EMBAGAI CALDERA, NGORONGORO VOLCANIC HIGHLANDS, TANZANIA: A POTENTIAL VOLCANIC SOURCE FOR OLDUVAI GORGE BED III

Embagai is a Pleistocene volcanic caldera located in the Ngorongoro Volcanic Highlands (NVH) of northern Tanzania. It has been postulated as a potential source volcano for volcaniclastic materials in Olduvai Gorge Beds II and III. As most tephra in Bed III are altered and reworked, it is difficult to determine their compositions based on bulk geochemistry. By using Electron Probe Microanalysis (EPMA) on individual phenocrysts within both Olduvai tuffs and from lavas and tuffs at potential source volcanoes, we can infer a stronger geochemical relationship between the two. In addition, this can allow for better relative dating of the beds based on dating of their eruptive parents.

Phenocryst mineralogy from several sites at Embagai was compared to phenocrysts from tuffs within Olduvai Bed II and Tuff IIIA (Tuff 1) of Bed III (unpublished data from McHenry). Augite, titanomagnetite, and andradite garnet were the primary minerals used for comparison. Nepheline and perovskite were also compared. Overall, augites from Bed II did not overlap well with the Embagai augites, and the lack of anorthoclase feldspar (and presence of nepheline) in Embagai tuffs and lavas points towards a different source. However, augites from Tuff IIIA show significant overlap in Fe and Mg with samples from the north, east, and west sides of Embagai, as well as its caldera floor. For titanomagnetites (Ti vs. Fe), Olduvai Tuff IIIA is compositionally similar to those from the northern rim of Embagai, while samples from the caldera floor tend to be slightly richer in Ti. Andradite garnets in Tuff IIIA are also similar in composition to Embagai andradites. Nepheline is abundant in both. Perovskites (Ti vs. Ca) are similar in composition to the western flank and potentially the northern rim of Embagai. This similarity suggests that Embagai is a likely source for Olduvai Bed III, while another source might be required for Bed II.