Terrane and core complex architecture of the Otago Schist in the Dunstan and Cairnmuir Mountains, New Zealand, from U-Pb and (U-Th)/He zircon dating

ABSTRACT

We report ten new zircon (U-Th)/He ages from psammitic greyschists collected in a southwest-northeast profile in the Otago Schist across the Dunstan and Cairnmuir Mountains, New Zealand. Six of the samples have accompanying new detrital zircon U-Pb ages. In this part of the Otago Schist, a broad antiformal core of garnet-biotite-albite zone schist defines a range-scale footwall block to the gently northeast-dipping Thomsons Gorge Fault and the gently southwest-dipping Cromwell Gorge faults. Hanging wall blocks above the faults comprise lower-grade chlorite zone schists. Nine schist samples from footwall and hanging wall blocks close to the low-angle faults exhibit a weighted mean (U-Th)/He age of 82 ± 3 Ma. We interpret that both low-angle fault systems were active prior to c. 82 Ma and confirm that they were extensional. We interpret the antiformal schist core as the late Early Cretaceous (c. < 122–109 Ma) ductile extensional lower plate of a metamorphic core complex. The lower plate (footwall) core was juxtaposed against the upper plate (hanging wall) schists by the low-angle normal decollement faults. Detrital zircon U-Pb ages indicate Carboniferous, Permo-Triassic, Jurassic and Cretaceous maximum depositional ages of schist protoliths which demand more complex terrane and metamorphic interpretations for the Otago Schist than previously recognised.

KEYWORDS:

• New Zealand
• Haast Schist
• Otago Schist
• tectonics
• zircon
• thermochronology
• (U-Th)/He dating
• U-Pb dating

Acknowledgements

We thank Delia Tamsen for assistance in the field, Ben Durrant for thin sections, and John Simes for mineral separation. Pre-submission reviews by Adam Martin and Matt Sagar, and journal reviews by Matthias Bernet and Francisco Pereira helped us improve the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Supplemental Files 1–3 are available in the Figshare online repository at https://doi.org/10.6084/m9.figshare.21343041. Supplemental File 1 contains (U-Th)/He zircon data, Supplemental File 2 U-Pb zircon data, and Supplemental File 3 whole rock X-ray fluorescence data for samples from the Dunstan and Cairnmuir Mountains.

Additional information

Funding

Lee was supported by Central Washington University. Mortimer acknowledges funding from the New Zealand Ministry of Business Innovation and Employment’s Strategic Science Investment Fund [grant number C05X1702] to GNS Science’s ‘Understanding Te Riu-a Māui/Zealandia’ Research Programme.