Timing of mylonitisation west of the Alpine Fault, central Westland, New Zealand

Abstract

Detailed mapping of Fraser Formation has revealed a different relationship between mylonitic and gneissic rocks than found by previous workers. Mylonitic zones envelop areas of amphibolite facies gneisses and granitoid rocks, ranging up to several square kilometre~, throug~ out the area. Mylonitisation does not mcrease m intensity towards either the Fraser _Fault or the Alpine Fault. Numerous lamprophync and trachytic dikes intrusive into Fraser Formation have not been observed to cut the foliation in the mylonite zones and several of the dikes have been truncated

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

“…U value, according to methods described by Compston et al (1984). through central (Adams & Nathan 1978;Tulloch 1979;White 1987;Rattenbury 1987Rattenbury , 1991Tulloch & Kimbrough 1989;Waight et al 1998b;Jongens 2006) and north Westland (Hunt & Nathan 1976). Additionally, post-110 Ma felsic (e.g.…”

“…Additionally, the age demonstrates that mylonitization of this Westland unit cannot be exclusively related to Early Cretaceous core complex formation (cf. Kimbrough et al 1994) and is therefore likely to be related to the adjacent Alpine Fault, as speculated by Rattenbury (1987Rattenbury ( , 1991. Combined with the biotite K/Ar cooling ages of 44-61 Ma (Rattenbury 1987) this would constrain the ductile mylonitization that affected the trachyte dike to have occurred between an interval with 72 and 44 Ma as the oldest and youngest possible ages, respectively.…”

“…Kimbrough et al 1994) and is therefore likely to be related to the adjacent Alpine Fault, as speculated by Rattenbury (1987Rattenbury ( , 1991. Combined with the biotite K/Ar cooling ages of 44-61 Ma (Rattenbury 1987) this would constrain the ductile mylonitization that affected the trachyte dike to have occurred between an interval with 72 and 44 Ma as the oldest and youngest possible ages, respectively. However, this age range precedes the onset of the Alpine Fault at 30-25 Ma (Cooper et al 1987;Schellart et al 2006;Furlong & Kamp 2009).…”

“…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. Basaltic, lamprophyric and trachyte dikes in the Fraser Complex were deformed during mylonitization.…”

Gneissic rocks of the Bonar Range, central Westland, New Zealand

“…These rocks have all undergone amphibolite facies metamorphism, have been variably mylonitised, and are faulted against low-grade Greenland Group metasediments (summarised in Nathan et al 2002). The juxtaposition with low-grade rocks has been attributed to Cretaceous low-angle detachment faulting in the Paparoa Range (Tulloch & Kimborough 1989), and for the Fraser Complex, high-angle fault movement associated with late Tertiary uplift on the Alpine Fault (White & Green 1986;Rattenbury 1987 In the Bonar Range of central Westland ( Fig. 1,2) a poorly documented group of high-grade gneissic rocks is also exposed.…”