The petrography of the Central North Island rhyolitic lavas

Abstract

The rhyolites of the Taupo-Rotorua region contain phenocrysts of plagioclase and magnetite, normally accompanied by hypersthene and quartz, and less commonly by hornblende and biotite. Three ferromagnesian assemblages are distinguished, namely, hypersthene rhyolites, hypersthene-hornblende rhyolites, and biotite-bearing rhyolites, which normally also contain hypersthene and hornblende. The occurrence of these three assemblages is statistically correlated with both total phenocryst contents and modal plagioclase/modal quartz ratios of the rhyolites in which they occur. The correlations are based on modal analyses of 246 rhyolites. The crystallisation of plagioclase and quartz from rhyolites of the composition of those from the Taupo-Rotorua area is discussed in relation to the system Ab-Or-Q. Graphs showing the progressive crytallisation of quartz and albite, as calculated from the phase diagram, are given. Using evidence to be presented in detail in later papers, it is noted that the modal quartz and plagioclase occurring in various flows and domes from localised areas, such as the Haroharo rhyolite complex of the Okataina ring structure, are highly correlated, and when plotted can be closely compared with the calculated trends. The correlations of the ferromagnesian assemblages with modal quartz and plagioclase can be largely accounted for by differences in crystallisation temperatures, except for some of the data concerning the hypersthene-hornblende rhyolites. The latter suggest that water pressure or rock composition, or both, are very important in controlling hornblende crystallisation. The above results are useful in showing some order in the phenocryst assemblages of the rhyolites, and further show that their crystallisation can be explained in terms of the experimental Ab-Or-Q system. They also form a basis on which more detailed mineralogical and chemical evidence, to be given in later papers, can be discussed and interpreted.