TRACKING THE INFLUENCE OF CRUSTAL CONTRIBUTIONS TO THE DEEP ROOT OF A CONTINENTAL ARC, FIORDLAND, NEW ZEALAND
Here we integrate U-Pb zircon geochronology with existing whole-rock initial isotope data from the arc root in Fiordland to evaluate the relative contributions of depleted mantle and pre-existing crust through time during Cretaceous arc magmatism. Our results show that whole rock initial values decrease from +3 to +5 during early, low flux magmatism at 160-120 Ma to lower values of 0 to +3 during peak magmatic flux at 118-115 Ma. The decrease in initial is also correlated with a sharp increase in whole rock Sr/Y values (from ≤60 to >300) over the same time interval. We interpret the decrease in initial to result from increased crustal melting during peak magmatic flux due to underthrusting of continental crust beneath Fiordland. The observed negative correlation between initial and Sr/Y values supports this hypothesis, as it suggests an increase in garnet stability and a decrease in plagioclase stability in the source region of the magmas, which we attribute to downward vertical movement of crustal material into the arc root. Ongoing work is aimed at systematic isotopic and geochemical characterization of the arc root with respect to time (apatite and whole-rock Rb-Sr and Sm-Nd isotopes) to clarify temporal changes in the relative contributions of magmas in the roots of the Jurassic to Cretaceous arc in Fiordland.