Potassium‐argon age studies of metamorphism/uplift/cooling in haast schist coastal sections south of dunedin, otago, New Zealand

Abstract

K‐Ar total‐rock and partial mineral concentrate (micas, stilpnomelane, albite, and actinolite) ages of Haast Schist, from coastal sections south of Dunedin, show a gradual decrease, from 177 to 132 Ma (Mid Jurassic ‐ mid Cretaceous), in a sedimentary sequence metamorphosed from pumpellyite‐actinolite facies to greenschist facies (biotite zone). In relation to total‐rock ages, the mineral concentrate ages usually have the pattern actinolite > total rock ≥ “light phyllosilicate” (micas) > “dark” phyllosilicate (stilpnomelane) ≥ feldspar (albite), but age discordances are only 2–3%. This pattern is a consequence of varying Ar retentivities of the mineral phases and reflects progressive closure of isotopic systems during cooling of different levels of the metamorphic sequence. The metamorphism is at least 177 Ma old (Mid Jurassic). Subsequent cooling associated with post‐orogenic uplift then extended over 45 Ma to mid‐Cretaceous times. Uplift was completed with the onlap of shallow marine/ terrestrial sediments on to eroded schist basement in mid ‐ late Cretaceous times. The rate of basement uplift is c. 0.2 km/Ma (mm/yr) and compares well with similar estimates at Danseys Pass, North Otago. Mid‐Cretaceous hydrothermal alteration occurred very locally at 120 Ma, during the mid‐stage of uplift and cooling. © The Royal Society of New Zealand 1993.