A multiple-parameter approach to andesitic tephra correlation, Ruapehu volcano, New Zealand

A multi-parameter approach was used to correlate andesitic tephras in a complex tephra sequence ranging in age from ca. 23 to ca. 75 ka on the eastern ring plain of Ruapehu volcano, North Island. Field properties, combined with ferromagnesian mineral assemblages and mineral compositions, were required to map and correlate this sequence. Three tephra units could be identified based on their unique physical appearance, but other tephras could not be correlated on this basis alone. Homblende and olivine proved to be valuable marker minerals enabling further distinction of two of the marker units recognised by field properties, as well as defining two further marker tephras. Unweathered titanomagnetite crystals, present in all of the tephras, were subjected to major-element analysis by electron microprobe. Canonical discriminant function analysis (DFA) of these analyses enabled the grouping and discrimination of tephra units, further aiding the identification of defined marker units, as well as defining new marker units, The titanomagnetite chemistry showed a strong relationship to the ferromagnesian mineralogy, showing that the ferromagnesian phenocrysts formed from the same melt or under the same melt conditions prior to eruption of each tephra, Canonical DFA was also applied to homblende and olivine mineral analyses to identify further marker beds and to confirm identifications of previously defined units. This statistical analysis was found to be invaluable in reducing the large amount of compositional data from this study into a useable form for andesitic tephra correlation and mapping.