Cretaceous high‐P granulites at Milford Sound, New Zealand: metamorphic history and emplacement in a convergent margin setting

Abstract

Granulite facies orthogneiss of the Arthur River Complex (ARC) at Milford Sound, western Fiordland records a complex Early Cretaceous magmatic and orogenic history for the Pacific Gondwana margin that culminated in the emplacement and burial of a dioritic batholith, the Western Fiordland Orthogneiss (WFO). Enstatite‐bearing mafic to intermediate protoliths of the ARC and WFO intruded the middle to upper crust. The early deformation history of the ARC is preserved in the Pembroke Granulite, where two‐pyroxene S1 assemblages that reflect P<8 kbar and T >750 °C were only patchily recrystallized during later deformation. S1 is cut by garnet‐bearing, leucogabbroic to dioritic veins, which are cut by distinctive D2 fractures involving anorthositic veins and garnet–diopside–plagioclase‐bearing reaction zones. These zones are widespread in the ARC and WFO and record conditions of P≈14 kbar and T >750 °C. Garnet–clinopyroxene‐bearing corona reaction textures that mantle enstatite in both the ARC and WFO reflect Early Cretaceous burial by approximately 25 km of continental crust. Most of the ARC is formed from the Milford and Harrison Gneisses, which contain steeply dipping S4 assemblages that envelop the Pembroke Granulite and involve garnet, hornblende, diopside, clinozoisite, rutile and plagioclase, with or without kyanite. The P–T history of rocks in western Fiordland reflects pronounced Early Cretaceous convergence‐related tectonism and burial, possibly related to the collision and accretion of island arc material onto the Pacific Gondwana margin.