Geology and tephrochronology of Raoul Island, Kermadec Group, New Zealand

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Lloyd, E.F.; Nathan, S. 1981 Geology and tephrochronology of Raoul Island, Kermadec Group, New Zealand. Wellington: DSIR. New Zealand Geological Survey bulletin 95 105 p.

Abstract: Raoul Island (latitude 29 deg. 14'50'', longitude 177 deg. 55'07''), is an active volcano and the northernmost island of the Kermadec Group. The exposed succession consists entirely of Quaternary volcanics. Only the oldest rocks (Boat Cove Formation) show any sign of deformation. These are basalts and basaltic andesites. Subaerial flows and minor tephras of basalt to basaltic andesites were next to be erupted. These are the D'Arcy formation rocks. A change to explosive volcanism of basaltic andesites led to the development of a large strato volcano (lower Hutchison formation). Collapse and infilling of the caldera by tephras and lahars followed (upper Hutchison formation). By early Holocene subaerial volcanism was confined to the eastern end of this caldera (Moumoukai formation). 3800 yrs BP a dramatic change to highly explosive eruptions of dacite tephra and smaller phreatic eruptions occurred (Ngaio group). Chemically and petrographically the eruptive rocks on Raoul are typical island arc tholeiites. They can be divided into 3 major groups. The high-Al group is characterised by 15% Al2O3 and abundant phenocrystic plagioclase. The high-Fe group is characterised by 12% FeO. The dacite pumices have 66-70% SiO2. The stratigraphy shows major changes in the magma composition with time. The Boat Cove formation consists entirely of high-Al basalt and basaltic andesite. High-Fe basaltic andesite first appears in the D'Arcy formation and comprises most of Hutchison formation. Dacite pumice first appears at the base of the Moumoukai formation and became dominant about 3800 years ago