Potassium-argon ages of schists from Chatham Island, New Zealand Plateau, Southwest Pacific

Abstract

The pre-Cretaceous basement rocks of Chatham Island consist of approximately 4750 m of interbedded felsic volcanic greywackes, black shales, terrigenous greywackes, and minor intermediate tuffs and cherts metamorphosed to the pumpellyite-actinolite facies. In part they resemble semi-schists and schists of textural zones I–III of the Otago belt, South Island, and have undergone three phases of folding synchronous with metamorphism. K-Ar ages for 11 total rock samples and ages for 10 mica samples of the Chatham Schist fall in the range 155–193 m.y., mostly between 157 and 162 m.y., and provide a minimum mid-jurassic age for the metamorphism associated with the jurassic-Cretaceous Rangitata Orogeny in the Chatham Islands. This is significantly older than the Lower Cretaceous estimate suggested by correlations with the Otago Schists of the South Island, N.Z., and implies either that rocks formed in an early Rangitata metamorphism (mid-jurassic) in the Chatham Islands were unaffected by later metamorphic events (Lower Cretaceous) occurring in the South Island, or that the post-orogenic cooling histories are different for the two areas. The ages for the Chatham Schist cannot be correlated with those obtained from other Rangitata metamorphic or igneous rocks from the Subantarctic islands of the Campbell Plateau, the South Island, New Zealand, and western Marie Byrd Land, Antarctica, which consistently yield Cretaceous ages (80–145 m.y.). An exception is the Bounty Islands granite (189 m.y.) which appears to predate the Rangitata metamorphism. However, dates in the age range 160–180 m.y. have been obtained from some intrusive rocks in the eastern part of West Antarctica.