The northern Tanzanian sector of the Gregory Rift plays a key role in the understanding of different scientific problems that span the fields of petrology, volcanology and anthropology. There are only few areas on Earth, where continental rifting is currently active. The East African Rift System is one of these areas, and the Gregory Rift is one of the most multifaceted parts of it, where early and recent episodes of the continental rift development are recorded in exposed igneous and sedimentary rocks. One of the challenges related to continental rifting is to explain the broad compositional and isotopic diversity of generated magmas. The genesis of alkali-rich carbonatites at the Oldoinyo Lengai volcano is one of the most frequently studied aspects of this chemical diversity. However, the carbonatites constitute a volumetrically insignificant part of the volcano’s total volume. In order to fully understand the role of carbonatite genesis in relation to continental rifting processes, it is therefore also important to investigate the associated, and considerably less-studied, silicate rocks. In addition to this, several anthropologically important sites occur in the Gregory Rift, which record the premodern human evolution at the Pliocene-Holocene transition.
In this thesis, the silicate rocks from the northern Tanzanian sector of the Gregory Rift are used to investigate the temporal and compositional evolution of the area, with special emphasis on the evolution of silicate magmas at the currently active Oldoinyo Lengai volcano.
For a better understanding of magma generation in a continental rift environment, isotope characteristics of a representative suite of rocks belonging to the northern sector of the Gregory Rift were investigated. The oldest of these (3.8-1.4 Ma) are exposed in the Rift Escarpment, and the youngest rocks are represented by different rocks from Oldoinyo Lengai and from the Lake Natron Engaruka-Monogenetic Volcanic Field (LNE-MVF). The results show that Rift Escarpment magmas were produced from a heterogeneous mantle source, where the heterogeneity is the result of carbonatite-related metasomatism.
The eruptive centres of the considerably younger LNE-MVF produced the most primitive (melilititic) magmas in the area. These magmas originate from a homogeneous mantle source, while the more evolved (nephelinitic and basanitic) magmas from the same area are derived from a slightly more heterogeneous source.
The silicate lavas from Oldoinyo Lengai define two separate geochemical trends, which reflect two distinct stages of volcanism at Oldoinyo Lengai. During the early stage, the volcano erupted various nephelinitic and phonolitic magmas with enriched isotopic compositions. The evolution of these magmas was controlled by crystal fractionation. A total fractionation of approximately 90% crystals allows to successfully reproduce the observed geochemical trend. The second stage of volcanism, which is currently ongoing, is characterized by eruptions of nephelinitic magmas with phenocrysts of wollastonite,
combeite and melilite. These magmas are also intimately associated with the alkali-rich carbonatites. The evolution of magmas belonging to Stage II is controlled by crystal fractionation up until the point of carbonatite exsolution, after which the silicate magmas evolve by a combination of processes such as fractionation, remelting and assimilation of cumulates left over from the first stage of volcanism and/or magma mixing with crystal mushes. The pronounced differences in geochemistry as well as in the isotope characteristicsof the volcanic rocks belonging to the first and the second stage of evolution indicate different primary magmas for the two series. A combination of isotope mixing calculations and fractional crystallization modelling indicates that the primary magma(s) of the second stage can be explained by interaction (partial remelting and assimilation) of cumulates left over in the crust from the first stage of volcanism with magma(s) that are geochemically and isotopically similar to the primitive magmas erupted in the neighbouring LNE-MVF.
The second part of the thesis provides the first steps aimed at establishing a tephrostratigraphic framework for this part of the Gregory Rift. The recently discovered Engaro Sero Footprint Site hosts one of the best-preserved sets of fossilized hominid footprints in the world, which are preserved in a sequence of volcaniclastic sedimentary rocks. In this thesis, it is shown that the footprint-bearing horizon consists of a primary volcanic ash-fall that has been slightly reworked by water, and that it was produced during a voluminous eruption of the Oldoinyo Lengai volcano. The sedimentary unit, which covered the footprints and helped to preserve them, consists of the wind-blown material derived from the same eruption intermixed with locally derived detrital material. A combination of field observations, regional stratigraphy and a possible correlation with ash layers preserved in the lacustrine sequence of Lake Emakat, located 20 km SW of the volcano, indicates an early Holocene age of the footprints.
In a sediment core from Lake Emakat, seven distinct ash layers of late Pleistocene – early Holocene age were described interbedded with a sequence of lacustrine sediments. These ash horizons have nearly identical mineralogy to that found in most recent and pre-modern pyroclastic rocks of Oldoinyo Lengai. Based on information from this sediment core, an average recurrence rate 810 years between voluminous explosive eruptions can be calculated for Oldoinyo Lengai during this time span. This data fill an important
informational gap regarding the eruptive history of the volcano. The main reason for this is that the written accounts for the area only cover the last ~150 years, and reliable age determinations are sparse at best
