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 kilometres, throughout the area. Mylonitisation does not increase in intensity towards either the Fraser Fault or the Alpine Fault. Numerous lamprophyric 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 by mylonitic fabric. These dikes therefore give a maximum age of mylonitisation, of probably middle Cretaceous, rather than a minimum age as previously believed. Structural data from the mylonites are consistent with dextral strike-slip movement.

New potassium-argon ages are presented from Fraser Formation mylonites and gneisses. Four hornblende ages range from 228 to 298 Ma, three whole-rock ages from 45 to 91 Ma, and five biotite ages from 44 to 61 Ma. These results do not discriminate between gneisses and mylonites, and reflect regional temperature changes within Fraser Formation. Excess argon contamination of hornblende has resulted in anomalously old ages, and hornblende closure probably occurred after Late Jurassic-Early Cretaceous. Whole-rock ages vary according to whether their mafic mineralogy is dominated by hornblende or biotite. The closely clustered biotite ages may date a period of cooling in the Middle Eocene. Mylonitisation of Fraser Formation occurred at temperatures within the accepted K-Ar biotite closure temperature range but the K-Ar biotite ages indicate that these temperatures have not been attained since 44 Ma. Early Tertiary mylonitisation is, however, inconsistent with strike-slip movement of the Alpine Fault having occurred in the middle-late Tertiary. Either the mylonitisation is unrelated to Alpine Fault movement, despite compatible structural data, or, more likely, the K-Ar biotite ages have been significantly affected by excess argon contamination.

Keywords:

• mylonitisation
• lamprophyre
• dikes
• Alpine Fault
• Fraser Fault
• Fraser Formation
• K/Ar
• absolute age
• Westland