MANTLE SOURCE AND MAGMATIC EVOLUTION OF THE LATE PLIOCENE OGOL LAVA BEDS OF LAETOLI, TANZANIA

Lava flows and several small cones are exposed in a 50 square kilometer region that includes the Laetoli Archeological Site in northern Tanzania. The flows are of interest to archeological studies because of their potential as a source for stone tools (see Adelsberger et al., this volume). The flows and cones are also geologically interesting because they were erupted in a region west of the major fault that bounds the Ngorongoro volcanic center and the western margin of the East African Rift system (Gregory Rift).

The flows are typically several meters thick and have well-defined flow-tops and interiors. Most flows are vesicular and contain <10% phenocrysts of olivine and pyroxene in a fine-grained matrix of pyroxene and plagioclase groundmass; plagioclase phenocrystss are rare, but amphibole is present as a minor phase in a several samples. The cones, measuring up to 100 meters in height and one kilometer in diameter, consist of agglomerate and scoria; volcanic bombs are up to 2 meters in length. Small xenoliths of gneiss and pyroxenite are present in some flows and volcanic bombs. The volcanic rocks are classified as basalt on a TAS diagram and are futher classified as alkali-olivine basalt on the basis of normative nepheline (0-10%) and olivine (10-20%). Many of these basalts have high Mg#'s ([Mg]/[Mg+Fe] = 0.57-0.65), (Ni (200-250 ppm), and Cr (500-650 ppm) indicating that they are relatively primitive. On chondrite-normalized trace element diagrams the samples are enriched in the LREE and Nb suggesting that they have not undergone significicant crustal contamination. On most major and trace element bivariate plots the samples plot along a narrow trends that can be modelled by fractional crystallization of single parental melts. The most primitive lavas (Mg#>0.6) exhibit a wide range of silica undersaturation (normative nepheline contents), but are characterized by relatively narrow ranges of Zr/Y, Zr/Nb, and La/Ce suggesting that the melts were produced from a single, relatively enriched mantle source by variable degrees of partial melting. The Ogol lavas (2.4 Ma) are younger and less undersaturated than the older nephelinites of Sadiman volcano (4.1-3.3 Ma) and are contemporaneous with the Pliocene-Recent alkali basalts of the Ngorongoro (2.5-2.0 Ma) and Lemagrut (2.3-1.9 Ma), reflecting an overall decrease in alkalinity with time.