New Zealand Journal of Geology and Geophysics abstracts
Weak chemical and mineralogical zonation in the Kaingaroa Ignimbrite, Taupo
Volcanic Zone, New Zealand
S. W. BERESFORD*
J. W. COLE
S. D. WEAVER
Department of Geological Sciences
University of Canterbury
Private Bag 4800
Christchurch, New Zealand
*Present address: Department of Earth Sciences, Monash University, Clayton
3168, Australia.
Abstract The 0.23 Ma Kaingaroa Ignimbrite is a composite,
multiple flow-unit ignimbrite erupted from the Reporoa Caldera, Taupo Volcanic
Zone, New Zealand. Reporoa Caldera evolved from a single vent during the
initial stages of the Kaingaroa eruption, to a trapdoor caldera with the
opening up of fractures along the eastern margin and asymmetric eruption of
early phases of the ignimbrite. Finally, plate collapse occurred during the
later stages of the eruption. Kaingaroa Ignimbrite pumice clasts range in
composition from dacite to rhyolite. Five pumice types have been identified
based largely on the geochemical variation of Rb and Sr and ferromagnesian
mineral composition. Pumice types A, B, C, and D exhibit variations in
mineralogy, trace element chemistry, and isotopic composition consistent with
derivation from a weakly zoned magma chamber. A subordinate dacitic juvenile
component (type E) appears to be unrelated to the Kaingaroa pumices by crystal
fractionation or assimilation, and is interpreted as a distinct magma batch
that was incorporated into the eruption. The asymmetrical removal of magma from
the chamber during caldera collapse is inferred to have resulted in a greater
degree of drawdown along the eastern margin of the chamber, as suggested by the
current spatial distribution of post-caldera rhyolites.
Keywords geochemistry; isotopic zonation; Kaingaroa
Ignimbrite; Reporoa Caldera; Taupo Volcanic Zone
New Zealand Journal of Geology & Geophysics, 2000, Vol. 43:
639-650
0028-8306/00/4304-0639 $7.00/0 (c) The Royal Society of New Zealand
2000
PDF file of entire paper: medium quality (1156K); (scanned from paper original: notes about this process)
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