Spatial and Temporal Variations in Melting Within the Lithosphere Above the East African Mantle Plume - Evidence from the Mt. Meru/Mt. Kilimanjaro Region, N. Tanzania

The temporal variation in composition of Quaternary magmatism in northern Tanzania primarily reflects the diverse style of metasomatic enrichment that occurred within the SCLM as a consequence of the southward propagation of the EAR over the plume head. Low Β factors ( ~1.2) and thick lithosphere preclude a significant amount melt generation in the East African plume head beneath this part of the EAR. The earliest volcanic activity in this part of northern Tanzania occurred between 8 Ma and 2 Ma and is represented by regional flood lavas, the Monduli volcanic centre and early activity at Kilimanjaro. These magmas are mildly alkaline and volatile-poor. Fractionated heavy rare-earth element ratios and relative depletions at Ti, Rb and K are consistent with melting of a phlogopite/amphibole-bearing garnet peridotite source. This early melting appears to have been initiated by conductive heating of H₂O-rich metasomatic veins in the basal lithosphere (at depths up to ~150 km). Subsequent localised crustal extension at ~1.2 Ma immediately precedes the generation of small-volume silicate melts (melilitites and melanephelinites), which are characterized by relatively high Zr/Hf. These melilitites are typically associated with the eruption of carbonatite tuffs. The most recent phase of volcanism (<0.2 Ma), on the eastern flank of the EAR in northern Tanzania, is represented by strongly alkaline magmas at Mt. Meru, and mildly alkaline magmas at Mt. Kilimanjaro. The Meru lavas are characterised by similar compositions to CO₂-rich glasses that occur in mantle xenoliths from the nearby Olmani cone. The genesis of the Meru magmas requires wide-scale melting of CO₂-rich metasomatised lithosphere ( ~100 km) to produce the large volume (>150 km³) of erupted magmas. Conduction models show that conductive heating of the lithosphere by a mantle plume over the ~10 Ma volcanic history of the region can produce the required melting of the lithosphere