BULK AND MINERAL COMPOSITIONS OF VOLCANICS FROM PLIO-PLEISTOCENE NGORONGORO VOLCANIC HIGHLAND (NVH), A POSSIBLE SOURCE FOR STONE TOOL MATERIALS AT OLDUVAI GORGE, TANZANIA

Samples from Ngorongoro Crater (~2.4 Ma), Lemagrut Volcano (2.0? Ma), Olmoti Crater (~1.85 Ma) and Sadiman Volcano in southeastern NVH and 13 fluvial cobbles from 1.75 Ma Bed II Olduvai Gorge (Tanzania) were analyzed for mineral and whole rock chemistry. The aim is to characterize the eruptive centers and determine if they are the sources for the fluvial cobbles that were also used for stone tools by early hominids.

Electron microprobe data show the highland to be characterized by complex fractionation histories of alkaline basic magmas that fractionate from basalt to trachyte and tephrite to phonolite. Zr, a fractionation index, ranges from 134.86 to 740.90 ppm in the less fractionated to the most fractionated samples. Zoned crystals and embayed grain boundaries are produced by multiple magma injections and/or disequilibrium melting. Basalt and trachyte whose pyroxenes are Ca43Mg33Fe24 define the Ngorongoro Crater. Trachy-basalt and trachy-andesite from Lemagrut contain pyroxenes that are Ca48Mg23Fe29-Ca48Mg32Fe20 while basalt-trachy-andesites from Olmoti have pyroxenes that are Ca27Mg30Fe43 in composition. Olivine from these centers is Fo33-47. Kaersutitic amphibole and basaltic hornblende occur as phenocrysts in most centers. Oxides are titanomagnetite and ilmenite.

Varying SiO2, TiO2 and alkali enrichment demonstrates magma fractionation. SiO2 is 45.5 to 63.3% in Ngorongoro Crater, 44.9 to 55.5% at Lemagrut, and 48 to 53.3% at Olmoti. TiO2 is 1.2 to 3.7% in Ngorongoro Crater, 1.4 to 3.6% in Lemagrut and 2.2 to 2.9% at Olmoti. LREEs range from 150-500x chondritic while heavy REEs are 7-30x. Large ion lithophiles vary smoothly across the centers possibly reflecting a common magma chamber. REE’s from the bulk cobbles are similar but not identical to those from the three centers.

Three distinct cobble groups are defined. One group resembles the Lemagrut trachy-basalt-trachy-andesite. The second group is tephrite-phonolite and resembles material tentatively assigned to the Sadiman volcano (a nepheline phonolite eruptive center adjacent to Lemarut). The third group is foidite unlike samples of the three centers. Eight cobbles of tool making material used by hominids at 1.75 Ma, were from Lemagrut (3) and Sadiman (5) while the other 5 are from Ngorongoro (2) and other sources (3).