island-arc intersected a mantle plume ‘hot spot’. The mafic-intermediate intrusives occur synchronous with, and outlast, the high-Mg basalt magmatism, suggesting that this latter event was transient in the context of the overall tectonic regime. The final magmatic event in the terrane is the intrusion of 640–620 Ma syn-orogenic granites that are coeval with 640–600 Ma upper amphibolite-granulite facies metamorphism. The broader geodynamic setting consisted of the Congo-Tanzania-Bangweulu Block of east Africa to the west and the Azania Block to the east of the Neomozambique Ocean, both of which contain Archean-Paleoproterozoic detritus. Based on the island arc-like geochemical characteristics of the 700–650 Ma mafic intrusive rocks and the lack of detrital zircons > 1000 Ma, we suggest that the Ntaka Terrane represents a Neoproterozoic intra-oceanic island arc which was accreted to either East Africa or Azania shortly before full continental collision (the East African Orogeny) at ca. 640–620 Ma. Data from the Ntaka Terrane correlate with the Vohibory Terrane (Madagascar), Eastern Granulites (northern Tanzania) and Cabo Delgado Nappe Complex (Mozambique), suggesting these areas represent a set of broadly correlative oceanic terranes.