Geochemistry and mineralogy of laetoli area tuffs: Lower laetolil through naibadad beds

Abstract

Tuffs from the Lower Laetolil, Upper Laetolil, Ndolanya, and Naibadad Beds of the Laetoli and Kakesio areas of Tanzania are characterized and compared to serve as correlation tools for this important paleoanthropological region. These beds contain variably-altered tuffs of original carbonatitic, nephelinitic, phonolitic, trachytic, and rhyolitic composition, tracking changes in volcanic source area and local diagenetic conditions. Previous studies (e.g., Hay 1987) show compositional differences between tuffs from different Laetoli area beds, but do not provide the tuff-specific information needed to create a high-resolution tephro-stratigraphic framework. This study details differences in mineral composition between individual tuffs (where distinguishable) within each bed. The Lower Laetolil tuffs have similar compositions but can generally be distinguished by their andradite compositions. Titanomagnetite and perovskite compositions help distinguish individual Upper Laetolil tuffs, and divide the bed compositionally into lower (UL 1–4) and upper (UL 5–8) units. Augite compositions distinguish the individual Ndolanya tuffs analyzed in this study, whereas glass compositions distinguish individual Naibadad tuffs. The variable degree of alteration observed within and between sites at Laetoli makes the comparison of mineral assemblages an unreliable means for identifying tuffs. Glass, nepheline, and melilite were likely significant components of the original Laetolil Beds tuff mineral assemblages, but are now recognized in few (if any) samples except by the presence of likely alteration products. Feldspar and biotite also show signs of degradation. A comparison of glass and phenocryst compositions between the Naibadad tuffs and the tuffs of lower Bed I at Olduvai Gorge does not support a direct correlation between the two sites. The presence of quartz and rhyolitic and trachytic glass in both does suggest a common source, likely Ngorongoro Crater.