Tracking The Metamorphic Evolution Of The Root To A Magmatic Arc: Coronas And Shear Zones In The Western Fiordland Orthogneiss, Doubtful Sound, New Zealand

The Western Fiordland Orthogneiss (WFO) is an arc-related composite batholith comprising metadiorite and metagabbro that intruded the lower crust during the early Cretaceous (126 and 109~Ma). The development of localised garnet granulite (high-P) mineral assemblages within the WFO (P ≈ 13~kbar, T\approx 780~°C) immediately followed batholith emplacement. The P--T evolution of the WFO has been a topic of debate in recent years; issues such as the depth of intrusion and the post-emplacement evolution remain controversial. In this study we combine detailed petrology and calculated mineral equilibria to help shed some light on the evolution of the WFO from emplacement, through to garnet granulite formation and finally exhumation. Corona reaction textures and discrete shear zone assemblages record changing mineral paragenesis. Five discrete metamorphic events (M1-5) can be distinguished. M1 represents amphibolite facies hydration of igneous assemblages. M2 is synonymous with a progressive dehydration of M1 hornblende in gabbroic rock compositions and involves symplectic intergrowths of clinopyroxene, biotite and quartz after igneous and M1 minerals. M3 garnet granulite is only observed within spatially localised zones of dehydration termed Garnet Reaction Zones (GRZ). Within the GRZs garnet-clinopyroxene-K-feldspar- quartz coronas enclose all earlier formed mineral assemblages. M4 mineral assemblages are restricted to major shear zones and represent the recrystalisation and hydration of garnet granulite assemblages to form garnet amphibolite. Late stage fracturing and hydration induce the development of greenschist facies M5 mineral assemblages replacing the earlier formed textures. P--T pseudosections calculated in the chemical system NCKFMASH using THERMOCALC are consistent with M1 assemblages having formed during post-magmatic isobaric cooling at low-P. Cooling was followed by burial of the terrain and the development of transitional granulite-garnet granulite facies M2 symplectites. Further cooling at high-P resulted in the formation of garnet granulite facies assemblages in M3 GRZs. The generation of garnet granulite was followed by a switch from burial to exhumation and cooling with concomitant development of crustal scale M4 garnet amphibolite facies shear zones. A dynamic history is inferred for the batholith: mid crustal intrusion, followed by cooling and burial contemporary with further magmatic flux. The development of localised garnet granulite reflects burial of the WFO to P ≈ 14~kbar, and was proceeded by a switch in the tectonic regime that resulted in cooling and exhumation of the terrane along crustal-scale extensional shear zones.