A younger age constraint on high‐grade metamorphism near George Sound in Fiordland, New Zealand

Abstract

A younger age limit to high‐grade metamorphism in part of western Fiordland, New Zealand, is obtained from an unmetamorphosed hornblende gabbro that intrudes kyanite‐margarite‐bearing amphibolite in the mountains east of George Sound. Laser ablation ICP‐MS analyses and cathodoluminescence imaging show that zircon grains from the hornblende gabbro form two distinct populations: (1) brightly luminescent grains with a pooled age of 345.8 ± 2.9 Ma and dully luminescent rims, and (2) dully luminescent whole grains with a pooled age of 118.5±1.2 Ma. On the basis of a lack of metamorphic assemblages, and distinct zircon rare earth element concentrations, the hornblende gabbro is interpreted to have been emplaced at 118.5 ± 1.2 Ma. This age puts a younger limit to kyanite grade metamorphism in this area and demonstrates that it did not re‐equilibrate during subsequent extension, as has been suggested for other parts of Fiordland.