4.2 Ga zircon xenocryst in an Acasta gneiss from northwestern Canada: Evidence for early continental crust
- Tsuyoshi Iizuka*1,
- Kenji Horie2,
- Tsuyoshi Komiya3,
- Shigenori Maruyama3,
- Takafumi Hirata3,
- Hiroshi Hidaka4 and
- Brian F. Windley5
- 1Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama Meguro-ku, Tokyo 152-8551, Japan
- 2Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
- 3Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama Meguro-ku, Tokyo 152-8551, Japan
- 4Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
- 5Department of Geology, University of Leicester, Leicester LE1 7RH, UK
Abstract
Evidence for the existence of continental crust older than 4.06 Ga has so far been obtained only from zircons in the Yilgarn Craton of Western Australia. In this paper we report the first occurrence of a very old zircon with a U-Pb age of 4.2 Ga in the Acasta Gneiss Complex of northwestern Canada, based on a laser ablation-inductively coupled plasma–mass spectrometry and sensitive high-resolution ion microprobe study. The U-Pb data reveal that the 4.2 Ga zircon occurs as a xenocryst in a 3.9 Ga granitic rock. Trace element compositions of the xenocryst suggest that it crystallized from a granitic magma. Our results, suggesting the existence of granitic rocks outside the Yilgarn Craton at 4.2 Ga, imply that granitic continental crust was more widespread than previously thought, and that it was reworked into Early Archean continental crust.
- Acasta Gneiss Complex
- Hadean
- ancient zircon
- crustal reworking
- laser ablation-inductively coupled plasma-mass spectrometry
- sensitive high-resolution ion microprobe
- U-Pb dating
Footnotes
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↵*tiiduka{at}geo.titech.ac.jp
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↵GSA Data Repository item 2006056, Table DR1, Pb isotopic data for zircons; Figure DR1, geological map of the Acasta Gneiss Complex showing the location of AC012; Figure DR2, cathodoluminescence images of zircons; Figure DR3, Raman spectra for an apatite inclusion and the enclosing zircon AC012/07; and Appendix DR1, analytical procedures, is available online at http://www.geosociety.org/pubs/ft2006.htm, or on request from editing{at}geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
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- Accepted 24 November 2005.
- Received 3 August 2005.
- Revision received 22 November 2005.
- Geological Society of America
