New Zealand Journal of Geology and Geophysics abstracts
Late Miocene marine tephra beds: recorders of rhyolitic volcanism in
North Island, New Zealand
PHIL SHANE
Department of Geology
University of Auckland
Private Bag 92 019
Auckland, New Zealand
TASHA BLACK
Auckland Institute and Museum
Private Bag 92 018
Auckland, New Zealand
STEVE EGGINS
Department of Geology
Australian National University
Canberra 0200, Australia
JOHN WESTGATE
Department of Geology
University of Toronto
Scarborough Campus
1265 Military Trail
Scarborough, Ontario M1C 1A4, Canada
Abstract A deep-sea sequence of 72 rhyolitic tephra beds, now
exposed at Mahia Peninsula in the Hawke's Bay region of the east coast, North
Island, New Zealand, provides a record of late Miocene volcanism of the
Coromandel Volcanic Zone (CVZ): the precursor to large-scale explosive
volcanism of the Quaternary Taupo Volcanic Zone (TVZ). The geochemical
signature of the glasses in the Miocene tephra has been protected from
hydrothermal alteration and prolonged subaerial exposure that have affected
proximal CVZ deposits. The tephra beds are primarily eruption-driven sediment
gravity flows that have been emplaced into a trench-slope basin, some
300 km from active volcanoes. Their occurrence is consistent with
long-distance fluvial transport followed by a point-source discharge into the
deep-sea environment, and has no implications for the paleogeographic location
of the basins relative to the volcanic arc.
The tephra beds are calc-alkaline rhyolites with SiO2 contents in the range
72-78 wt% (recalculated on a volatile-free basis), and are broadly similar
to glassy rocks of the CVZ. Their major oxide, trace element, and REE
compositions are indistinguishable from glasses of TVZ rhyolites. The trace
element and REE compositional variability in the late Miocene tephra beds,
which were erupted over an estimated duration of c. 0.5-2.4 m.y., is
no greater than that of large silicic eruptives of the last 350 ka, and is
suggestive of a long-lived source and/or similar magmatic processes. However,
the individual tephra beds are products of discrete homogeneous magma
batches.
New fission-track ages of the Miocene tephra beds suggest the main period of
volcaniclastic deposition occurred in the interval c. 9-7.5 Ma. This
corresponds well with the initiation of rhyolitic volcanism in the CVZ at
c. 10 Ma, and a major period of caldera formation that took place to
c. 7 Ma. The ages suggest a sediment accumulation rate of between
0.23 and 1.2 m/ka (av. 0.4 m/ka), and a frequency of eruption of
tephra beds between 1 per 7 to 36 000 yr (av. 1 per 21 000 yr).
Although these are minimum estimates of eruptive frequency, they are similar to
rates estimated from distal records of the Quaternary TVZ, considered to be one
of the most active rhyolite centres on Earth. Overall, rhyolitic volcanism of
the TVZ appears to be a continuation of a volcanic regime that commenced at
least 10 m.y. ago.
Keywords Coromandel; rhyolite; tephra; geochronology;
fission-track; geochemistry; Miocene; east coast; Mahia Peninsula
New Zealand Journal of Geology and Geophysics, 1998, Vol. 41: 165-178
0028-8306/98/4102-0165 $7.00/0 (c) The Royal Society of New Zealand
1998
PDF file of entire paper: medium quality (2009K); (scanned from paper original: notes about this process)
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