K-Ar ages of low grade metasediments of the Greenland and Waiuta Groups in Westland and Buller, New Zealand

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Abstract
Potassium-argon (K-Ar) whole-rock ages of Greenland and Waiuta Group slates from Buller and Westland yield a wide range of ages from 298 to 438 m.y. The younger ages ranging from 298 to 370 m.y. almost certainly reflect the thermal overprint associated with Tuhuan igneous and/or metamorphic events, especially at about 300 m.y., whilst an older group of ages in the range 395–438 m.y. reflect a pre-Devonian event. It is argued that the older ages are not “detrital”, (i.e., related to the age of the sedimentary source area), or related to diagenetic recrystallisation during sedimentation, but date the metamorphism and perhaps formation of slaty cleavage in the sediments during the uppermost Ordovician. Accordingly a pre-Upper Ordovician age for the Greenland Group is suggested.