Cretaceous chronology of the Lower Buller Valley, South Island, New Zealand

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Abstract
Twenty-six new potassium-argon whole rock and mineral ages of Cretaceous igneous and metamorphic rocks in the Lower Buller Valley provide time limits for deposition of the nonmarine Pororari Group, and allow a chronology for late Rangitata events on the Foreland Province to be established. Whole rock and biotite ages of the Bedins Porphyry range from 105–109 m.y., in agreement with earlier rubidium-strontium dating (110 m.y.), and indicate a late Aptian to early Albian age for emplacement of the Berlins Porphyry and eruption of the contemporaneous Stitts Tuff Member at the base of the Ohika Formation. Mineral ages of the Buckland Granite and associated hornfels range from 96–99 m.y., and reflect uplift and unroofing of the Buckland pluton, and rapid deposition of the granite-derived part of the Pororari Group in middle to late Albian time. Whole rock and mineral ages of lamprophyre dikes range between 78–84 m.y., which are slightly younger than ages from other dated lamprophyres in Westland and West Otago. A single basalt sill was dated at 89–90 m.y. A dated tuff bed in the lower part of the Kyeburn Formation (East Otago) falls within the age lange of the Stitts Tuff Member, indicating the contemporaneity of both acid volcanism and rapid sedimentation in widely separated parts of the South Island. Plutonism and uplift between 110–95 m.y. are closely related in the Lower Buller Valley, and are interpreted as a late Rangitata event.