Abstract
Detrital zircon U–Pb geochronology and heavy mineral data from four main rivers in coastal southern Tanzania - the Rufiji, Matandu, Mbwemkuru and Ruvuma rivers - are compared with the bedrock geology of their catchment and with data from the Mesozoic strata of the Mandawa Basin, also in coastal Tanzania. The objective is to evaluate the source-to-sink pattern of sedimentation through time, between the Mesozoic and present day.The recent river sediments display variations in both heavy mineral assemblages and U–Pb zircon populations, reflecting their different catchment areas. The Matandu and Mbwemkuru rivers transport sediments characterised by amphibole-dominated heavy mineral assemblages, whereas the great Rufiji and Ruvuma rivers are characterised by more stable heavy mineral assemblages. The detrital zircon populations show age peaks at c. 2900–2500, 2000–1800, 1000, 800 and 700–500 Ma, the common denominators are the Late Mesoproterozoic (c. 1000 Ma) and Late Neoproterozoic (700–500 Ma) age fractions. The Rufiji River and the Matandu River display similar zircon age distributions, both containing abundant Palaeoproterozoic zircons, interpreted as recycled from Karoo successions in a part of the Selous Basin which is drained by both rivers. The Mbwemkuru and Ruvuma rivers contain mainly Late Mesoproterozoic and Late Neoproterozoic aged zircons that were supplied from the Unango and Marrupa complexes and the Cabo Delgado Nappe Complex/Eastern Granulites. The more diverse zircon population in the Rufiji and Matandu river is likely reflects recycling of older sedimentary successions in addition to sediments supplied from basement lithologies.
Highlights
Provenance evaluation of fluvial sediments allows us to indirectly examine erosion and sediment generation in the hinterland, by comparing the results with data from the rock record of the palaeo-drainage system (Alizai et al, 2011)
We have investigated heavy mineral assemblages and isotopic U–Pb zircon ages in sand from four major rivers along the southern coast of Tanzania: the Rufiji River, the Matandu River, the Mbwemkuru River and the Ruvuma River (Fig. 1)
Because zircon is a highly resistant mineral to both physical and chemical alteration it can survive several erosiontransport-deposition cycles, homogenising material from various protosources over long periods of time (Andersen et al, 2016). These recycled zircons will reflect the source of the sandstones, and preserve the memory of an ancient source-to-sink system. This is an important limitation for using detrital zircon geochronology in provenance studies because identifying recycled grains is in most cases not possible
Summary
Provenance evaluation of fluvial sediments allows us to indirectly examine erosion and sediment generation in the hinterland, by comparing the results with data from the rock record of the palaeo-drainage system (Alizai et al, 2011). Because zircon is a highly resistant mineral to both physical and chemical alteration it can survive several erosiontransport-deposition cycles, homogenising material from various protosources over long periods of time (Andersen et al, 2016). These recycled zircons will reflect the source of the sandstones, and preserve the memory of an ancient source-to-sink system. This is an important limitation for using detrital zircon geochronology in provenance studies because identifying recycled grains is in most cases not possible
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