MORPHOLOGY AND CHEMISTRY OF OLIVINE PHENOCRYSTS OF MANGAMATE TEPHRA, TONGARIRO VOLCANIC CENTER, NEW-ZEALAND

Four members of Mangamate Tephra [MT], erupted c. 10,000 years BP froM Mt Tongariro, are identifiable in tephra sequences of both the Mt Tongariro and Mt Ruapehu ring plains, and are morphologically and mineralogically distinct from underlying andesitic tephras derived from Mt Ruapehu. Mangamate Tephra comprises dominantly grey angular lithic lapilli units, contrasting strongly with older and younger, distinctly pumiceous lapilli units from Mt Ruapehu. Three members of MT contain forsteritic olivine (fo75-88). Two olivine habits are identified. Type [I] non-skeletal (granular or polyhedral) olivines are subhedral equant to tabular crystals in thin section and occur only in Waihohonu Lapilli Member. Type [II] skeletal olivines are euhedral elongate, incomplete crystals with hollow interiors and hopper and H-shaped chain morphologies. They are found in Poutu Lapilli, Waihohonu Lapilli and Oturere Lapilli members. Type [II] skeletal olivines, not previously reported in Central North Island tephras, provide a useful marker mineral for discriminating Mt Tongariro-sourced from Mt Ruapehu-sourced tephras erupted over this time period. Ferromagnesian assemblages identified for four members of Mangamate Tephra are: Opx > Cpx >> O1 +/- Hb [Poutu Lapilli] O1 > Cpx >> Opx +/- Hb [Waihohonu Lapilli] Opx >> Cpx > O1 +/- Hb [Oturere Lapilli] Hb >> Opx > Cpx +/- O1 [Te Rato Lapilli] Olivine morphology and ferromagnesian assemblage are useful for correlation of distal andesitic tephras to member level where field criteria are equivocal. A correlative of MT known from the Waikato region, previously of uncertain member status, is provisionally identified as Waihohonu Lapilli Member. Skeletal fo-rich olivine indicates melt supercooling and rapid changes in melt conditions just before the eruption of MT. Comparison of ferromagnesian mineral assemblages in MT and older tephras of Tongariro Volcanic Centre indicate introduction of new magma into Tongariro Volcanic Centre c. 10,000 years BP.