New Zealand Journal of Geology and Geophysics abstracts
Stratigraphy and geochemistry of the Turoa area, with implications for andesite
petrogenesis at Mt Ruapehu, Taupo Volcanic Zone,
New Zealand
T. E. WAIGHT*
R. C. PRICE+
Victorian Institute of Earth and Planetary Sciences
School of Earth Sciences
La Trobe University
Bundoora, VIC 3083, Australia
+School of Science and Technology
The University of Waikato
Private Bag 3105
Hamilton, New Zealand
R. B. STEWART
Institute of Natural Resources
Massey University
P.O. Box 11-222
Palmerston North, New Zealand
I. E. M. SMITH
Department of Geology
The University of Auckland
Private Bag 92019
Auckland, New Zealand
J. GAMBLE
Department of Geology
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand
*Present address: Danish Lithosphere Centre, Øster Voldgade 10, 1350
Copenhagen K, Denmark.
Email: tew@dlc.ku.dk
Abstract Detailed mapping and sampling on Turoa ski-field and
surrounding areas on Mt Ruapehu has enabled identification of individual flow
packets that represent small scale eruptive events during the major
cone-building episodes previously identified on Ruapehu by Graham &
Hackett. The area is dominated by plagioclase-pyroxene-phyric andesitic lavas
of Mangawhero Formation, which are petrographically and geochemically typical
of post-120 ka Ruapehu lavas. Although the lavas are from a relatively
small area of the volcano, geochemical and isotopic compositions show a range
in variation similar to that observed for the entire volcano. Examination of
geochemical variations between individual flow packets and also between
sequential lava flows indicates complex processes of assimilation and influx of
"new", variably evolved and fractionated magmas into high level magma chambers.
These chambers are most likely heterogeneous, and individual eruptions may also
sample compositionally distinct regions of the same chamber. It is probable
that a complex plumbing system exists beneath Ruapehu with at least two levels
of magma storage, evolution, and crustal interaction. Consequently, attempts to
numerically quantify such a complex system using models such as
assimilation-fractional crystallisation processes are inherently
oversimplifications.
Keywords Ruapehu; andesite; isotopes; New Zealand;
geochemistry; stratigraphy
New Zealand Journal of Geology & Geophysics, 1999, Vol. 42:
513-532
0028-8306/99/4204-0513 $7.00/0 (c) The Royal Society of New Zealand
1999
PDF file of entire paper: medium quality (4039K); (scanned from paper original: notes about this process)
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