Abstract:
Sm-Nd garnet and U-Pb zircon ages for eclogite and granulite from the Breaksea Orthogneiss provide a detailed chronology for pluton
emplacement and subsequent thermal history of the lower arc crust exposed in Fiordland, New Zealand. The 147Sm-143Nd ages for ~1 cm
garnet grains in eclogite yield a 108.2 ± 1.8 Ma (7 points) age and similar sized grains of garnet from granulite interlayered with eclogite
yield a ca. 110.5 ± 1.6 Ma (8 points) age. Both samples retain sparse domains with older ages of 123–121 Ma. Distinct Ca, Lu, and Hf zoning
in garnet indicate that eclogite and granulite cooled rapidly enough to negate significant diffusion. The Ca zoning is interpreted to indicate
significant garnet recrystallization during the granulite facies event, ca. 110 Ma. The older garnet ages are indistinguishable from the oldest
206U/238Pb zircon ages, ca. 123 and 120 Ma, in granulite orthogneiss that yielded two age populations; these granulites have younger age
populations of 111.1 ± 1.4 and 115.2 ± 1.3 Ma, respectively. Zircon from orthogneiss samples nearby yield single age populations indicating
additional intrusions ca. 115 and late metamorphic zircon growth ca. 95 Ma. The zircon and garnet ages combined with pressure-temperaturetime
paths document magma intrusion into the lowermost arc crust, near isothermal exhumation of Breaksea rocks at ~2.2 km/m.y. from
~65 km to 40–45 km depths, followed by continued high heat flow with granulite facies metamorphism. The latter high temperatures were
synchronous with granulite facies metamorphism in the adjacent Malaspina pluton, indicating that high-temperature metamorphism affected
>600 km2
of lower crust in the continental magmatic arc.
The complex age results for U-Pb zircon and Sm-Nd garnet dating indicate the need for comprehensive data sets from multiple rocks
for deciphering the intrusive and subsequent thermal history of the lower crust. The study detailed here clearly indicates that Sm-Nd garnet
geochronology can provide useful ages for high-temperature rocks when large grains cool at rates of >10 °C/m.y. The geochronological
results indicate that voluminous magmatism was closely followed by high-temperature metamorphism. This is a common phenomenon in
the lower crust of magmatic arcs and a signature for high magmatic flux through the lower crust.
