SHRIMP U‐Pb geochronology of Cretaceous magmatism in northwest Nelson‐Westland, South Island, New Zealand

Abstract

Ion microprobe U‐Pb zircon ages have been obtained from four samples of Cretaceous granitoid and two samples of volcanogenic sediment from the northwest Nelson‐Westland region of the South Island of New Zealand. Crow Granite, which intrudes lower Paleozoic metasedi‐mentary rocks in the Buller Terrane on the eastern side of the Karamea Batholith, has given a crystallisation age of 137 ± 3 Ma (2a). This age is typical of the Jurassic‐Early Cretaceous plutonic rocks that dominate the Median Tectonic Zone, and raises the possibility that the Western Province and the Median Tectonic Zone were linked some 20 m.y. earlier than previously proposed. The “Gouland granod‐iorite”, which forms a large pluton at the northeastern margin of the Karamea Batholith, has a crystallisation age of 119 ± 2 Ma (2a). This age is similar to the Separation Point Batholith (118 Ma), and the distinctive chemistry of the batholith (high Na, Al, Sr, and low Y) is also displayed by the Gouland granodiorite. The “Big Deep granite”, which intrudes Devonian granites of the Karamea Batholith in the Upper Buller Gorge, has a crystallisation age of 110 ± 3 Ma (2a), similar to the age of Buckland Granite (Rahu Suite) in the Paparoa Batholith. Berlins Porphyry in the Lower Buller Gorge also has a mid‐Cretaceous age of 111 ± 2 Ma (2a). Two samples of Stitts Tuff from the Lower Buller Gorge have given crystallisation ages of 101 ± 2 Ma (2a) and 102 ± 3 Ma (2a). These ages constrain the timing of initiation of fault‐bounded sedimentary basins on the West Coast, related to a major period of crustal extension before the opening of the Tasman Sea Basin.

Keywords:

• Cretaceous
• granite
• geochronology
• U‐Pb isotopes
• zircon
• New Zealand