Paper No. 12
Presentation Time: 11:15 AM
EPISODIC METAMORPHISM AND COOLING IN THE ROOT OF A CONTINENTAL ARC, FIORDLAND, NEW ZEALAND
Marbles from shear zones in the exhumed root of a Jurassic-Cretaceous arc in Fiordland contain both metamorphic zircon and titanite that record the timing of granulite-facies metamorphism (>800°C) and cooling though 650°C associated with progressive orogenic collapse. We present 206
U ages and thermometry data from eight samples distributed over ca. 400 km2
from the middle and lower crust in central Fiordland, which allows us to evaluate timescales of metamorphism and cooling of lower arc crust. New SHRIMP-RG ages for metamorphic zircon yield bimodal distributions with peak crystallization ages at ca. 115-112 Ma (Nancy Sound, Doubtful Sound, Resolution Island), and ca. 106 Ma (Doubtful Sound). Ti-in-zircon thermometry yields temperatures of 750-900°C, consistent with peak metamorphism at granulite-facies conditions. New titanite Pb/U ages record cooling through ca. 650°C and are also bimodal with peak populations at 105-103 Ma (Nancy Sound and Doubtful Sound), and 98-95 Ma (Resolution Island-Vancouver-Wet Jacket region). Six of eight samples display minor occurrences of both older and younger populations that are resolvable at the 2σ level of uncertainty. For example, titanites from the Nancy Sound yielded ages of 105.0 ± 2.0 and 96.6 ± 1.7 Ma; whereas, titanites from the Resolution Island yielded similar ages of 103.1 ± 1.5 Ma and 96.8 ± 1.1 Ma.
The 400 km2 spatial distribution pattern of titanite ages is difficult to reconcile with simple monotonic arc cooling models (either top-down or bottom-up). Another possibility is that cooling ages reflect NW-SE-oriented extension related to intraplate rifting; however, the spatial distribution of ages does not appear to correlate with the locations of known lower crustal extensional shear zones. The occurrence of older and younger populations within individual samples is also difficult to reconcile with simple cooling along faults. We suggest that metamorphic zircon and titanite cooling ages may record punctuated reheating and cooling associated with short thermal events at ca. 115-112 Ma, 106-103 Ma and 98-95 Ma. We speculate that the spatial distribution of cooling ages in the lower crust may delineate regions of lithospheric root detachment after peak magmatic construction of the lower arc crust from 118-115 Ma.