Abstract
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 plesiomorphes), 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 ...
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