Paper No. 11
Presentation Time: 11:30 AM
TIME SCALES OF CONTINENTAL ARC ROOT CONSTRUCTION AND DEEP CRUSTAL MAGMATIC FLUX RATES: INSIGHTS FROM U-PB ZIRCON GEOCHRONOLOGY OF A TRIASSIC-CRETACEOUS ARC, FIORDLAND, NEW ZEALAND
New U/Pb zircon ages from the exhumed root of a Triassic-Cretaceous arc exposed in Fiordland, New Zealand allow us to determine time scales of arc root construction and reconstruction of magmatic flux rates into the lower and middle crust. Presented here are 14 new 206
U SHRIMP-RG ages which span >1200 km2
of Mesozoic lower and middle crust. Mafic to intermediate plutonic rocks were collected from the Misty and Malaspina plutons which occupy structural positions above and below the lower crustal extensional Doubtful Sound shear zone, respectively. Together, these plutons comprise >54% of the arc root. Eight samples from the Misty orthogneiss yielded error-weighted average 206
U zircon ages ranging from 114.5 to 117.1 Ma, with a calculated error-weighted average age of 115.96 ± 0.60 Ma (2σ) from all Misty zircons analyzed. Similarly, six samples from the Malaspina orthogneiss yielded error-weighted average 206
U zircon ages from 115.4 to 120.3 Ma, with a calculated error-weighted average age of 117.41 ± 0.53 Ma (2σ) for all Malaspina zircons analyzed. When combined with existing zircon dates, these new data suggest that >60% of the arc root was emplaced during a short-lived (<5 Ma), flare-up event that overlapped with arc construction.
Integration of the new 206Pb/238U ages with regional and detailed maps permit estimation of apparent magmatic flux rates to the Fiordland arc root. Average magmatic flux rates are calculated assuming an estimated paleothickness of 25 km and using durations of magmatism determined from new zircon geochronology. The calculated average magmatic flux rates are roughly 125 and 98 km3/ km-strike/Ma for the Misty and Malaspina, respectively, where thickness (km3) was calculated by multiplying the area of the plutons by the paleodepths. These rates are similar though somewhat higher than those reported from shallow to middle crustal exposures in the Sierra Nevada batholith and Coast Plutonic complex.