SHRIMP U-PB GEOCHRONOLOGY OF CRETACEOUS MAGMATISM IN NORTHWEST NELSON-WESTLAND, SOUTH ISLAND, NEW-ZEALAND

Ion microprobe U-Pb zircon ages have been obtained from four samples o f Cretaceous granitoid and two samples of volcanogenic sediment from t he northwest Nelson-Westland region of the South Island of New Zealand . Crow Granite, which intrudes lower Paleozoic metasedimentary rocks i n the Buller Terrane on the eastern side of the Karamea Batholith, has given a crystallisation age of 137 +/- 3 Ma (2 sigma). This age is ty pical of the Jurassic-Early Cretaceous plutonic rocks that dominate th e Median Tectonic Zone, and raises the possibility that the Western Pr ovince and the Median Tectonic Zone were linked some 20 m.y. earlier t han previously proposed. The ''Gouland granodiorite'', which forms a l arge pluton at the northeastern margin of the Karamea Batholith, has a crystallisation age of 119 +/- 2 Ma (2 sigma). This age is similar to the Separation Point Batholith (118 Ma), and the distinctive chemistr y of the batholith (high Na, Al, Sr, and low Y) is also displayed by t he Gouland granodiorite. The ''Big Deep granite'', which intrudes Devo nian granites of the Karamea Batholith in the Upper Buller Gorge, has a crystallisation age of 110 +/- 3 Ma (2 sigma), similar to the age of Buckland Granite (Rahu Suite) in the Paparoa Batholith. Berlins Porph yry in the Lower Buller Gorge also has a mid-Cretaceous age of 111 +/- 2 Ma (2 sigma). Two samples of Stitts Tuff from the Lower Buller Gorg e have given crystallisation ages of 101 +/- 2 Ma (2 sigma) and 102 +/ - 3 Ma (2 sigma). These ages constrain the timing of initiation of fau lt-bounded sedimentary basins on the West Coast, related to a major pe riod of crustal extension before the opening of the Tasman Sea Basin.