Identification and correlation of thinly bedded tephra: The Tirau and Mairoa Ashes

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Abstract
The Tirau and Mairoa Ashes are two volcanic ashes named during early field surveys in the Waikato and King Country districts of the North Island of New‐Zealand. To examine the relationship between the two, samples from the type localities and at geographically intermediate sites were investigated using multicomponent methods (mineral assemblages, rhyolitic character), single component methods (chemical analysis and magnetic susceptibility of glass shards), and single particle methods (refractive index of glass shards). The Mairoa Ash is appreciably andesitic. The Tirau Ash, earlier considered rhyolitic, is shown to have a significant andesitic component, particularly lower in the profile. These features, although apparent with the multicomponent methods, are made more obvious by the single component methods, which more closely represent compositional relationships at the time of eruption. The value of single particle methods is demonstrated by the recognition of two rhyolitic glasses in the Mairoa Ash from studies of the refractive index of the glass shards.

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