Abstract This paper shows how heavy minerals and single-grain varietal studies can be conducted on silt (representing c. 50% of world's sediments) sediments to obtain quantitative data as efficiently as for sand-sized sediments. The analytical workflows include heavy mineral separation using a wide grain-size window (15–355 μ) analysed through integrated optical analysis, Raman spectroscopy, QEMSCAN microscopy and U–Pb dating of detrital zircon. Upper Jurassic–Cretaceous silt-sized sediments from the Mandawa Basin of central-southern Tanzania have been selected for the scope of this research. Raman-aided heavy mineral analysis reveals garnet and apatite to be the most common minerals together with durable zircon, tourmaline and subordinate rutile. Accessory but diagnostic phases are titanite, staurolite, epidote and monazite. Etch pits on garnet and cockscomb features on staurolite document the significant effect of diagenesis on the pristine heavy mineral assemblage. Multivariate statistical analysis highlights a close association among durable minerals (zircon, tourmaline and rutile, ZTR) while garnet and apatite plot alone reflecting independence between the three groups of variables with garnet increasing in Jurassic samples. Raman data for garnet end-member analysis document different associations between Jurassic (richer in A, Bi and Bii types) and Cretaceous (dominant A, Ci and Cii types) samples. U–Pb dating of detrital zircon and their statistical integration with the above-mentioned datasets provide further insights into changes in provenance and/or drainage systems. Metamorphic rocks of the early and late Pan-African orogeny terranes of the Mozambique Belt and those of the Irumide Belt acted as main source of sediment during the Jurassic. Cretaceous sediments record a broadening of the drainage system reaching as far as the Usagran–Ubendian Belt and the Tanzanian Archean Craton.

To further advance our understanding of the way in which a portion of the African superswell in eastern Africa formed, and also to draw attention to the importance of eastern Africa for the plume versus plate debate about mantle dynamics, upper-mantle structure beneath eastern Africa is reviewed by synthesizing published results from three types of analyses applied to broadband seismic data recorded in Tanzania, Kenya, and Ethiopia. (1) Joint inversions of receiver functions and surface wave dispersion measurements show that the lithospheric mantle of the Ethiopian Plateau has been significantly perturbed, much more so than the lithospheric mantle of the East African Plateau. (2) Body wave tomography reveals a broad (≥300 km wide) and deep (≥400 km) low-velocity anomaly beneath the Ethiopian Plateau and the eastern branch of the rift system in Kenya and Tanzania. (3) Receiver function stacks showing Ps conversions from the 410 km discontinuity beneath the eastern branch in Kenya and Tanzania reveal that this discontinuity is depressed by 20–40 km in the same location as the low-velocity anomaly. The coincidence of the depressed 410 km discontinuity and the low-velocity anomaly indicates that the low-velocity anomaly is caused primarily by temperatures several hundred degrees higher than ambient mantle temperatures. These findings cannot be explained easily by models invoking a plume head and tail, unless there are a sufficient number of plume tails presently under eastern Africa side-by-side to create a broad and deep thermal structure. These findings also cannot be easily explained by the plate model. In contrast, the breadth and depth of the upper-mantle thermal structure can be explained by the African superplume, which in some tomographic models extends into the upper mantle beneath eastern Africa. Consequently, a superplume origin for the anomalous topography of the African superswell in eastern Africa, in addition to the Cenozoic rifting and volcanism found there, is favored.

New and published whole-rock major-element contents of xenolithic peridotites, combined with mineral trace-element, 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, and 3 He/ 4 He isotopic compositions, are used to unravel the metasomatic history of lithospheric mantle sampled by volcanic pipes in the Tanzanian section of the East African Rift. The deepest portion of the mantle beneath Labait (craton margin) exhibits high-μ (HIMU)–like 87 Sr/ 86 Sr (0.7029), 143 Nd/ 144 Nd (0.51286), and 3 He/ 4 He (5.9), which may reflect the plume in this region. Within the Mozambique belt, recent calcio-carbonatite melt metasomatism has overprinted the mantle lithosphere signature beneath Olmani, leading to high whole-rock Ca/Al and low SiO 2 , and remarkably homogeneous 87 Sr/ 86 Sr (0.7034–0.7035) and 143 Nd/ 144 Nd (0.51281–0.51283) of clinopyroxenes. Identical Sr and Nd isotope values are also reported for clinopyroxenes from peridotite xenoliths from the northern portion of the Gregory Rift in Tanzania (Pello Hill and Eledoi), which have a strong rift magma overprint. The silicate and carbonatite metasomatic melts are likely to be related to recent plume-derived magmatism of the East African Rift, and thus 87 Sr/ 86 Sr and 143 Nd/ 144 Nd values of the clinopyroxene from these samples can be used to define the rift isotopic signature beneath northern Tanzania. Some mantle regions beneath Lashaine and Labait escaped the recent rift-related overprint and have highly variable Sr-Nd isotope systematics. Labait clinopyroxenes show a near-vertical array on a 87 Sr/ 86 Sr versus 143 Nd/ 144 Nd plot, indicating highly variable time-integrated rare earth element (REE) patterns and low time-integrated Rb/Sr. Lashaine peridotites range to much higher 87 Sr/ 86 Sr at a given 143 Nd/ 144 Nd, and several plot in the right quadrants in the 87 Sr/ 86 Sr versus 143 Nd/ 144 Nd diagram, suggesting the influence of a (subducted?) Archean upper continental crust component on the lithospheric mantle beneath Lashaine. Their variable whole-rock SiO 2 and high Na 2 O contents, and clinopyroxene with high Sr/Y, low Sm/Nd, and variable Zr/Sm are consistent with this interpretation. Silicate lavas from the eastern branch of the East African Rift show increasingly evolved Sr and Nd isotope composition from north to south and hence increasing input of ancient metasomatized lithosphere (“EM1” and “EM2” components), similar to that beneath Lashaine and Labait, and well outside the suggested range in isotope compositions of the heterogeneous Kenya plume ( 87 Sr/ 86 Sr = 0.7029–0.7036; 143 Nd/ 144 Nd = 0.51275–0.51286). In the western branch, the “anomalous” Sr signature identified in Lashaine peridotites is prominent in silicate lavas and may indicate that the lithospheric mantle beneath that area was similarly enriched during ancient subduction. By contrast, the Sr-Nd isotope systematics of carbonatites reflect EM1 but not EM2 inputs, suggesting that such melts in the East African Rift neither derive from nor have interacted with subduction-modified mantle regions.