New Zealand Journal of Geology and Geophysics abstracts

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K-Ar ages, paleomagnetism, and geochemistry of the South Auckland volcanic field, North Island, New Zealand

R. M. BRIGGS

Department of Earth Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand

T. OKADA
T. ITAYA

Hiruzen Research Institute
Okayama University of Science
1-1 Ridai-cho
Okayama, 700, Japan

H. SHIBUYA

Department of Earth Sciences, CIAS
University of Osaka Prefecture
Sakai, 593, Japan

I. E. M. SMITH

Department of Geology
University of Auckland
Private Bag 92 019
Auckland, New Zealand

Abstract  The South Auckland volcanic field is one of the Pliocene-Quaternary intraplate basaltic fields in northern North Island. It consists of at least 97 monogenetic volcanic centres covering an area of c. 300 km2, 38 km south of Auckland. Fifty-nine of the volcanic centres are characterised by mainly magmatic or effusive activity that constructed scoria cones and lava flows, while 38 are mainly phreatomagmatic or explosive that produced tuff rings and maars. Rock types consist of basanites, hawaiites, nepheline hawaiites, transitional basalts, and ol-tholeiitic basalts, with relatively minor amounts of nephelinites, alkali basalts, Q- tholeiitic basalts, and nepheline mugearites. Forty-three new K-Ar ages are presented, which range from 0.51 to 1.59 Ma, and show two peaks of activity at 0.6 and 1.3 Ma. Paleomagnetic determinations at 26 selected sites agree well with the paleomagnetic reversal time scale and support the K-Ar age data. Age data from each of the volcanic fields of Okete, Ngatutura, South Auckland, and Auckland, which constitute the Auckland intraplate basaltic province, show that they have developed within a time span of 0.3-1.1 Ma. After activity ceased in any particular field, a new field then developed 35-38 km to the north. These consistent time/space patterns indicate the possibility of a mantle source migrating northwards at c. 5 cm/yr. There is no correlation of rock composition with time, which is consistent with observations in the Northland intraplate province, but is not consistent with the formerly invoked rising diapir model.

Keywords  South Auckland volcanic field; Pleistocene; volcanic rocks; K-Ar; paleomagnetism; geochemistry

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