TRACING THE CONTROLS OF AN IGNIMBRITE FLARE-UP, TAUPŌ VOLCANIC ZONE, NEW ZEALAND

The central Taupō Volcanic Zone of New Zealand has been one of the most productive rhyolitic volcanic centers over the last 2 Ma. The flux of volcanism has fluctuated through time with periods of particularly intense volcanic activity termed “ignimbrite flare-ups”. The most voluminous is the 350-240 ka flare-up, which includes at least seven caldera-forming eruptions each with erupted volumes ~50-2000 km³ DRE, totaling >3000 km³ DRE of rhyolite in just ~100 ka. This period demonstrates the extraordinary rate of magma production that is possible and provides an opportunity to examine the processes of rhyolitic magma generation over the 10s to 100 ka temporal scale. Here, we aim to determine the mantle versus crustal processes controlling this ignimbrite flare-up.

Using a combination of whole-rock (via XRF) and matrix-glass (via LA ICP-MS) trace-element compositions, we interrogate the temporal trends for the magma bodies that fed the Whakamaru, Matahina, Kaingaroa, Chimp, Pokai, Mamaku, and Ohakuri caldera-forming eruptions. We analyze ten to twenty pumice clasts per deposit. Trends are analyzed by principal component analysis multivariate statistics. Preliminary Pb, Nd, and Sr isotopes for a subset of samples are compared to previously reported mantle and crustal compositions to determine the proportion of mantle and crustal contributions.

The trace elements reveal several temporal trends. The oldest flare-up eruptions are relatively depleted in middle and heavy rare earth elements (REEs), Y, and Zr, compared to the youngest eruptions. This indicates a gradual transition in the lower crustal/mantle source. Fractionation of feldspars is clearly traced by comparing Sr, Rb, and Ba concentrations between glass and whole-rock compositions. The feldspar signal is decoupled from the REEs, suggesting that the REEs trace a different stage of the magmatic history than the Sr, Rb, and Ba. The beginning and end of the flare-up have variable and elevated plagioclase fractionation signals, whereas the middle eruptions have a weaker signal. Isotopes suggest that crustal assimilation was elevated during the flare-up. These results reveal the interplay between crustal and mantle processes that began the ignimbrite flare-up, drove it, and ultimately caused the system to relapse into relative quiescence.