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

ABSTRACT 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 By Joseph Frank Sadorski Master of Science in Geology 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 addition rates into the lower and middle crust. Presented here are 15 new 206Pb/238U SHRIMP-RG ages from rocks spanning 1200 km2 of Mesozoic lower and middle crust. Mafic to intermediate plutonic rocks were collected from the Malaspina and Misty plutons which occupy structural positions below and above the lower crustal extensional Doubtful Sound shear zone, respectively. Together, these plutons comprise 54% of the arc root. Seven samples from the Malaspina pluton yielded error-weighted average 206Pb/238U zircon ages from 115.4 to 118.0 Ma, with a calculated error-weighted average age of 117.4 ± 0.5 Ma (2σ) for all Malaspina zircons analyzed. Notably, the northwestern lobe of the Malaspina pluton contained an older component of 120.3 ± 1.4 Ma and coupled with observed vertical fabrics unique to this region, is now designated as a distinct phase of the Malaspina pluton, termed here the NW Malaspina pluton. Eight samples from the Misty pluton yielded error-weighted average 206Pb/238U zircon ages ranging from 114.5 to 116.9 Ma, with a calculated error-weighted average age of 116.0 ± 0.6 Ma (2σ) from all Misty 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. Integration of the new 206Pb/238U zircon ages with regional and detailed maps permit the estimation of Areal Addition Rates (AAR) to the Fiordland arc root. The calculated AAR rates are 213.08, 39.0, and 267.87 km2/Ma for the Malaspina, NW Malaspina, and the Misty plutons, respectively. These rates are similar though somewhat higher than those reported from middle to upper crustal exposures in the Californian Arc.