GARNET SM-ND AGE CONSTRAINTS ON GRANULITE METAMORPHISM IN THE LOWER CRUST OF THE FIORDLAND MAGMATIC ARC, NEW ZEALAND

We report new garnet Sm-Nd ages for granulite facies metamorphism in the northernmost pluton of the Western Fiordland Orthogneiss (WFO) in New Zealand. Zircon U-Pb ages from the Worsley pluton indicate that the Worsley is one of the oldest in the WFO at ca. 126-120 Ma (Milan et al. 2016). Zircon and Titanite thermochronology data suggest that thermal weakening of the lower crust occurred during a short burst of mafic to intermediate magmatism (Misty and Malaspina plutons of the southern WFO) from 118 to 115 Ma. This was followed by garnet granulite facies metamorphism (Stowell et al. 2014), partial melting, and lower crustal flow from 115 to 108 Ma, and extensional collapse (Schwartz et al. 2016). However, it is still unclear exactly what role the earlier-emplaced Worsley Pluton had on the lower crust, and how and when the pluton was affected by garnet granulite facies metamorphism.

New Sm, Nd, U, and Pb isotope data from the Worsley Pluton place constraints on the timing of the granulite metamorphism that affected the pluton. New data are from the north side of Bligh Sound ~4 km inland from the mouth (15NZ25) and from the southeastern slope of Mount Longsight approximately 4 km east-northeast of the first sample (P76580). A zircon U-Pb age of 115.2 ±0. 9 was recently determined from laser ablation of sample 15NZ25, and two garnet Sm-Nd ages of 114.3 ± 3.0 Ma and 104.9 ± 3.0 Ma were determined from sample P76580 using mass spectrometry. Previous U-Pb zircon ages (SHRIMP-RG) from Bligh Sound samples 15NZ2 and 15NZ27 are 113.7 ± 3.6 and 119.3 ± 2.4 respectively (Schwartz et al. 2017). Our new ages indicate that granulite facies metamorphism may have occurred between 0 and ~20 Ma after pluton emplacement, which may help to further constrain the timing of metamorphism.