| Summary: | A continuum of rhyolite compositions has been observed throughout the Taupo Volcanic Zone (TVZ) over the past 550 kyr. reflecting changes in the ƒH2O, ƒO₂, and P-T conditions in a lower crustal 'hot-zone' (10-30 km) where these evolved melts are generated by crystal fractionation of successively intruded basaltic magmas. The rhyolite compositional continuum is bound by two distinct end-member types: R1 is characterized by hydrous minerals (hornblende ± biotite), low FeO*/MgO (calc-alkaline series), low MREE, Y, and Zr, and high Sr; and R2 is characterized by anhydrous minerals (orthopyroxene ± clinopyroxene), high FeO*/MgO (tholeiitic series), high MREE, Y, and Zr, and low Sr.
Slab-derived aqueous fluid components (Ba, Cl) correlate well with oxygen fugacity, and other well defined characteristics of silicic magmas in the Taupo Volcanic Zone (TVZ) between a cold-wet-oxidizing magma type (R1: amphibole ± biotite; high Sr, low Zr and FeO*/MgO, depleted MREE) and a hot-dry-reducing magma type (R2: orthopyroxene ± clinopyroxene; low Sr, high Zr, and FeO*/MgO, less depleted MREE). Oxygen fugacity was obtained from analysis of Fe-Ti oxides and ranges between -0.039 to +2.054 log units (ΔQFM; where QFM = quartz + fayalite + magnetite buffer) and is positively correlated with the bulk-rock Ba/La ratio, indicating that slab-derived fluid is the oxidizing agent in the rhyolites. Chlorine contents in hornblende also correlate with the bulk-rock Ba/La ratio. Hence, high fluid-flux typically correlates with the R1 and low fluid-flux with R2 rhyolite magma types. A geochemical evolution and distribution can be tracked in time and space throughout the central region of the TVZ from 550 ka to present and has revealed two distinct magmatic cycles that vary in length. The first cycle included widespread R1 type magmatism across the central TVZ beginning ca. 550 ka and was directly associated with previously unreported dome-building and ignimbrite-forming volcanism, and led to a voluminous (>3000 km³) ignimbrite 'flare-up' between ca. 340 and 240 ka. These magmas also display the highest K₂O and Pb isotopic compositions compared to those erupted more recently, and is consistent with a peak in slab-derived sediment input. The second cycle began roughly 180 ka, erupting ca. 800 km³ of magma, and continues to the present. The duration, rate, and composition of melt production within these cycles appears to be governed by the flux of fluid/sediment released from the subducting slab, while the distribution of melts may be governed more by extension along the central rift axis.
The Matahina Ignimbrite (~160 km³ rhyolite magma; 330 ka) was deposited during a caldera-forming eruption from the Okataina Volcanic Centre, TVZ. The outflow sheet is distributed primarily from the northeast to southeast and consists of a basal plinian fall member and three ash-flow members. Pumice clasts are separated into three groups defined by differences in bulk geochemistry and mineral contents: high CaO, MgO, Fe₂O₃T, TiO₂, and low Al₂O₃, +hornblende (A2), low CaO, MgO, Fe2O3T, TiO2, ±hornblende (A1), and a subset to A1, which has high-K, +biotite (B). Two types of crystal-rich mafic clasts were also deposited during the final stages of the eruption. The distinct A and B rhyolite magma types are petrogenetically related to corresponding type A and B andesitic magma by up to 50% crystal fractionation under varying ƒO₂-ƒH₂O conditions. Further variations in the low- to high-silica rhyolites can be accounted for by up to 25% crystal fractionation, again under distinct ƒO₂-ƒH₂O conditions. Reconstruction of the P-T-ƒO₂-ƒ’H₂O conditions of the andesite to rhyolite magmas are consistent with the existence of a compositional and thermal gradient prior to the eruption. Magma mingling/mixing between the basalt to andesite and main compositionally zoned rhyolitic magma occurred during caldera-collapse, modifying the least-evolved rhyolite at the bottom of the reservoir and effectively destroying the pre-eruptive gradients.
A detailed examination of the diverse range of calcic-amphibole compositions from the ca. 330 ka Matahina eruption (ca. 160 km³ rhyolitic magma) of the Okataina Volcanic Complex, Taupo Volcanic Zone, including crystal-rich basalt to dacite pumice from post-collapse deposits, reveals several pre- and syn-eruption magmatic processes. (1) Amphibole phenocrysts in the basaltic-andesite and andesite crystallized at the highest pressures and temperatures (P: up to 0.6±0.06 GPa and T: up to 950°C), equivalent to mid-crustal depths (13-22 km). Inter- and intra-crystalline compositions range from Ti-magnesiohornblende → Ti-tschermakite → tschermakite → magnesiohornblende and some display gradual decreases in T from core to rim, both consistent with magma differentiation by cooling at depth. (2) The largest amphibole crystals from the basaltic-andesite to andesite display several core to rim increases in T (up to 70°C), indicating new hotter magma periodically fluxed the crystal mush. (3) The dominant population of amphibole (magnesiohornblende) from the rhyolite is small and bladed and crystallized at low P-T conditions (P: 0.3 GPa, T: 765°C), equivalent to the eruptive P-T conditions. Amphibole (tschermakite-magnesiohornblende) from the dacitic and low-silica rhyolitic pumice form two distinct populations, which nucleated at two different T (High: 820°C and Low: 750°C). These compositional variations, governed primarily by differences in T conditions during crystal growth, record the mixing of two distinct amphibole populations that approached a thermal equilibrium at the eruptive T. Therefore, the diversity in amphibole compositions can be reconciled as an exchange of crystals+liquid between the basaltic-andesite to dacite from the mid-crust and rhyolite from the upper-crust, which quenched against one another, modifying the dacite to low-silica rhyolite compositions as the eruption progressed.
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