URANIUM-LEAD ZIRCON AGES FROM THE MEDIAN TECTONIC ZONE, NEW-ZEALAND

The Median Tectonic Zone (MTZ) of New Zealand is a generally north tre nding belt of Mesozoic subduction-related I-type plutonic, volcanic, a nd sedimentary rocks in South Island and Stewart Island that separates Permian strata of the Eastern Province Brook Street Terrane from lowe r to mid-Paleozoic Gondwana margin assemblages of the Western Province . High-precision isotope dilution U/Pb ages of zircons from 30 rocks a re reported. Pre-digestion leaching of zircon in hydrofluoric acid yie lded significantly more concordant residues by removing common Pb and dissolving more soluble high-U domains that have been more affected by relatively recent Pb loss. The results show that MTZ magmatism ranges in age from at least Early Triassic to Early Cretaceous (247-131 Ma), with a pronounced gap in the Middle Jurassic. Triassic plutons tend t o occur on the eastern side of the MTZ, and they intrude volcanic/sedi mentary sequences of the MTZ in Nelson and eastern Fiordland. These se quences are in turn intruded by Late Jurassic - Early Cretaceous gabbr onoritic to granitic plutonic rocks (c. 147-137 Ma), which volumetrica lly dominate the MTZ. Several Late Jurassic Early Cretaceous intrusive suites cannot be definitely linked to the 147-137 Ma group. These inc lude a Rotoroa Complex gabbronorite (155 Ma), 157-151 Ma granitoid plu tons at Lake Manapouri, and 134-131 Ma plutons at Lake Monowai. Quartz ofeldspathic gneiss and crosscutting Late Carboniferous biotite leucog ranite on Pepin Island, and Early Carboniferous tonalites (c. 337-343 Ma) in eastern Fiordland probably represent Western Province, but coul d alternatively represent local exposures of MTZ basement, or a hither to unrecognised terrane. A parautochthonous origin for MTZ with respec t to the Western Province is suggested by isolated minor Triassic intr usions that are chemically similar to MTZ plutons. Minor inheritance i n several of the Late Jurassic - Early Cretaceous MTZ zircon populatio ns also indicate the presence of old sialic crust beneath the are. Pos sible intrusive and sedimentary provenance links to several Eastern Pr ovince terranes caution against distinction of the MTZ as a discrete t errane in the strict sense. The essential features of the MTZ were est ablished by c. 126-105 Ma as indicated by stitching of the MTZ to the Western Province by granites of the Separation Point Suite of that age in Nelson, eastern Fiordland, and Stewart Island. The Triassic - Earl y Cretaceous magmatic history outlined for the MTZ resembles that of t he Thurston Island area of West Antarctica.