THE FRASER COMPLEX - HIGH-GRADE METAMORPHIC, IGNEOUS AND MYLONITIC ROCKS IN CENTRAL WESTLAND, NEW-ZEALAND

Diverse, mappable, and variably mylonitised high-grade gneisses and granitoids, and lamprophyre, trachyte, and basalt dikes comprise the Fraser Complex, which is a new name proposed to replace the existing Fraser Formation. Sillimanite-bearing pelitic gneisses have an upper amphibolite facies metapelitic mineralogy, indicating temperatures > 600-degrees-C. Quartzofeldspathic gneisses, lacking sillimanite, are migmatitic, and hornblende gneisses have a diverse epidote-amphibolite mineralogy. The high-grade gneisses form a metamorphic suite within the Fraser Complex. Granite, granodiorite, and tonalite are intrusive into the high-grade gneiss suite, and may have been derived from partial melts of the high-grade gneisses. Swarms of camptonite lamprophyre, trachyte, and basalt dikes intrude the high-grade gneisses and the granitoids. Mylonite zones now envelop the gneisses, granitoids, and dikes, and mylonitic deformation occurred at mid-greenschist facies conditions and resulted in some retrogressive alteration of the protoliths. The Fraser Complex forms part of a regional gneissic and granitic basement to the Greenland Group in Westland. The juxtapostion of the Fraser Complex, with Mesozoic high-grade metamorphism, against the low-grade Ordovician Greenland Group sediments can be explained by vertical movement along the Fraser Fault rather than by invoking large-scale normal detachment faulting, as has been suggested in North Westland.