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 Rotoechu 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 allows 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 (An40-60) and magnesian orthopyroxene (En60-70), and they display high eruption temperatures (870-940ᄚC) 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 (An30-40), low-Mg orthopyroxene (En50-60), and display lower eruption temperatures (755-830ᄚC) 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 from 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 1predate 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.