Rj. Muir et al., SHRIMP U-PB GEOCHRONOLOGY OF CRETACEOUS MAGMATISM IN NORTHWEST NELSON-WESTLAND, SOUTH ISLAND, NEW-ZEALAND, New Zealand Journal of Geology and Geophysics, 40(4), 1997, pp. 453-463
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.