New Zealand Journal of Geology and Geophysics abstracts
Volcanic stratigraphy and phase chemistry of the 11 900 yr BP Waiohau
eruptive episode, Tarawera Volcanic Complex, New Zealand
Josie Speed
Phil Shane
Department of Geology
University of Auckland
Private Bag 92019
Auckland, New Zealand
email: pa.shane@auckland.ac.nz
Ian Nairn
45 Summit Rd
RD5
Rotorua, New Zealand
Abstract The 11 900 yr BP Waiohau eruptive episode
is a moderately sized rhyolitic event of the Tarawera Volcanic Complex. Subdivision
of the pyroclastic fall deposits previously mapped as Waiohau Tephra has allowed
the chronology of the eruption to be deciphered. At least 15 events (units
A-O) with volumes in the range <0.01-1.7 km3 can be recognised
in proximal to medial settings. The eruption commenced with a series of minor
pyroclastic falls and surges, which rapidly changed to a major phase of pyroclastic
surges (unit D) that reached up to 14 km from the vent with ash clouds recognisable
up to 30 km. Eastern Dome is considered to have been extruded early in the
episode. Several prominent plinian phases (unit E, L, M) followed, two of
which are estimated to have had column heights of c. 30 km. One event was
associated with the destruction of an obsidian dome and dispersal of obsidian
lapilli (unit F). These plinian events were accompanied by thin pyroclastic
flows punctuated by minor magmatic and phreatomagmatic fall eruptions. There
is no evidence for prolonged quiescence during these eruptions. The climactic
pyroclastic fall phase (unit M) was followed by a major failure of the northeast
flanks of the Tarawera massif, either by a series of block and ash flows
or debris avalanche. Effusive activity resulted in the emplacement of Waikakareao
lava flows, and voluminous and widespread pyroclastic flows (unit N) to the
north and northeast. The Waiohau episode concluded with effusive activity
producing the Pokuhu flow and Kanakana Dome. A new volume estimate for the
combined units A-O is 4.5 km3. This is a minimum because it does
not properly incorporate distal ash (>50 km) poorly preserved in paleosols.
Previous volume calculations are considered overestimates because they included
tephric loess. Effusive activity represents 3.9 km3, and pyroclastic
debris in the northern sector adds another 2.4 km3, but much of the latter
is xenolithic. Deposits from the entire Waiohau eruptive episode were derived
from a hypersthene (+ minor hornblende) rhyolite magma that lacked compositional
(SiO2 = 78 ± 0.2 wt%) and mineralogic gradients. Electron
microprobe analyses reveal that phenocrystic phases lack compositional variation.
The bulk of the magma also lacked physical gradients (T = 780-790°C;
log fO2 = c. -15) as determined by Fe-Ti oxide geothermometry.
Eastern Dome and the Pokohu lava display lower temperature (<740°C)
and log fO2 (c. -16). There is no evidence for mafic triggering
of the Waiohau eruption, unlike other eruptive episodes of Tarawera, and
the magma represents a separate batch, unrelated to earlier and later biotite-bearing
magmas. The Waiohau eruptive products appear to have been derived from a
rapidly convecting, low aspect ratio magma body that did not reside in the
crust for a prolonged time.
Keywords Waiohau eruptive episode; Okataina Volcanic
Centre; Taupo Volcanic Zone; pyroclastic fall; Tarawera
G01020 Received 22 June 2001; accepted 8 May 2002; published 27 September
2002
New Zealand Journal of Geology & Geophysics, 2002, Vol. 45: 395-410
0028-8306/02/4503-0395 $7.00/0 © The Royal Society of New Zealand
2002
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