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Journal of the Geological Society

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Potassium-argon age studies of a low-grade, progressively metamorphosed greywacke sequence, Dansey Pass, South Island, New Zealand

Institute of Nuclear Sciences, DSIR, Lower Hutt, New Zealand
^* New Zealand Geological Survey, DSIR, Dunedin, New Zealand

39 potassium-argon age determinations from a progressively metamorphosed greywacke-schist sequence in southern New Zealand (prehnite-pumpellyite to greenschist facies), range from 199 to 154 Ma (early to late Jurassic). They show an inverse correlation of age with metamorphic grade. Concordant ages from varied lithologies at all metamorphic grades show that degassing of detrital potassium-bearing minerals was completed during metamorphism. The lowest grade metamorphic rocks, which yield the oldest ages, about 200 Ma, have only just exceeded the argon closure temperature and hence their ages date closely their time of metamorphism in the earliest Jurassic. Thus it is shown that the Jurassic-Cretaceous, Rangitata Orogeny in New Zealand commenced earlier, is more complex, and of longer duration than previously supposed. The remaining ages, from pumpellyite-actinolite and lower greenschist rocks, range uniformly through the Jurassic and are interpreted as cooling ages. From these data, a long-term uplift rate of 0.23 mm/yr is calculated for the Jurassic-Cretaceous.

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