Tauhara Volcano, Taupo Zone

Abstract

Tauhara Volcano consists of five chemically and petrographically distinct dacite cumulo-domes. The rocks are glomeroporphyritic. The dominant phenocrysts, occurring in various proportions, are plagioclase (An₄₀), quartz, orthopyroxene, augite, and calcic hornblende (in various states of oxidation). Five types of orthopyroxene are distinguished according to optical properties and habit. Other minerals occurring as phenocrysts are magnesian olivine, biotite, and magnetite. The groundmass mineral assemblage, in crystalline rocks, is plagioclase-orthopyroxene-clinopyroxene-cristobalite-magnetite. The Tauhara dacites are classified petrographically according to the relative percentages of plagioclase, pyroxene, and amphibole. The rocks contain basic cognate xenoliths (enclaves plésiomorphes), and xenocrysts from these, characteristic of calc-alkaline dome forming lavas. Five new chemical analyses show that the Tauhara dacites range in composition from 65.8–68.0% SiO₂ (water-free). A chemical analysis of a cognate xenolith (54.4% SiO₂ ) and 3 analyse3 of certain other dacites and rhyolites from the Taupo Zone are also presented. The Tauhara and other dacites from the Taupo Zone are chemically related to the vast quantities of rhyolite and lesser quantities of andesite and basalt in the Taupo Zone. Petrographic evidence suggests that mixing of acid and basic magmas may have played a significant part in the origin of the Tauhara dacite magma. This would explain the occurrence of magnesian olivine, derived from basic magma and certain phenocrysts of orthopyroxene (Fe₄₈) derived from acid magma. The preferred hypothesis is that the Tauhara dacite magma formed primarily through the fusion of rocks of either dacitic or more basic chemical composition and was then contaminated by igneous material.