New Zealand Journal of Geology and Geophysics abstracts
Geochemistry of the Willsher Group, southeast Otago, New Zealand,
and comparison with the Murihiku and Dun Mountain-Maitai Terranes
Barry P. Roser
Department of Geoscience
Shimane University
Matsue, Shimane 690-8504, Japan
roser@riko.shimane-u.ac.jp
Douglas S. Coombs
Department of Geology
University of Otago
P.O. Box 56
Dunedin, New Zealand
Abstract Triassic Willsher Group sandstones and
siltstones in the Kaka Point Structural Belt of southeast Otago have
geochemical provenance signatures and petrographic characteristics
indicative of derivation from a moderately evolved, partially
dissected, medium-K volcanic arc, probably of continental island arc
nature. Pre-Etalian and Etalian sandstones are andesitic to dacitic in
composition, whereas those in the Kaihikuan Stage are more felsic.
These features are demonstrated by increased silica contents, immobile
element ratios, and rare earth element parameters including LaN/YbN
and LaN/SmN ratios. Significant negative Eu
anomalies appear at the top of the Willsher succession. Comparison with
the chemistry of coeval and older sandstones from the adjoining Dun
Mountain-Maitai Terrane shows the Willsher Group sediments are
substantially more felsic, and close linkage between the two terranes
can be ruled out. Comparison with coeval intervals in the adjacent
Murihiku Terrane, including new rare earth element data presented and
discussed here, shows that pre-Etalian Willsher sandstones are more
felsic, but that Etalian and Kaihikuan equivalents are similar in
composition. In both the Willsher Group and the Murihiku Terrane,
sandstones become more felsic stratigraphically upward, although the
Willsher Group shows more variability within age groups, probably due
to local variations in source proportions. These features and others
previously described suggest the Kaka Point Structural Belt should be
regarded as a separate subterrane of a greater Murihiku Terrane.
Keywords Otago; Willsher Group; Kaka Point
Structural Belt; Triassic; sandstones; Murihiku Terrane; Dun
Mountain-Maitai Terrane; geochemistry; provenance; major and trace
elements; REE; terrane affiliation
G04030; Received 30 July 2004; accepted 12 January 2005; Online
publication date 22 August 2005
New Zealand Journal of Geology & Geophysics, 2005, Vol. 48:
415–434
0028–8306/05/4803–0415© The Royal Society of New Zealand 2005
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