Geochronology, paleomagnetism and magnetic fabric of metamorphic rocks in the northeast Fraser Belt, Western Australia

Abstract

The first zircon U–Pb SHRIMP dating on high-grade meta-igneous units in the northernmost parts of the Fraser Belt along the southern margin of the Western Australian Yilgarn Craton, reveal crystallisation ages between 1299 ± 10 and 1250 ± 23 Ma. A small number of older xenocrystic zircons, incorporated in some samples, indicate the presence of Late Paleoproterozoic crust in the region. Zircon that crystallised within a melt accumulated in the neck of a boudinaged mafic unit was dated at 1296 ± 4 Ma, indicating that the emplacement of the igneous protoliths took place syntectonically. The anisotropy of magnetic susceptibility of the granulites indicates minimum axes with a mean inclination of 4° towards 130°, corresponding to a nearly vertical southwest–northeast (50–230°) magnetic foliation. This is very close to the structural trend of the Fraser Belt suggesting that the magnetic fabric was acquired syntectonically, during the collision between the Yilgarn and Gawler Cratons. The paleomagnetic data on the granulites overlap with published poles for various 1.2 Ga units in the Albany Belt and the 1.2 Ga Fraser dykes, possibly suggesting that the remanence was acquired during the second stage of the Fraser tectonism. A younger magnetisation component resembles a pole of uncertain age published for Bremer Bay in the Albany Belt.

Key words:

• Fraser Belt
• geochronology
• magnetic fabric
• metamorphic rocks
• paleomagnetism, SHRIMP, uranium–lead dating
• Western Australia, zircon

Acknowledgements

We would like to acknowledge a research grant offered by the University of Western Australia for this work. SHRIMP work was carried on the Perth Consortium facilities at the John De Laeter Centre for Mass Spectrometry, Curtin University of Technology, Perth. SEM imaging was conducted at the Centre for Microscopy and Microanalysis at the University of Western Australia, Perth, and the paleomagnetic work at the Paleomagnetic Laboratory of the University of Western Australia. We also thank the staff of the Geological Survey of Western Australia, Kalgoorlie office, for their invaluable help (especially Sarah Jones), Neal McNaughton for help with the geochronology, and journal reviewers Simon Johnson and an anonymous reviewer. BDW publishes with the permission of the Executive Director, British Geological Survey (NERC).

Notes

^∗Appendix 1 [indicated by an asterisk (∗) in the text and listed at the end of the paper] is a Supplementary Paper; copies may be obtained from the Geological Society of Australia's website (<http://www.gsa.org.au>) or from the National Library of Australia's Pandora archive (<http://nla.gov.au/nla.arc-25194>).