GEOCHEMICAL ANALYSIS OF PLEISTOCENE VOLCANICS AROUND THE PENINJ AREA, NATRON BASIN, TANZANIA

The Humbu and Moinik Formations are paleontologically and anthropologically significant series of Pleistocene tuffs, clays, and sandstones in the Peninj Group of Tanzania’s Natron basin. Of note was the discovery of an Australopithecus boisei mandible below the base of the Humbu tuff, and several finds of fossils and tools above and below the Main Humbu Tuff, all important to the understanding of the Oldowan and Acheulean periods. The appearance of lavas below and within these units, including the Wa Mbugu basalt within the Humbu’s Main Tuff, has allowed scientists to radiometrically date these beds, with mixed results of 1.91 Ma, between 0.96 and 1.21 Ma (Isaac and Curtis, 1974), and 1.19 Ma ± 0.03 Ma (Deino et. al., 2006). Mapping of individual basalts in the region is difficult because of faulting.

Lava samples were collected from the Peninj Type Section along the Peninj River (Maritanane) and at the foot of or atop the escarpment overlooking the shore of Lake Natron south of Peninj. These included undisputed examples of the Sambu, Hajaro, and Wa Mbugu lavas (Type Section and south along the base of the escarpment) and others of unknown origin from the top of the presumed source volcano of the Humbu and less well constrained stratigraphic sections atop the escarpment. We compared the known samples to those with more questionable origins to help constrain the stratigraphic positions and geographic extent of these lavas.

Thin sections of most samples were examined by petrographic microscope, and fused beads were analyzed by X-ray Fluorescence for major and minor elements. Some samples were zeolitized, resulting in high Loss on Ignition and sodium values. Most samples plotted as basalts on a total alkali vs. silica diagram (after Le Bas et al., 1984), with some as trachybasalts. Multiple “supposed” Wa Mbugu samples had consistent compositions; however, two lavas from a single section separated by tens of meters of sediments (and the Main Humbu Tuff) were indistinguishable in major and minor elements. The most distinctive composition was for the Humbu source cap lava, which is significantly different from all other samples (including the Wa Mbugu samples). Despite its position at the Humbu source, this lava is not the Wa Mbugu. Further work with new samples and analysis by X-ray Diffraction will help to further constrain this system.