New Zealand Journal of Geology and Geophysics abstracts

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Tephrostratigraphy and geochemical fingerprinting of the Mangaone Subgroup tephra beds, Okataina Volcanic Centre, New Zealand

Victoria C. Smith
P. Shane
I. E. M. Smith

Department of Geology
University of Auckland
Private Bag 92019
Auckland, New Zealand

Abstract  The Mangaone Subgroup is a sequence of 14 plinian tephra beds erupted from the Okataina Volcanic Centre in North Island, New Zealand, that are bracketed between the regional marker beds of Rotoehu Tephra (50–60 ka) and Oruanui Tephra (26 ka). The Mangaone Subgroup tephra beds are separated by thin paleosols in ascending stratigraphic order: unit A, unit B, unit C, Pupuwharau Tephra (new), Pongakawa Tephra (new), Maketu, Te Mahoe, Hauparu, unit G, unit H, Mangaone, Awakeri, Omataroa, and unit L. Geochemical fingerprinting of the glass and phenocryst phases allows clear subdivision of the Mangaone Subgroup into two stratigraphic intervals. Units A–G are rhyodacites and low-SiO₂ rhyolites (71–75.5 wt% SiO₂ glass; 68–71 wt% SiO₂ whole rock), clinopyroxene bearing, with calcic plagioclase (An_40–60) and magnesian orthopyroxene (En_60–70), and they display high eruption temperatures (870–940°C) and oxygen fugacities (–logfO₂ = 11.66–10.40). Some of these units are compositionally heterogeneous in the glass phase (SiO₂ range up to 8 wt%), especially unit A, Ngamotu, Te Mahoe, and Hauparu tephra beds. Units A–G can easily be distinguished from other Taupo Volcanic Zone tephra erupted in the last c. 60 000 yr. Units H–L are high SiO₂ rhyolites (76–78 wt% SiO₂ glass; 71.5–75 wt% SiO₂ whole rock), which contain sodic plagioclase (An_30–40), low-Mg orthopyroxene (En_50–60), and display lower eruption temperatures (755–830°C) and oxygen fugacities (–logfO₂ = 14.7–12.93). Compositional differences in titanomagnetite and ilmenite can also be used to distinguish them from older units. The two compositional groups also form distinct T–fO₂ trends. The shift in chemical affinity follows the large volume Hauparu eruption, and coincides with the likely geographic shift in vent location to the eastern margin of the caldera complex. Tephra beds that both predate and postdate the Mangaone Subgroup also reflect changes in magma type and vent location. The distinctive geochemical character of tephra in the Mangaone Subgroup makes them valuable stratigraphic markers in the central North Island and surrounding oceans.

Keywords  Mangaone Subgroup; Okataina Volcanic Centre; Taupo Volcanic Zone; tephra; tephrostratigraphy; Pupuwharau Tephra; Pongakawa Tephra; new stratigraphic names

G01011 Received 4 May 2001; accepted 27 February 2002
New Zealand Journal of Geology & Geophysics, 2002, Vol 45: 207–219
0028–8306/02/4502–0207 $7.00/0 © The Royal Society of New Zealand 2002

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