New Zealand Journal of Geology & Geophysics abstracts
Influence of the crust and crustal structure on the location and
composition of high-alumina basalts of the Taupo Volcanic Zone, New
Zealand
J. Hiess
Research School of Earth Sciences
Australian National University
GPO Box 4 Canberra, ACT 2601, Australia
J. W. Cole
Department of Geological Sciences
University of Canterbury
Private Bag 4800 Christchurch 8140, New Zealand
K. D. Spinks
Mighty River Power Ltd
PO Box 445
Hamilton 3240, New Zealand
Abstract High-alumina basalts (HABs) that occur
throughout the central part of the Taupo Volcanic Zone (TVZ) are
associated particularly with faulting, and many occur where faults
intersect caldera margins. For convenience, the basalts are described
in terms of three geographic-tectonic segments: Okataina in the north,
Kapenga in the middle, and Taupo in the south. Evidence for mixing and
mingling between rising basaltic magmas and rhyolitic rocks and magmas
is common, including the frequent occurrence of xenocrysts and
xenoliths, quench textures, and melting around the rims of inclusions.
Chemically, the basalts are similar in terms of major element
compositions, suggesting relatively homogeneous PT conditions in the
mantle source, but variation between some trace elements suggests
different processes are operating in the crust with variable degrees of
contamination.
The model presented for HAB generation in the TVZ is for
partial melting of mantle peridotite in the upper mantle, with the melt
rising into the lower crust via dike swarms. In the upper crust, the
distribution of HAB is strongly influenced by location and structure.
In the Kapenga segment, there is little evidence for interaction
between basaltic and rhyolitic magma, other than at very shallow
levels, perhaps because the rhyolitic magma chambers (or pods) were
solid, allowing brittle deformation and rapid intrusion of basalt
dikes. At Okataina there is much greater mixing and mingling,
suggesting there was still partially molten rhyolitic magma chambers
beneath this area during basalt intrusion. Basalt in the Taupo segment
occurs outside the Taupo caldera complex and may be related to the
earlier Whakamaru caldera complex. The basalt is thought to rise
through the crust as a network of unrelated melt batches into a plexus
of discrete magma chambers and conduits, many of which are sited along
fault zones causing fissure eruptions at the surface.
Keywords high-alumina basalt; Taupo Volcanic
Zone; structural control; mixing and mingling; magma evolution
G07005; Online publication date 21 September 2007 Received 20 March
2007; accepted 28 August 2007
New Zealand Journal of Geology & Geophysics, 2007, Vol. 50:
327–342
0028–8306/07/5004–0327 © The Royal Society of New Zealand 2007
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