Granulite facies thermal aureoles and metastable amphibolite facies assemblages adjacent to the Western Fiordland Orthogneiss in southwest Fiordland, New Zealand

Abstract

In southwest New Zealand, a suite of felsic diorite intrusions known as the Western Fiordland Orthogneiss (WFO) were emplaced into the mid to deep crust and partially recrystallized to high-'P' (12 kbar) granulite facies assemblages. This study focuses on the southern most pluton within the WFO suite (Malaspina Pluton) between Doubtful and Dusky sounds. New mapping shows intrusive contacts between the Malaspina Pluton and adjacent Palaeozoic metasedimentary country rocks with a thermal aureole ∼200-1000 m wide adjacent to the Malaspina Pluton in the surrounding rocks. Thermobarometry on assemblages in the aureole indicates that the Malaspina Pluton intruded the adjacent amphibolite facies rocks while they were at depths of 10-14 kbar. Similar 'P-T' conditions are recorded in high-'P' granulite facies assemblages developed locally throughout the Malaspina Pluton. Palaeozoic rocks more than ∼200-1000 m from the Malaspina Pluton retain medium-P mid-amphibolite facies assemblages, despite having been subjected to pressures of 10-14 kbar for > 5 Myr. These observations contradict previous interpretations of the WFO Malaspina Pluton as the lower plate of a metamorphic core complex, everywhere separated from the metasedimentary rocks by a regional-scale extensional shear zone (Doubtful Sound Shear Zone). Slow reaction kinetics, lack of available H₂O, lack of widespread penetrative deformation, and cooling of the Malaspina Pluton thermal anomaly within 'c'. 3-4 Myr likely prevented recrystallization of mid amphibolite facies assemblages outside the thermal aureole. If not for the evidence within the thermal aureole, there would be little to suggest that gneissic rocks which underlie several 100 km² of southwest New Zealand had experienced metamorphic pressures of 10-14 kbar. Similar high-'P' metamorphic events may therefore be more common than presently recognized.