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

The Median Tectonic Zone (MTZ) of New Zealand is a generally north trending belt of Mesozoic subduction-related I-type plutonic, volcanic, and sedimentary rocks in South Island and Stewart Island that separates Permian strata of the Eastern Province Brook Street Terrane from lower to mid-Paleozoic Gondwana margin assemblages of the Western Province. High-precision isotope dilution U/Pb ages of zircons from 30 rocks are reported. Pre-digestion leaching of zircon in hydrofluoric acid yielded 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 to occur on the eastern side of the MTZ, and they intrude volcanic/sedimentary sequences of the MTZ in Nelson and eastern Fiordland. These sequences are in turn intruded by Late Jurassic - Early Cretaceous gabbronoritic to granitic plutonic rocks (c. 147-137 Ma), which volumetrically dominate the MTZ. Several Late Jurassic Early Cretaceous intrusive suites cannot be definitely linked to the 147-137 Ma group. These include a Rotoroa Complex gabbronorite (155 Ma), 157-151 Ma granitoid plutons at Lake Manapouri, and 134-131 Ma plutons at Lake Monowai. Quartzofeldspathic gneiss and crosscutting Late Carboniferous biotite leucogranite on Pepin Island, and Early Carboniferous tonalites (c. 337-343 Ma) in eastern Fiordland probably represent Western Province, but could alternatively represent local exposures of MTZ basement, or a hitherto unrecognised terrane. A parautochthonous origin for MTZ with respect to the Western Province is suggested by isolated minor Triassic intrusions that are chemically similar to MTZ plutons. Minor inheritance in several of the Late Jurassic - Early Cretaceous MTZ zircon populations also indicate the presence of old sialic crust beneath the are. Possible intrusive and sedimentary provenance links to several Eastern Province terranes caution against distinction of the MTZ as a discrete terrane in the strict sense. The essential features of the MTZ were established 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 - Early Cretaceous magmatic history outlined for the MTZ resembles that of the Thurston Island area of West Antarctica.