Garnet Sm-Nd and Zircon U-Pb Ages Track Pluton Emplacement, Granulite Facies Metamorphism, Partial Melting, and Extension in the Lower Crust, Fiordland New Zealand

Extensional collapse of over thickened magmatic arc crust depends on crustal thickness variations and architecture. The structural architecture of the lower- and mid-crustal at the onset of extension may be complex due to lithologic variation resulting from structural juxtaposition of pre-arc lithologies, intrusion of plutons, and local partial melting. Additional complexity is introduced by the dynamic nature of arcs. Thus, robust ages for deformation, metamorphism, intrusion, and partial melting are essential for unraveling arc evolution and discerning the role of arc plutonism and metamorphism in strain localization during both contraction and extension. Eclogite and granulite facies metamorphic minerals indicate that Mesozoic arc crust in Fiordland was ≥ 50 km thick ca. 130 Ma prior to extensional collapse. This mid- to lower-crust records a history of mafic to intermediate magmatism, high-grade metamorphism, lower crustal melting, and the formation of extensional detachments that border eclogite- and granulite-cored gneiss domes. U-Pb zircon and Sm-Nd garnet ages indicate that intrusion of voluminous plutons, including the Western Fiordland Orthogneiss (WFO), and subsequent metamorphism occurred sequentially from north to south. Pluton emplacement occurred at 0.6-1.1 GPa in the north to 1.0-1.2 GPa in the south. In northern Fiordland [Milford Sound], intrusion of 135-128 Ma gabbroic magma was followed by 0.6-1.1 GPa 2-pyroxene granulite metamorphism at 126-135 Ma, and then 1.2-1.4 GPa garnet granulite metamorphism and partial melting ca. 126-123 Ma. To the south, WFO plutons have a similar history from north to south: the low-P 125-120.1 Ma Worsley was metamorphosed to garnet granulite at 1.2-1.4 GPa, ca. 115 Ma; the low-P ca. 120 Ma Misty was metamorphosed to garnet granulite at 1.2 GPa, ca. 115 Ma; the high-P 117.8-113.2 Ma Malaspina was metamorphosed to garnet granulite at 1.0-1.4 GPa, ca. 113 Ma along Doubtful Sound and 111.9±1.6 Ma to the south on Resolution Island. New ages, and metamorphic and structural studies link periods of magmatism and metamorphism to specific spatial and temporal patterns of extensional deformation. The Malaspina pluton in central Fiordland underwent garnet granulite metamorphism <3.5 Myr after emplacement and then in part cooled to amphibolite facies conditions ca. 110 Ma. Fabrics record a progression from magmatic flow to high-T extensional deformation at garnet granulite & eclogite (700-800°C, 1.2-1.8 GPa) facies to lower-T extensional deformation at upper amphibolite facies (550-650°C, 0.7-0.9 GPa). Locally, the lower crust cooled thru 550-650°C without significant exhumation by 111 Ma. Elsewhere garnet ages indicate hot lower crust at 111 Ma. Within a 3.5 Myr timeframe the pluton cooled & crystallized, and extensional shear zones underwent a rapid (<3 Ma) transition from distributed melt-assisted deformation to cooler, localized upper amphibolite facies deformation at pluton margins. These results indicate that rheological variations and structural transitions linked to magmatism and partial melting may be more spatially heterogeneous and short-lived than previously believed.