IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A2_3C-O13

Room A5

Date/Time: July 23 12:00-12:15

Magmatic-tectonic triggers leading to the eruption of multiple isolated magma batches
and twin caldera collapse in the Taupo Volcanic Zone, New ZealandFlorence Begue1 , Darren M Gravley1 , Chad D Deering2 , Ben M Kennedy1 , Isabelle S Chambefort3
1

University of Canterbury, New Zealand, 2 University of Wisconsin-Oshkosh, USA, 3 GNS Science, NewZealandE-mail: darren.gravley@canterbury.ac.nz

Calderas, their associated structures, and the geometry of their pre-collapse magma reservoir(s) have been thesubject of much scientific intrigue, largely because they represent the largest scale of volcanic eruptions and ourknowledge has only been gleaned from ancient examples. However, detailed field, analogue, and petrochemical
studies of several calderas reveal an often complex arrangement of magma bodies that are chemically distinct
from each other. In addition, the arrangement and geometry of these magma bodies can be dictated by regional
tectonic structures that are activated during eruption and collapse. Here, we present new petrochemical evidence
for a complex subvolcanic plumbing system including multiple juxtaposed magma bodies that erupted over aremarkably short timeframe to form twin calderas 30 kilometers apart.

The Rotorua and Ohakuri calderas in the central Taupo Volcanic Zone (TVZ) formed

240 ka with theeruption of >245 km3 rhyolitic magma associated with the Mamaku and Ohakuri ignimbrites, respectively. As aconsequence, a large volcano-tectonic depression spanning the area between the two calderas formed. New
pumice glass and melt inclusion data (combined with the existing bulk-rock and mineral chemistry) supports
a petrogenetic model for six different magma batches extracted from the same source region, i.e. a laterally
extensive and continuous intermediate mush zone beneath the Rotorua and Ohakuri calderas. Minor geochemicaldifferences in the batches, including fluid mobile elements U, Cs, and Li and volatiles (CO2 and H2O), suggest
different extraction conditions for the emplacement of the magma batches. However, combining the chemistry withthe timing of eruption for each batch (recorded in the deposit stratigraphy) suggests a more tantalising picture
for the spatial distribution of the magma batches in the upper crust and a close interplay between tectonics,
magmatism and volcanism. In particular, the initial plinian airfall eruption from Ohakuri erupted simultaneouslywith the Mamaku ignimbrite from Rotorua and they share a similar trend in Cs and Li concentrations, and CO2
values that suggest, if volatile saturation was reached, melt inclusion entrapment at pressures ranging from 75
to 150 MPa. The chemistry suggests that the magma batches for the earliest eruptions may have been fed bya connected rhyolitic dyke on the western margin of the large volcano-tectonic depression. An elongated rhyolite
dome with the same 240 ka age and same chemistry also lies on the western margin of the depression between
the two calderas; further supporting the idea of a laterally extensive dyke. The evacuation of these magmaticallylinked melt batches via a laterally extensive dyke could have initiated rupture of regionally linked faults enough totrigger the eruption of the chemically distinct magma batches associated with the Ohakuri ignimbrite and collateralsubsidence of the area between the two calderas.

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