GEOCHRONOLOGY AND GEOCHEMISTRY OF A MESOZOIC MAGMATIC ARC SYSTEM, FJORDLAND, NEW-ZEALAND

The Median Tectonic Zone in Eastern Fiordland, SW New Zealand, compris es a tectonically disrupted belt of Mesozoic magmatic are rocks relate d to subduction along the palaeo-Pacific margin of Gondwana. New ion m icroprobe (SHRIMP) U-Pb zircon ages confirm that the bulk of the pluto nic rocks in eastern Fiordland range from Mid- Jurassic to Early Creta ceous (168-137 Ma) in age. Carboniferous age granitoids occur in SW Fi ordland, along the western side of and within the zone. Triassic pluto nic rocks appear to be restricted to the eastern side of the zone. The Mid-Jurassic-Early Cretaceous igneous rocks (collectively referred to as the Darran Suite) are cut by several plutons of Na-rich granitoid (Separation Point Suite) that give ages of c. 124 Ma, slightly older t han equivalent rocks in the NW part of the South Island. Early Cretace ous granulite facies orthogneisses (126-119 Ma) in western Fiordland ( Western Fiordland Orthogneiss) are considered to be the lower crustal equivalent of the Separation Point plutons. The majority of the Darran Suite rocks are I-type, hornblende-bearing calc-alkaline igneous rock s, most likely derived from melting in the mantle wedge above a subduc ting slab of oceanic lithosphere. In contrast, the Separation Point-ty pe plutons are Na-rich, alkali-calcic granitoids with high concentrati ons of Sr (typically >500 ppm and up to 1000 ppm) and low concentratio ns of Y (less than or equal to 5 ppm) and heavy REE (<10 times chondri tic). Isotopic compositions are primitive, with Sr-87/Sr-86 initial ra tios of c. 0.7038; and epsilon(Nd) values of c. +3 at 120 Ma. Their ge ochemistry is consistent with melting of a mafic protolith of garnet a mphibolite mineralogy. Mafic Darran Suite rocks have the appropriate c hemical and isotopic compositions to generate the Western Fiordland Or thogneiss and the higher level Separation Point type plutons. We sugge st that the sudden appearance of large volumes of Na-rich magma during the Early Cretaceous was triggered tectonically, perhaps by thrusting of the Median Tectonic Zone are beneath western New Zealand. Melting of basal are underplate at depths of >40 km would then have generated Na-rich granitoids, leaving residues of garnet+clinopyroxene+amphibole .