GEOCHRONOLOGY AND GEOCHEMISTRY OF LAVAS FROM OLMOTI CRATER TANZANIA: IMPLICATION FOR OLDUVAI BED I TEPHRA AND LAVA SOURCES

Lava and tephra interbedded in the artifact, hominin and vertebrate fossil bearing Bed I stratum (ca 1.9 to 1.78 Ma) of Olduvai Gorge, Tanzania (Leakey et al. 1971; Hay, 1976; Blumenschine et al. 2003) are considered to have been derived from nearby Olmoti Crater, part of the Ngorongoro Volcanic Highlands (NVH). This correlation is based on K-Ar and paleomagnetics data obtained from Olduvai lava (Hay, 1976). In 2004, McHenry, 2004 and Mollel and McHenry, 2004, presented further support of this correlation through geochemical fingerprinting of mineral suites and textural compositions linking Olduvai Tuff IF to an Olmoti source. Little is known on the evolutionary history of Olmoti. Here, we present geochemical and geochronological data for Olmoti Crater that further support the Olmoti source for upper Olduvai Bed I. Total alkali vs. silica content indicates that Olmoti ranges in composition from trachybasalt to trachyte. Petrographic and electron microprobe study indicate that the Olmoti mineral assemblage includes: augite + plagioclase + anorthoclase +/- amphibole, Ti-Oxide, aenigmatite, sodalite. Trachyte from Olmoti Crater has a mineral assemblage and textural composition that compare well with that of rock fragments incorporated in Olduvai Tuff. Major and trace elements as well as isotopic data for Olmoti and Olduvai Bed I lavas are indistinguishable, suggesting similar magma source. Sr isotopic data on five Olmoti lavas are constrained to 0.7045 to 0.7049 whereas those of three lavas from Olduvai Gorge are 0.7049 to 0.7050.

40Ar/39Ar laser incremental-heating data of lavas from the Olmoti Crater wall indicate that Olmoti was active from 2.04 +/- 0.09 to 1.79 +/- 0.03 Ma. Two trachybasalt lavas from Olduvai Bed I dated by the same technique yielded plateau ages of 1.86 +/- 0.02 and 1.809 +/- 0.02 Ma suggesting correlation with the upper part of Olmoti activity. The similarity in major elements and isotopic data are also in agreement with an Olmoti source. Tuff IF, the top of Olduvai Bed I, is coincident with an N-R geomagnetic polarity transition correlated here with the top of Olduvai Subchron (1.94 to 1.785 Ma: Berggren et al. 1995). The lower age of Olmoti is significant as it suggests overlap in age with activity from Ngorongoro Crater, and if confirmed is the first evidence for multiple source activity from the NVH.