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

Abstract

Diverse, mappable, and variably mylonitised highgrade 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 highgrade 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.

Magmatism during Gondwana break-up: new geochronological data from Westland, New Zealand

“…In south Westland, the 64.8 ± 0.8 Ma Arnott Basalt (Timm et al 2010) and a 61.4 ± 0.8 Ma rhyolite clast (Phillips et al 2005) remain the only accurately dated magmatic events during Tasman Sea spreading. Mafic dikes have also been found in the deformed Fraser Complex and Granite Hill Complex (Rattenbury 1991;Waight et al 1997). The vast majority of the dikes found in the Fraser Complex were deformed during mylonitization of the surrounding country rock (Rattenbury 1991) but this does not seem to be the case for dikes found on Granite Hill (Waight 1995;Waight et al 1997).…”

“…Mafic dikes have also been found in the deformed Fraser Complex and Granite Hill Complex (Rattenbury 1991;Waight et al 1997). The vast majority of the dikes found in the Fraser Complex were deformed during mylonitization of the surrounding country rock (Rattenbury 1991) but this does not seem to be the case for dikes found on Granite Hill (Waight 1995;Waight et al 1997).…”

“…1). The Fraser Complex was metamorphosed at kyanite grade (Rattenbury 1991) in the Cretaceous (Hiess et al 2010), and was subsequently mylonitized. K/Ar dating of biotite (Rattenbury 1987) suggests cooling through the biotite closure temperature (c. 450°C, Villa 1998) at c. 44-61 Ma.…”

“…The complex is composed of amphibolite-facies orthogneiss and paragneiss (Rattenbury 1991). The protolith may represent a metamorphosed equivalent of regional granitic and gneissic basement to Greenland Group sediments (Rattenbury 1991). More recently it has been suggested to represent metamorphosed Buller Terrane sedimentary rocks and intrusives, including 110 Ma Rahu Suite granitoids as the youngest unit identified within the protolith (Hiess et al 2010).…”

“…The complex is composed of amphibolite-facies orthogneiss and paragneiss (Rattenbury 1991). The protolith may represent a metamorphosed equivalent of regional granitic and gneissic basement to Greenland Group sediments (Rattenbury 1991).…”