T89 Continental Arcs #1: Tectonopetrologic Processes Controlling Arc Tempos and Evolution

Monday, 20 October 2014
8:00 AM - 12:00 PM
121

TRACKING THE INFLUENCE OF CRUSTAL CONTRIBUTIONS TO THE DEEP ROOT OF A CONTINENTAL ARC, FIORDLAND, NEW ZEALAND

Paper number: T89-13
Program: Technical Sessions
Time: 10:45 AM – 11:00 AM

Authors

Abstract

The exhumed root of the Jurassic to Cretaceous arc in central Fiordland, New Zealand preserves a well-exposed section of >2000 km2 of middle and lower arc crust, nearly 70% of which was emplaced during a brief, high-flux event from 118-115 Ma. A key problem in understanding the growth of continental crust in magmatic arcs centers on the mechanisms by which high-flux events are triggered. Previous workers have proposed that high-flux events may be triggered by underthrusting of continental crust thereby inducing melting by introducing ‘fertile’ crust to elevated geothermal gradients. By contrast, high flux events may also be triggered by mantle upwelling as a result of delamination and/or lithospheric extension.

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.