THE TEMPO OF CONTINENTAL MARGIN ARC CONSTRUCTION AND EXTENSIONAL OROGENIC COLLAPSE: A DEEP-CRUSTAL PERSPECTIVE FROM FIORDLAND, NEW ZEALAND

The exhumed root of the Fiordland orogen offers a deep-crustal view into the life cycle of a continental-margin arc from magmatic construction to extensional orogenic collapse. We integrate U-Pb and Sm-Nd geo- and thermochronologic data from zircon, garnet, titanite, and rutile with structural observations from >1200 km²of the arc root to document the tempo of deep-crustal magmatism, high-temperature metamorphism, extension and cooling in central Fiordland.

Results from U-Pb SHRIMP-RG zircon geochronology reveal two principal phases of Cretaceous, deep-crustal plutonism at ca. 125-120 Ma and 118-115 Ma. The latter phase of magmatism resulted in the emplacement of nearly 70% of the arc root and constituted a major flux of high Sr/Y magmas. Metamorphic zircon from Paleozoic host orthogneisses, trondhjemitic leucosomes and garnet reaction zones yield ages of 119-117 Ma, and metamorphic zircons from calc-silicates yield ages of 116-112 Ma. Garnet TIMS Sm-Nd ages from Cretaceous orthogneisses, garnet-reaction zones and trondhjemitic leucosomes range from 115-110 Ma. Outcrop- and microstructural-scale observations indicate that granulite-facies shearing initiated at ca. 116-110 Ma resulting in widely distributed zones of lower crustal flow. These data signify that regional granulite-facies metamorphism was partially synchronous with deep-crustal magmatism, partial melting and high-temperature, crystal-plastic deformation.

Titanite LASS chronology of calc-silicates and mafic orthogneisses yielded uniform ages of 113-110 Ma, suggesting rapid cooling of the arc root from ca. 900 to ~650°C in <10 Ma. Metamorphic zircon rims from Doubtful Sound indicate a pulse of amphibolite-facies metamorphism at 106 Ma that may coincide with the development of organized networks of upper amphibolite-facies shear zones. In the Resolution Island region, metamorphic zircons yield ages of 93-89 Ma implying a separate, localized pulse of amphibolite-facies metamorphism in the southwestern sector of central Fiordland. Rutile ages from both regions as old as 89 Ma signifying slow cooling (~5°C/Ma) and residence of the arc root above amphibolite-facies conditions for >20 Ma (110-89 Ma). These conditions and slow cooling facilitated extensional orogenic collapse leading to the opening of the Tasman Sea at 83 Ma.