The Petrography of the Central North Island Rhyolitic Lavas

Abstract

This account is based on the systematic examination of the phenocrystic mineralogy of the rhyolitic lavas (referred to as rhyolites) associated with the four volcanic centres of the Taupo Volcanic Zone, namely the Mokai Ring Structure, Lake Taupo Volcanic Centre, Rotorua Caldera, and Okataina Volcanic Centre. From the results of over 200 modal analyses, it has been possible to subdivide the Mokai rhyolites into seven groups of varying extent, and the Okataina rhyolites into five complexes. The rhyolites of each group or complex are interpreted to be the eruptive products of the same high-level magma reservoir. Evidence suggests that the foci of eruptions in the Mokai structure have migrated eastwards with time. A close correlation exists between the quartz and plagioclase phenocrysts in the rhyolites from each of the groups and complexes of the Mokai and Okataina centres, and also for the Lake Taupo and Rotorua rhyolites. This is interpreted to indicate that the rhyolites from each group, etc., crystallised under similar water vapour pressure conditions, from magmas of fairly restricted compositions. Differences exist among groups or complexes, in the proportions of plagioclase crystallising before being joined by quartz, this being mainly the result of small differences in magma compositions between these groups or complexes. Significant correlations also exist between the average diameter and modal abundance of both the quartz and plagioclase phenocrysts, and provide additional evidence against a xenocrystic origin of these phenocrysts. It is believed that the rhyolites of the four volcanic centres have similar crystallisation histories before their eruption, with the exception of the Haroharo rhyolites (and their associated pumice deposits) within the Okataina centre. These Haroharo eruptives differ in a number of respects from the other rhyolites of the region, the most interesting difference being the occurrence of phenocrystic cummingtonite (in place of the normal calcic hornblende) in most of the pumice deposits and some of the lavas. It is concluded that the crystallisation of quartz and feldspar in the rhyolites of the Taupo Volcanic Zone, considered in relation to the rhyolite chemistry, can be broadly explained in terms of the Ab-Or-Q-H₂O system (Tuttle and Bowen, 1958).