Potassium-argon age of some New Zealand lamprophyre dikes near the Alpine Fault

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Abstract
Lamprophyre dikes in north-west Otago and north Westland yield whole rock potassium-argon ages mainly lying between 136 and 107 m.y. These ages correspond roughly to the ages inferred by others for the wall rock, suggesting that the ages give the latest major cooling event and that the dikes were possibly intruded earlier. In the dikes, the mica and amphibole ages are younger than their host whole rock age, while the plagioclase ages are older. The mineral ages are best explained as due to loss of argon from micas and amphiboles, and absorption by plagioclase and possibly other tectosilicates, the whole rock remaining approximately a closed system. The redistribution of argon is thought to have occurred at slightly lower temperatures than that causing outgassing of the whole rock. The whole rock dates support an earlier suggestion that the dikes were intruded as a single belt that has since been separated into two parts by a 120 km displacement along the Alpine Fault. The Alpine Fault displacement, which is dextral, was probably in two major periods: one in the Early Cretaceous, and the other in the Late Cenozoic.