Terrane characterisation and timing of metamorphism in the Otago Schist, New Zealand, using Rb‐Sr and K‐Ar geochronology

Abstract

A Rb‐Sr whole‐rock study of the Otago Schist, southern South Island, New Zealand, parallels earlier K‐Ar dating and indicates a Jurassic age for schist metamorphism and initiation of uplift, and a Cretaceous age for completion of uplift and final closure of the isotopic systems. Initial ⁸⁷Sr/⁸⁶Sr ratios provide a useful fingerprinting method to enable determination of the terrane affinity (Torlesse or Caples) of the schist. Torlesse rocks range in initial ⁸⁷Sr/⁸⁶Sr from 0.7065 to 0.7098, consistent with a cratonic/felsic‐volcanic provenance. Caples rocks range in initial ⁸⁷Sr/⁸⁶Sr from 0.7035 to 0.7055, consistent with an intra‐oceanic volcanic arc provenance. The data provide strong support for geochemical correlation of the Caples Terrane with Pelorus and Waipapa Groups in the northern part of New Zealand.

Key words:

• Rb‐Sr
• K‐Ar
• initial 87Sr/86Sr
• Otago Schist
• Torlesse
• Caples
• terranes
• low‐grade metamorphism