ION MICROPROBE DATING OF PALEOZOIC GRANITOIDS - DEVONIAN MAGMATISM INNEW-ZEALAND AND CORRELATIONS WITH AUSTRALIA AND ANTARCTICA

Precise ion microprobe U-Pb zircon ages have been obtained from a repr esentative set of Paleozoic igneous cocks from the Western Province of the South Island, New Zealand. Granitoid rocks forming the Karamea Ba tholith and related plutons in the Buller terrane all yield crystallis ation ages that are indistinguishable within a +/-5 Ma uncertainty at 375 Ma (Middle-Late Devonian). Previous workers have suggested that th e batholith was emplaced over a long time interval and comprises rocks ranging in age from 370 to 310 Ma, with the bulk in the Early Carboni ferous. Granitoid rocks with Carboniferous ages (similar to 330 Ma) do occur further to the west and younger Cretaceous granitoids occur wit hin the Karamea Batholith, but these are not volumetrically significan t. A sample from the ultramafic-mafic Riwaka Complex in the adjacent T akaka terrane gave a crystallisation age of 376.9 +/- 5.6 Ma (2 sigma) , indicating emplacement coeval with the Karamea Batholith. The Paleoz oic granitoids contain a large amount of inherited zircon, with distin ct 390-, 500-600- and 1000-Ma components. The 390-Ma age corresponds t o widespread plutonism in the Lachlan Fold Belt in SE Australia. The 5 00-600-Ma (Ross-Delamerian age) and 1000-Ma (Grenville age) age compon ents have also been observed in granitoids from the Lachlan Fold Belt and in Ordovician metasedimentary rocks from SE Australia and New Zeal and. The inherited zircons in the Karamea Batholith could be derived f rom continental basement at depth, from the incorporation of upper-cru stal material into the granitoid magmas, or both. The Devonian granito ids in New Zealand can now be correlated with rocks of similar age in northern Victoria Land (Admiralty Intrusives) and Marie Byrd Land (For d Granodiorite) in West Antarctica, in NE Tasmania, and in the central part of the Lachlan Fold Belt in SE Australia. On a reconstruction of the SW Pacific margin of Gondwana, prior to later break-up, it is pos sible to trace out a semi-continuous magmatic belt in excess of 2000 k m in length.