Geology of crystalline rocks of northern Fiordland: Details of the granulite facies Western Fiordland Orthogneiss and associated rock units

Abstract

Abstract A c. 700 km2 area of northern Fiordland (South Island, New Zealand) is described in which Early Cretaceous high-pressure metamorphic rocks and virtually unmetamorphosed plutonic rocks occur. The dominant rocks are orthogneisses developed from synmetamorphic basic-intermediate intrusive complexes, the youngest and most widespread of which is the Early Cretaceous Western Fiordland Orthogneiss (WFO). The latter has undergone granulite facies metamorphism and occurs throughout much of western Fiordland. In the study area, the WFO protolith intruded a country rock of amphibolite facies metasediments and orthogneisses. Fragments of the country rock are rafted within WFO and are represented by George Sound Paragneiss and Rafted Granitoid Gneisses. External country rock is represented by Arthur River Complex and Jagged Gneiss (possibly related to the Anita Ultramafites); it may also include Indecision Creek Complex and Mount Anau Complex. The George Sound Paragneiss is correlated with the Central Fiordland Metasediments of Oliver & Coggon. WFO was emplaced synkinematically in a subductionrelated magmatic arc. A collisional event during or immediately following magma emplacement resulted in crustal thickening equivalent to onloading of a 20 km thick section over rocks already buried at mid-crustal depths. This event was responsible for peak load pressures of c. 12–13 kbar recorded throughout WFO and in at least part of the country rock. Intrusive relations and a shared metamorphic and structural history indicate that WFO granulites are not tectonically exotic relative to associated amphibolite facies rocks in western Fiordland. The steeply dipping Surprise Creek Fault juxtaposes high-pressure metamorphic rocks of western and central Fiordland against virtually unmetamorphosed gabbroic rocks of the Early Cretaceous Darran Complex. This structure is a major tectonic break and separates fundamentally different crustal levels of a disrupted Early Cretaceous magmatic arc. Arc magmatism, collision, and subsequent great uplift are key events controlling Mesozoic Fiordland evolution.