Ion microprobe UPb zircon geochronology of granitic magmatism in the Western Province of the South Island, New Zealand

Abstract

Zircon UPb isotopic systems are described for three mid-Palaeozoic and four late Mesozoic granites from the Western Province of the South Island, New Zealand. The granites chosen are representative of the major plutons in this region. Two distinct episodes of Palaeozoic magmatism have been identified: Middle Devonian (∼ 380 Ma) and Early Carboniferous (∼ 330 Ma). The Mesozoic granites are all mid-Cretaceous in age, and range from ∼ 120 to ∼ 110 Ma. The O'Sullivans Granite, forming a large part of the Karamea Batholith, has a crystallisation age of 377.7 ± 4.1 Ma, and the Windy Point Granite from the Paparoa Batholith has an age of 328.6 ± 4.4 Ma. Further west, the granite at Cape Foulwind has a similar age of 327.3 ± 6.2 Ma. From east to west, the Cretaceous granites appear to decrease in age. The Separation Point Batholith is dated at ∼ 118 Ma. The oldest phase at Separation Point gave an age of 116.6 ± 1.9 Ma and the youngest phase, the Pearse Granodiorite, gave the same age within error at 119.4 ± 2.3 Ma. The Mount Olympus pluton in NW Nelson gave an age of 111.4 ± 2.0 Ma and the same age is found for the Buckland Granite, which forms most of the Paparoa Batholith, at 109.6 ± 1.7 Ma. Inherited zircon components as old as 1700 Ma have been identified in many of the granites, and distinct components are present at 1000 and 500–600 Ma. This pattern of zircon ages has also been observed in granites from the Lachlan Fold Belt and in Ordovician sediments from SE Australia and New Zealand. However, it is not clear whether the old zircons are derived from the lower crust, from the incorporation of upper-crustal material, or both. The Palaeozoic granites can be broadly correlated with rocks of similar age in Antarctica, SE Australia and Tasmania that formed part of the Gondwana supercontinent. The Cretaceous granites are probably part of an extensive Mesozoic magmatic arc that can be traced from South America through West Antarctica and on into New Zealand.