The volcanology, petrology and geochemistry of Mount Pihanga volcano, Tongariro volcanic centre, New Zealand

Abstract

Mount Pihanga is a prophyritic calcalkaline, andesitic stratovolcano resulting from convergent plate margin magmatism. Mount Pihanga (1325 m.) is located about 7 kilometres south of Turangi, at the northernmost end of the Tongariro Volcanic Centre, an andesitic centre which lies at the southwestern end of the Taupo Volcanic Zone (TVZ). Within this thesis a map of Pihanga volcano is presented. The general geology, volcanology and stratigraphy is described. Then detailed geochemical and petrological analysis, description, and interpretation of the collected samples is applied to determining the various influences on the petrogenesis of the Pihanga magma. Pihanga Andesite is separated from andesites of different chemistry and mineralogy by the normal faulting of the Rotopounamu Graben. These faults and another through the centre of Mount Pihanga follow the recent regional NNE - SSW trend forming the graben structure of the Taupo Volcanic Zone. An older NW - SE structural lineament is formed by the vents of Pihanga, Kakaramea-Tihia, and Tongariro. The oldest Pihanga deposits discussed are drill core samples of lava taken from the Poutu dam site. The stratigraphy of the slip at the summit of Mount Pihanga reveals the most recent cone building episodes in the Pihanga history. An explosive phreatomagmatic eruption style is indicated with little lava but alternating breccia and pyroclastic deposits. A crater formed in the summit of the cone was subsequently infilled by tephras. These are deeply weathered and altered and the oldest recognised belongs to the Kawakawa Tephra Formation (22 590 years BP Carter et al., 1995; Wilson et al., 1995) and so place a minimum age on Pihanga activity. Pihanga Andesite is distinguished in hand sample by the grey colour, the exceptionally large green/black pyroxenes and the white plagioclases. Petrographic descriptions and modal analyses determine the phenocryst assemblage to be dominated by clino- and orthopyroxenes, followed by plagioclase, iron oxides, ± olivine, Cr- spinel, apatite. Analysed Pihanga lavas are mostly basic andesite with SiO2 contents ranging from 53 - 57.63 wt.%. Pihanga Andesite are geochemically distinguished by their high Cr, Ni, Sr, MgO; and low Al2O3. The geochemical evidence suggests that Pihanga lavas are relatively primitive andesites that have felt the minor effects of some contamination. Microprobe studies on Pihanga thin sections show chemical groupings of both pyroxenes and plagioclase, indicating several generations of Pihanga phenocrysts have occurred. The variable phenocryst composition may be a result of interaction of magma batches with different volatile contents, chemical compositions, and temperatures. The presence of igneous xenoliths and phenocrystic disequilibrium features in Pihanga lavas is indicative of magma mixing and accumulation. Petrography, mineral chemistry, and bulk rock chemistry of Pihanga Andesite can be used to infer the petrogenesis history of the magma. The involvement of slab derived fluids promoted partial melting within the mantle wedge. The primary magma was subsequently influenced by fractional crystallization, magma mixing and phenocryst accumulation. Some secondary assimilation with fractional crystallization (AFC) is likely to have occurred.