NEW GARNET SM-ND AGES RECORD TIMING OF ECLOGITIC GARNET GROWTH IN FIORDLAND, NEW ZEALAND

Granulite and eclogite facies rocks are exposed in the root of an exhumed magmatic arc in Fiordland. Discontinuous pods and layers of eclogite are enveloped within partly retrogressed garnet granulite around the entrance to Breaksea Sound. These rocks of the Breaksea Orthogneiss (BO) are the highest pressure rocks in the Western Fiordland Orthogneiss (WFO). Protolith and metamorphic ages of the BO are poorly known and the relationship between the eclogite and granulite is poorly understood.

We report the first garnet Sm-Nd (UNC, TIMS) ages of eclogite from the BO: 123.1±2.8 (5 pts.) and 108.2±1.8 Ma (7 pts.). The older age is indistinguishable from a ²⁰⁶U-²³⁸Pb zircon age of 123.2±1.3 Ma (Stanford-USGS SHRIMP) compatible with cumulate igneous garnet or eclogitic garnet growth triggered by intrusion of the granulite protolith. Granulite metamorphism of the Malaspina Orthogneiss adjacent to the BO east of the Resolution Island Shear Zone occurred at 111.9±1.6 Ma (garnet Sm-Nd). Similarity of this age to the younger BO eclogite age indicates synchronous metamorphism across the shear zone.

Chondrite-normalized rare earth element (REE) concentrations (USGS-Denver LA-ICPMS) in garnet from eclogite and granulite show enriched HREE relative to LREE. The cores of eclogitic garnet show a pronounced concave down HREE pattern indicating likely equilibrium with a HREE-rich phase, possibly zircon. However, garnet rims show flat HREE and likely did not grow with zircon or other HREE-rich phases. Garnet cores and rims show no Eu anomaly, compatible with a lack of plagioclase. Garnet cores and rims from nearby granulite have enriched HREE relative to LREE but flat HREE patterns lacking any indication of equilibrium with zircon. REE patterns for cores of eclogite garnet, the garnet age of 123.1±2.8 Ma, and likely lack of coeval plagioclase indicate that initial garnet may have grown with zircon: indicating a possible high-pressure igneous cumulate origin. REE patterns for rims of eclogite garnet and the garnet age of 108.2±1.8 Ma indicate that garnet rims likely grew without zircon at granulite facies conditions. Garnet in granulite has REE patterns which suggest that this garnet grew without zircon during metamorphism.

Results illustrate the utility of multiple chronometers tied to mineral compositions for dating high T processes.