Late Jurassic‐Early Cretaceous metamorphic age of Fraser Complex migmatite, Westland, New Zealand

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Abstract
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 upper intercept date of 735 ± 100 Ma indicates inheritance of Proterozoic zircon. The mid‐Mesozoic age for high‐grade metamorphism does not support the existence of a regional Precambrian gneiss basement, incorporating the Fraser Complex and other gneissic rocks in Westland, as suggested by earlier workers. Early Paleozoic Greenland Group, of subgreenschist 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 New Zealand Western Province where lower plate amphibolite facies metamorphic rocks are separated from upper plate Greenland Group strata by detachment faults. Although the mid‐Mesozoic age of metamorphism reported here suggests an affinity with the Mesozoic Median Terrane, fault juxtaposition with Greenland Group strata, petrologic affinity of associated granitoids with those of the Western Province, and Proterozoic zircon inheritance, suggest that the Fraser Complex originated within the Western Province. The Fraser Fault may have originated as a low‐angle normal fault that was subsequently deformed by Cenozoic transpression on the Alpine Fault.