The Fraser Complex: High‐grade metamorphic, igneous and mylonitic rocks in central Westland, New Zealand

Abstract

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°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.

Keywords:

• petrology
• geochemistry
• gneisses
• granitoids
• dikes
• mylonites
• Fraser Complex
• Westland