NEW 206PB/238U ZIRCON AGES RECORD MAGMATISM AND METAMORPHISM IN THE CRUSTAL ROOT OF A MAGMATIC ARC, FIORDLAND, NEW ZEALAND

The root of a Cretaceous magmatic arc is exposed around the entrance to Breaksea Sound, Fiordland. The arc collapsed along extensional shear zones allowing partial exhumation, rifted during opening of the Tasman Sea, and exhumed along the Alpine fault. The root is comprised of eclogite and granulite of the Breaksea Orthogneiss (BO) and Resolution Orthogneiss (RO) of the Western Fiordland Orthogneiss suite (WFO).

New zircon ²⁰⁶Pb/²³⁸U ion probe (Stanford-U.S.G.S SHRIMP-RG) ages fall into pre- syn- and post-granulite facies groups when compared to garnet-clinopyroxene granulite in the nearby Malaspina Orthogneiss. 2 BO samples contain zircons with 2 spot age populations: weighted average age (Wt Ave) = 120.2±1.0 Ma (N= 13, MSWD=0.36) and Wt Ave=115.9±0.7 Ma (N=21, MSWD=0.73). REE patterns do not show HREE flattening, suggesting an igneous origin. The older ages are interpreted to date intrusion of gabbroic magma into the root prior to granulite facies metamorphism. The young population is indistinguishable from the Wt Ave=115.5±1.3 Ma (N=7, MSWD=1.08) obtained from a nearby sample of RO. CL images for the young population of BO zircons exhibit blurred sector and oscillatory zoning or no igneous textures. Therefore, we interpret the new ca. 115 Ma zircon ages as metamorphic. Gabbroic orthogneiss cross-cutting eclogite and granulite in the BO and gabbroic orthogneiss in the RO have Wt Ave=92.6±2.1 and 94.4±0.8 Ma, respectively. Zircon from these RO rocks exhibit oscillatory CL zoning that may be igneous in origin.

Eclogite and granulite mineral assemblages in the BO have been inferred as synchronous and a result of different bulk compositions. Garnet Sm-Nd and zircon U-Pb ages indicate that garnet granulite metamorphism in the Malaspina was 115-112 Ma. A 111.9±1.6 Ma garnet age from ca. 10 km south of Breaksea on Resolution I. is indistinguishable from the new 115.9±0.7 Ma zircon age, compatible with the zircon equilibration during metamorphism. The new ages are compatible with multiple generations of magmatism, deformation, and metamorphism, and underline the complexity of lower crustal processes. New 94.4±0.8 and 92.6±2.1 Ma ages are the youngest known in the WFO and indicate young pre– to syn-tectonic magmatism that could be related to the early phases of rifting and opening of the Tasman Sea.