Lake Taupo Late Pleistocene tephras: a petrochemical study

Abstract

Poihipi, Okaia, Tihoi, Waihora and Otake tephras were erupted from the Lake Taupo Volcanic Centre between c. 42 000 and 20 000 years B.P. Defined as rhyolitic tephra formations they are preserved in the Taupo district. Hypersthene + hornblende is the dominant ferromagnesian assemblage of these tephras, and differences in proportions of ferromagnesian minerals are useful for initial tephra identification. Electron microprobe analyses of the components; glass, hornblende, orthopyroxene, magnetite and ilmenite; fully characterize each tephra, some components being more useful for characterization than others. Characterization of the two older tephras is assisted by factors resulting from their weathering. Tihoi, Waihora and Otake tephras are stratigraphically and chemically separated from Okaia and Poihipi tephras, and as a whole the five Late Pleistocene tephras (and probably the Kawakawa Tephra Formation) are chemically distinct from Holocene tephras erupted from the Lake Taupo Volcanic Centre.