Transactions of the Royal Society of Edinburgh: Earth Sciences

Research Article

Some observations on the use of zircon U-Pb geochronology in the study of granitic rocks

Second Hutton Symposium: The Origin of Granites and Related Rocks, Australian Academy of Science, Canberra.

Ian S. Williamsa1

a1 Ian S. Williams, Research School of Earth Sciences, The Australian National University, GPO Box 4, Canberra City, ACT 2601, Australia

ABSTRACT

In situ, microscale, U-Pb isotopic analyses of zircon using the SHRIMP ion microprobe demonstrate both the potential and the limitations of zircon U-Pb geochronology. Most zircons, whether from igneous or metamorphic rocks, need to be considered as mixed isotopic systems. In simple, young igneous rocks the mixing is principally between isotopically disturbed and undisturbed zircon. In polymetamorphic rocks, several generations of zircon growth can coexist, each with a different pattern of discordance. A similar situation exists for igneous rocks rich in inherited zircon, as these contain both melt-precipitated zircon and inherited components of several different ages. Microscale analysis by ion probe makes it possible to sample the record of provenance, age and metamorphic history commonly preserved within a single zircon population. It also indicates how the interpretation of conventionallymeasured bulk zircon isotopic compositions might be improved.

KEY WORDS

  • Dalgety;
  • discordance;
  • inheritance;
  • ion probe;
  • Kameruka;
  • monazite;
  • Mount Sones;
  • radiogenic Pb;
  • Watergums;
  • Watersmeet