Rejuvenation and Repeated Eruption of a 1.0 Ma Supervolcanic System at Mangakino Caldera, Taupo Volcanic Zone, New Zealand

The Mangakino volcanic center, Taupo Volcanic Zone, New Zealand was highly active during a period of caldera-forming activity from 1.21 to 0.95 Ma. The Kidnappers eruption (~ 1200 km3 DRE) from Mangakino produced the most widespread ignimbrite on Earth and was closely followed by the smaller (ca. 200 km3 DRE) Rocky Hill eruption. Plagioclase 40Ar/39Ar ages from the Kidnappers and Rocky Hill are identical within analytical uncertainty at 1.0 Ma, and zircon U-Pb ages from both deposits yield closely similar age spectra, with peaks at ~1.0 Ma and subordinate tails back to ~1.2 Ma. The Rocky Hill ignimbrite is, however, separated from the Kidnappers ignimbrite by an erosion surface and both ignimbrites display distinctive field and petrographic characteristics suggesting that they were erupted during separate events. We present a petrological and geochemical study of minerals and glass from juvenile clasts in both ignimbrites, which suggest the Kidnappers magmatic system was rejuvenated and tapped for a second time to produce the Rocky Hill ignimbrite. Pumice whole rock and glass chemistry from the Kidnappers ignimbrite show three compositional groupings (types A, B and C), consistent with groupings previously identified in the Kidnappers fall deposit, and are inferred to have erupted from three discrete melt-dominant magma bodies. Two of the compositional groups (A and B) are also present in the Rocky Hill deposits. The range in glass chemistry within each compositional group is reduced in the Rocky Hill deposits suggesting that a more homogenized melt was later tapped or extracted from a crystal mush. Hornblende model T-P estimates from both the Kidnappers and Rocky Hill eruption deposits range from 770-860 °C and 90-220 MPa, corresponding to magma storage depths of ca. 4 to 8 km. Products of both eruptions have the same common phenocryst assemblages of plagioclase, quartz, hornblende, orthopyroxene, biotite and Fe-Ti oxides, all with overlapping chemical characteristics. However, the Rocky Hill pumices contain larger (up to 4 mm) abundant hornblende and rare biotite crystals, suggesting a change in the systems conditions and further crystal growth occurred between the two eruptions. Common crystal clots with brown glass selvedges showing fluid-affected compositions (low K2O and Rb) in the Rocky Hill pumices indicate incorporation of marginal material rarely observed within Kidnappers pumices, again implying that the eruptions are distinct events. We propose that the Kidnappers and Rocky Hill magmas were extracted from a common source zone (including crystal mush) and two independent melt-dominant magma bodies were re-established prior to the Rocky Hill eruption.