HR: 12:05h
AN: V22A-08    [Abstracts]
TI: A Coupled Lu-Hf and O Isotope in Zircon Approach to Crustal Evolution
AU: * Hawkesworth, C
EM: c.j.hawkesworth@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ United Kingdom
AU: Kemp, T
EM: tony.kemp@geo.sc.niigata-u.ac.jp
AF: Department of Geological Science, Niigata University, Niigata, 950-2181 Japan
AB: A complete picture of crustal evolution involves integration of the different records preserved in sedimentary and igneous rocks, and Hf and O isotopes in detrital and inherited zircons offer a unique way to do this. The igneous record preserves the ages of the specific events in which new continental crust was generated. The sedimentary record comprises mixtures of material from different sources, and so it is a record of the average composition of the upper crust at different times, rather than of specific periods of crustal growth. Zircons offer the most representative record of the geological history of the crust because they are common in the rocks of the upper crust, they are robust, and they crystallised from magmas that were derived from sedimentary or igneous source rocks. The ages of zircons from the Lachlan Fold Belt in SE Australia have established major magmatic episodes at ~500 and 1000 Ma, but these did not involve the generation of new crust. The Hf model ages reflect the time when new continental crust was generated, and the zircons from magmas from igneous and sedimentary rocks (low and high 18O respectively) yield different Hf model ages. The zircons from magmas from igneous sources have Hf model ages indicating periods of new crustal growth at 1.8 and 3.0 Ga. In contrast, the zircons from magmas from sedimentary source rocks have Hf model ages peaking at ~2.0 Ga, and so they represent mixtures of rocks generated at 1.8 and 3.0 Ga. O isotopes therefore have a key role in distinguishing major crust forming events from peaks of model Hf ages that reflect mixing during erosion and sedimentation. The data constrain the rates at which new crust dominates the sedimentary record, and they are used to reassess models for the generation and evolution of the continental crust.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005