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The Rotoiti eruption from the Taupo Volcanic Zone (TVZ) in northern New Zealand produced voluminous pyroclastic deposits. The ferromagnesian mineral assemblage in these dominantly consists of cummingtonite + hornblende + orthopyroxene with uniform magnesium/iron ratios; a second assemblage of biotite + hornblende + orthopyroxene, also with uniform Fe/Mg ratios, appears midway through the eruption sequence and, thereafter, increases in abundance. These contrasting mineral assemblages, together with pumice clast and groundmass glass compositions, provide evidence for mingling of two discrete magmas. Similarities in the chemical characteristics of the two magmas suggest that they developed from a similar source. The eruption initially tapped relatively homogeneous magma that was erupted throughout most of this phase of activity. The middle stages of the eruption included some mixed magma. The final stages of the eruption were dominated by a second magma composition, which was probably injected into the bottom of the main magma body as the eruption proceeded. The source that fed the eruption was complex, and discrete magma bodies existed and evolved separately prior to the eruption. We conclude that eruptions in the TVZ are fed from a diffuse upper-crustal zone of partially interconnected, and at times physically separate, magma bodies rather than from centralized and necessarily large long-lived magma chambers.

The Petrology of the Rotoiti Eruption Sequence, Taupo Volcanic Zone: an Example of Fractionation and Mixing in a Rhyolitic System