K-Ar geochronology of torlesse supergroup metasedimentary rocks in canterbury, new zealand

Abstract

K‐Ar ages of 144 slates and semischists are reported from the Torlesse Supergroup, mostly in the Canterbury region. From local schist enclaves at upper prehnite‐pumpellyite to pumpellyite‐actinolite mineral facies, K‐Ar ages are compared of intercalated mudstone protolithologies of continent‐derived turbidite, and oceanic‐derived pelagic/volcanic, associations. These reveal metamorphism sufficiently thorough to reset detrital K‐bearing mineral components in the former. Two main events are separately recorded within the Late Permian and Middle‐Late Triassic parts of the Torlesse accretionary prism, respectively at 240 ± 10 Ma (Early Triassic) and 210 ± 10 Ma (latest Triassic to earliest Jurassic). The Early Triassic metamorphism is correlated with a stratigraphic break in the Torlesse succession, which reflects a period of non‐deposition and possibly uplift. The pattern of Late Trias sic‐Early Jurassic metamorphism, and subsequent younger, cooling ages, is similar to those of Triassic Torlesse sequences in North Otago and Wellington regions, and they are also correlated with a major, Early‐Middle Jurassic, stratigraphic break. The Jurassic Clent Hills Group may represent a local Rakaia Terrane cover sequence on the earlier‐consolidated parts of the accretionary prism, whilst elsewhere, in the Pahau Terrane, Late Jurassic‐Early Cretaceous Torlesse sedimentation recommenced. © 2003 Taylor & Francis Group, Ltd.