TERRAPUB Earth, Planets and Space
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Earth Planets Space, Vol. 56 (No. 2), pp. 283-288, 2004

LETTER

K-Ar ages of the Auckland geomagnetic excursions

Nobutatsu Mochizuki1, Hideo Tsunakawa1, Hidetoshi Shibuya2, Takahiro Tagami3, Ayako Ozawa3, John Cassidy4, and Ian E. M. Smith4

1Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan
2Department of Earth Sciences, Kumamoto University, Kumamoto 860-8555, Japan
3Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
4Department of Geology, University of Auckland, PO Box 92019, Auckland, New Zealand

(Received January 15, 2004; Revised February 20, 2004; Accepted February 20, 2004)

Abstract: K-Ar age determinations were made on two monogenetic volcanoes in the Auckland volcanic field, New Zealand, which have recorded the Auckland geomagnetic excursions. For the Wiri volcano with the north-down intermediate paleomagnetic direction, five samples gave a weighted mean age of 27 ± 5 (1s) ka. For the Hampton Park volcano with the west-up intermediate direction, three samples gave a weighted mean of 55 ± 5 (1s) ka. Since these two K-Ar ages are distinguished at 2s level, it is inferred that at least two geomagnetic excursions can be recognized in Auckland. The age of the Hampton Park is barely distinguished from the established age range of the Laschamp excursion (39-45 ka) at 2s level. The age of the Wiri coincides with the age of c. 30 ka in which excursions have been found from sedimentary and volcanic records. The reported excursions from volcanic rocks show a VGP cluster in the central to northern Pacific region which is distinct from the VGP paths or clusters during polarity reversals.
Key words: K-Ar age, Auckland geomagnetic excursion, Auckland volcanic field.


Corresponding author E-mail: nmochi@geo.titech.ac.jp


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