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New constraints on the age and evolution of the Wishbone Ridge, southwest Pacific Cretaceous microplates, and Zealandia–West Antarctica breakup

¹ Institute of Geological and Nuclear Sciences, Private Bag 1930, Dunedin, New Zealand
² IFM-GEOMAR Leibniz Institute for Marine Sciences, Wischhofstrasse 1-3, D-24148 Kiel, Germany
³ Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand
⁴ Research School of Earth Sciences, Australian National University, Canberra, Australia
⁵ Tethys Geoconsulting GmbH, Wischhofstrasse 1-3, D-24148 Kiel, Germany
⁶ Institute of Geological and Nuclear Sciences, P.O. Box 30368, Lower Hutt, New Zealand

We present analytical results from four dredge locations across the eastern Zealandia continental margin and adjacent ocean crust. The 115 Ma dacites dredged from the West Wishbone Ridge (WWR) are isotopically primitive, weakly adakitic, slab-derived lavas. The 97 Ma A-type granites and a basalt from the easternmost Chatham Rise enlarge the known area of postsubduction Gondwana magmatism. Amphibolite-grade schists from a fault block south of the Chatham Rise provide a critical bridge between the Zealandia and West Antarctica belts of Jurassic–Cretaceous accretionary prism rocks. The new recognition of the WWR as a remnant of a 115 Ma intraoceanic subduction system means that previous hypotheses of the WWR as a fracture zone or spreading ridge require modification. The dacite ages constrain the start of Osbourn Trough spreading, which caused breakup of the Hikurangi-Manihiki igneous plateau, to before 115 Ma. We speculate that, after 115 Ma, the WWR was rifted by an intraoceanic spreading center that developed along its southeast side. Impingement of this spreading center against the Gondwana margin led to widespread 95–100 Ma postsubduction magmatism, variable lithospheric stretching, and ultimately continental splitting of Zealandia and West Antarctica across basement trends.

Key Words: Cretaceous • southwest Pacific • Gondwana • dating • geochemistry • tectonics

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