New Zealand Journal of Geology and Geophysics abstracts
Geochemical fingerprinting of basaltic tephra deposits in the
Auckland Volcanic Field
PHIL SHANE
IAN SMITH
Department of Geology
The University of Auckland
Private Bag 92019
Auckland, New Zealand
email: pa.shane@auckland.ac.nz
Abstract Small volume (<2 km3) basaltic
volcanoes have been active throughout the late Quaternary in the Auckland
Volcanic Field (AVF) in northern New Zealand. The main pyroclastic products of
these centres are phreatomagmatic surges containing abundant accidental ejecta
and magmatic fall deposits of limited aerial extent. Previously they had
received no attention from a tephrostratigraphic perspective. Despite extensive
weathering, many of these deposits contain basaltic glass that can be used to
geochemically fingerprint the volcanic source and individual eruptive events.
The glasses are predominantly basanites with SiO2 contents in the range 42-50
wt%. Many individual emplacement units are compositionally homogeneous on the
basis of electron microprobe analysis (SiO2 +/- 0.5 wt%), and can be
distinguished on the basis of TiO2, CaO, K2O, and P2O5 contents. Individual
tephra beds can easily be characterised, however volcanoes are more
compositionally diverse. Although the volcanoes are considered monogenetic,
they display a wide range of different eruptive styles, and their pyroclastic
deposits can display a compositional range (SiO2 45-50 wt%) within short
stratigraphic sequences that show no evidence of hiatus. This increases the
difficulty in matching distal tephra deposits to their source. Glass
compositional data from 18 volcanoes show no evidence of spatial or temporal
trends within the AVF.
Keywords tephra; tephrochronology; Auckland Volcanic Field;
glass chemistry
New Zealand Journal of Geology & Geophysics, 2000, Vol. 43:
569-577
0028-8306/00/4304-0569 $7.00/0 (c) The Royal Society of New Zealand
2000
PDF file of entire paper: medium quality (875K); (scanned from paper original: notes about this process)
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