LATE JURASSIC EARLY CRETACEOUS METAMORPHIC AGE OF FRASER COMPLEX MIGMATITE, WESTLAND, NEW-ZEALAND

Discordant U-Pb zircon data from a migmatitic leucosome in amphibolite facies gneiss of the Fraser Complex define a 157 +/- 21 Ma concordia lower intercept date interpreted as the age of peak metamorphism. An u pper intercept date of 735 +/- 100 Ma indicates inheritance of Protero zoic zircon. The mid-Mesozoic age for high-grade metamorphism does not support the existence of a regional Precambrian gneiss basement, inco rporating the Fraser Complex and other gneissic rocks in Westland, as suggested by earlier workers. Early Paleozoic Greenland Group, of subg reenschist facies regional metamorphic grade, is separated from Fraser Complex by the Fraser Fault. This structural relationship is similar to that described from Cretaceous metamorphic core complexes in the Ne w Zealand Western Province where lower plate amphibolite facies metamo rphic rocks are separated from upper plate Greenland Group strata by d etachment faults. Although the mid-Mesozoic age of metamorphism report ed here suggests an affinity with the Mesozoic Median Terrane, fault j uxtaposition with Greenland Group strata, petrologic affinity of assoc iated granitoids with those of the Western Province, and Proterozoic z ircon inheritance, suggest that the Fraser Complex originated within t he Western Province. The Fraser Fault may have originated as a low-ang le normal fault that was subsequently deformed by Cenozoic transpressi on on the Alpine Fault.