STEPWISE COOLING AND EXHUMATION OF A GRANULITE TERRANE IN FIORDLAND, NEW ZEALAND

Fiordland, in southwestern New Zealand, is dominated by the Cretaceous granulite facies Western Fiordland Orthogneiss, which is overlain by a cover sequence of Paleozoic age. The Paleozoic cover sequence originally formed part of the active Pacific margin of Gondwana, which underwent extensional collapse and partial exhumation following magmatic thickening of the crust in the mid Cretaceous. The subsequent break-up of Gondwana resulted in a tectonically quiet period while Australia and New Zealand drifted apart. Final exhumation of Fiordland only took place after plate reconfiguration in the Miocene.

The first stage of cooling started soon after intrusion of the 126-116 Ma Western Fiordland Orthogneiss and its subsequent high pressure metamorphism. ⁴⁰Ar-³⁹Ar dating of hornblendes and micas indicate relatively rapid cooling to below ~350 °C by about 90 Ma for the Western Fiordland Orthogneiss and cooling to below ~350 °C by about 100 Ma for the Paleozoic cover sequence. Decompression textures in the Western Fiordland Orthogneiss show that cooling was associated with partial exhumation.

Multi-diffusion domain modelling of K-feldspar ⁴⁰Ar-³⁹Ar age spectra indicates that cooling slowed down considerably around 90 Ma and that temperatures remained virtually stable for more than 70 Ma. This heralded the onset of continental break-up and subsequent initiation of seafloor spreading in the Tasman Sea and suggests a tectonically quiet period for Fiordland during the opening of the Tasman Sea. The start of the second stage of cooling and final exhumation is well constrained at around 15 Ma (K-feldspar ⁴⁰Ar-³⁹Ar age spectra) which is in good agreement with apatite fission track ages of around 7 Ma. The change in cooling rate around 15 Ma is thought to indicate the start of compression across the precursor of the Alpine Fault.