Age and structure of the Permian Brook Street Terrane, Takitimu Mountains, New Zealand

Abstract

ABSTRACT We use a recently completed airborne magnetic and gamma-ray spectrometric survey, and new U/Pb and 40Ar/39Ar age determinations, to revise and clarify several aspects of Brook Street Terrane geology in the Takitimu Mountains. Steeply dipping and homoclinal Permian Takitimu Subgroup formations defined in the central part of the range can, with moderate confidence, be traced along strike to other parts of the range where they are more complexly folded and faulted. We report intrusion ages for three Brook Street plutonic suites: pyroxene-bearing White Hill Intrusives (306-270 Ma, U/Pb zircon), amphibole-bearing Mackinnon Peak Intrusives (261 ± 1 Ma, 40Ar/39Ar amphibole), and shoshonitic Wether Hill Dikes (254 ± 1 Ma, 40Ar/39Ar plagioclase). Geological relationships between these intrusions and the host Takitimu Subgroup reinforce the Early Permian age of the latter, which is distinct from the overlying Late Permian Productus Creek Group. The Wether Hill Dikes mark the cessation of igneous activity in the Brook Street Terrane at c. 254 Ma. A large (n = 499) dataset of detrital zircon U/Pb ages from the nonmarine Jurassic Barretts Formation confirms a local Median Batholith-Tuhua Intrusives provenance and supports a post-Permian fore-arc tectonic setting along the Gondwana margin.