EAPSI: Insights into the growth of continents from an innovative method of simultaneously determining uranium-lead age, hafnium isotopes, and trace elements in zircon

Hagen-Peter Graham A, Santa Barbara CA

Investigators

Abstract

Continental arcs are tectonic plate boundaries where an oceanic plate descends into the mantle beneath a continent. Magmas sourced from recycled continental crust, the underlying mantle, and the descending oceanic plate are emplaced in the crust as large granitic bodies and erupted at volcanoes. Magmas sourced from the mantle that crystallize in the crust of the overriding plate are new additions of continental crust, whereas magmas sourced from remelting of the overriding plate recycle and "distill" previously existing continental material. Both processes probably act in continental arcs, but the extent of each is currently debated. The granitic roots of the Ross orogen, a ~3500 km-long continental arc that was active around 500 million years ago, are exposed in Antarctica. Determining whether these huge tracts of granitic rocks were sourced from the mantle or from recycled crust has important implications for the role of continental arcs in generating large volumes of continental crust. Advanced analytical facilities housed at the National Taiwan University will be used to measure the isotope and trace element composition of the mineral zircon, which records important information about the source of the igneous rocks in which the minerals form. This research will be done in collaboration with Dr. Sun-Lin Chung at the National Taiwan University. This project will employ uranium-lead geochronology coupled with the hafnium isotope and trace element compositions of the mineral zircon to study the age and source of arc granites from the Ross orogen. A routine for the simultaneous measurement of trace elements, U-Pb, and Hf isotopes in zircons by laser-ablation inductively-coupled plasma mass spectrometry will be developed. This research will employ an innovative method to explore the role of continental arc magmatism as a mechanism of crustal growth. This NSF EAPSI award is funded in collaboration with the National Science Council of Taiwan.