Anti-clockwise P–T paths in the lower crust: an example from a kyanite-bearing regional aureole, George Sound, New Zealand

The George Sound Paragneiss (GSP) represents a rare Permo-Triassic unit in Fiordland that occurs as a km-scale pillar to gabbroic and dioritic gneiss of c . 120 Ma Western Fiordland Orthogneiss (WFO). It is distinguished from Palaeozoic paragneiss common in western Fiordland (Deep Cove Gneiss) by SHRIMP and laser-ablation U–Pb ages as young as c . 190 Ma and ¹⁷⁶Hf/¹⁷⁷Lu >0.2828 for detrital zircon grains. The Mesozoic age of the GSP circumvents common ambiguity in the interpretation of Cretaceous v. Palaeozoic metamorphic assemblages in the Deep Cove Gneiss. A shallowly dipping S₁ foliation is preserved in the GSP distal to the WFO, cut by 100 m scale migmatite contact zones. All units preserve a steeply dipping S₂ foliation. S₁ staurolite and sillimanite inclusions in the cores of metapelitic garnet grains distal to the WFO preserve evidence for prograde conditions of T  <   650 °C and P <  8 kbar. Contact aureole and S₂ assemblages include Mg-rich, Ca-poor cores to garnet grains in metapelitic schist that reflect WFO emplacement at ≈760 °C and ≈6.5 kbar. S₂ kyanite-bearing matrix assemblages and Ca-enriched garnet rims reflect ≈650 °C and ≈11 kbar. Poorly oriented muscovite–biotite intergrowths and rare paragonite reflect post-S₂ high- P retrogression and cooling. Pseudosection modelling in NCKFMASH defines a high- P anti-clockwise P–T history for the GSP involving: (i) mid- P amphibolite facies conditions; preceding (ii) thermal metamorphism adjacent to the WFO; followed by (iii) burial to high- P and (iv) high- P cooling induced by tectonic juxtaposition of cooler country rock.