Abstract

Potassium-argon ages from the Alpine schist belt in south-east Nelson, New Zealand, range from 6 to 143 m.y.; the youngest ages are from rocks closest to the Alpine fault. The K-Ar ages cannot be consistently correlated with metamorphic grade, and there is evidence that degassing of minerals has occurred on a small scale, producing a smooth radiogenic argon concentration gradient spatially related to the Alpine fault. Degassing does not appear to have involved net argon loss or gain from the schist belt as a whole.

The setting of the K-Ar ages in the regional context of the Mesozoic Rangitata orogeny allows a reassessment of previous models of metamorphism and uplift of the schist belt, and a new model of two-stage uplift and metamorphism in Late Jurassic and late Cenozoic times is proposed. Metamorphism of schist occurred in Jurassic time followed by uplift and cooling during mid-Cretaceous time. In late Cenozoic time, vigorous vertical movements of the Alpine schist belt initiated degassing of radiogenic argon, resulting in a spectrum of disturbed “Rangitata” ages from 4 to 140 m.y. along a narrow belt close to the Alpine fault. It is estimated that 5,000 m of uplift commenced 4 ± 2 m.y. ago.

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