New Zealand Journal of Geology and Geophysics abstracts

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Age/isotopic characterisation of the Waipapa Group in Northland and Auckland, New Zealand, and implications for the status of the Waipapa Terrane

C. J. Adams

Institute of Geological & Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand

R. Maas

Department of Earth Sciences
La Trobe University
Bundoora, Victoria 3083
Australia*

*Present address: Department of Geology, University of Melbourne, Parkville, Vic 3010, Australia.

Abstract  Rb-Sr ages of low-grade metasediments of the Waipapa Group (probable Permian–Jurassic) of Northland and Auckland range from Late Triassic (c. 209–217 Ma) to Middle Jurassic (c. 164–174 Ma), reflecting deformation and metamorphism during the Rangitata Orogeny. K-Ar ages indicate Middle Jurassic (c. 165–175 Ma) uplift and cooling of the western Omahuta-Puketi Forest block, and later, Middle–Late Jurassic (c. 150–160 Ma) cooling of the Helena Bay to Hunua Ranges eastern coastal block. In the intervening Western Bay of Islands block, there is evidence of local Early Cretaceous (c. 135 Ma) metamorphism. Rb-Sr whole-rock isochron age (t) and initial ⁸⁷Sr/⁸⁶Sr ratio (i) data support the assignment of Waipapa Group of the westernmost, Omahuta-Puketi block to the Caples Terrane. The remaining data have a consistent (t)-(i) signature, with (i) values 0.7039–0.7049, unlike Permian–Cretaceous Torlesse Supergroup metasediments with more radiogenic (i) values, typically >0.7060. They thus support the retention of a separate, Waipapa Terrane which continues southwards through central North Island to the Kaimanawa Ranges, and probably to Kapiti Island, and schist equivalents in eastern Marlborough.

Keywords   Northland; Auckland; Waipapa Group; Waipapa Terrane; Triassic; Jurassic; Cretaceous; K-Ar geochronology; Rb-Sr geochronology; Sr-isotopes; metamorphic rocks

G02048; Received 16 September 2002; accepted 26 January 2004; online publication date 20 May 2004
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47: 173–187
0028–8306/04/4702–0173 © The Royal Society of New Zealand 2004

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