Abstract:
The extinct multiple volcano, Tauhara, late Pleistocene in age, situated northeast of Lake Taupo in the North Island belt of active volcanism, consists of five coalescing cumulo-domes of dacitic composition. Their internal fan-shaped flow structure indicates that they were constructed by continuously extruded viscous lava.
The cumulo-domes are petrographically distinctive. The uniform glomeroporphyritic texture of the rock types and the properties of the minerals indicate that they were derived from the one magma body. All the rock types contain phenocrysts of plagioclase, quartz, orthopyroxene, clinopyroxene (augite) and amphibole in various proportions and textures. Other important minerals are magnesian olivine and biotite. The rocks contain basic cognate xenoliths, characteristic of domical protrusions.
Variation diagrams indicate that the rare dacites of Tauhara and elsewhere are related chemically to the moderately abundant andesites, and to the abundant rhyolites of the volcanic belt. A continuous series, with accumulative types, from non-porphyritic aluminous basalt through andesite and dacite to rhyolite is established.
From quantitative, chemical, mineralogical and petrographic evidence it is concluded that the magma body giving rise to the Tauhara dacites was formed by mixing and assimilation of fused acid material by basaltic magma. In this heterogenous magma crystal settling and conventional mixing occurred, resulting in the various mineral assemblages and compositions of the Tauhara dacites.
