METHODS OF IDENTIFYING LATE QUATERNARY RHYOLITIC TEPHRAS ON THE RING PLAINS OF RUAPEHU AND TONGARIRO VOLCANOS, NEW-ZEALAND

On the ring plains of Ruapehu and Tongariro volcanoes, distal rhyoliti c marker tephras provide a valuable stratigraphic framework. However, identification of many of these tephras has been imprecise. Here we pr ovide a quantitative approach for identifying tephras within the ring- plain sequences. We extend from simple canonical discriminant function models of glass chemistry to show how these, in conjunction with othe r geological information, can be used in a practical field-based study . In two stratigraphically distinguishable groups (10-22 ka and 22-65 ka), we established discriminant models for possible tephra correlativ es from standard glass analyses. Testing analyses from unknown tephras against the models classified 34 of the 41 samples with probabilities >0.75 to tephras that were consistent with mineralogical and stratigr aphic evidence. Unknowns with lower probabilities of classification ha d several possible correlatives. Some of these were improved when the tephras classified with >0.75 probability, and which were consistent w ith stratigraphic and other evidence, were added to the discriminant m odels. The classifications were improved because of an increased numbe r of samples for each tephra and also because the added analyses were produced by the same EMP operator under the same instrument conditions . Classifications of other unknowns were improved by considering them as mixed tephras. In addition to more rigorously correlating several t ephras previously identified in this area, we have identified four tep hras in the area for the first time-the Okaia, Omataroa, and Hauparu T ephras and the Rotoehu Ash. These occur as microscopic accumulations o f rhyolitic glass shards within weathered andesitic tephra deposits.