U–Pb zircon geochronology of Silurian–Devonian granites in southeastern Australia: implications for the timing of the Benambran Orogeny and the I–S dichotomy

Abstract

Despite extensive geochemical study and their importance to granite studies, the geochronology of Silurian to early-Devonian granitic rocks of southeastern Australia is poorly understood. In order to provide an improved temporal framework, new ion microprobe U–Pb zircon ages are presented from these rocks, and previous work is critically reviewed. Geochronological control is best in the Berridale Batholith, where S- and I-type granites have a close spatial relationship. In this region, there is a small volume of I-type granite that crystallised at 436 Ma, followed closely by a large volume of S-type granite at 432 Ma. I-type granite is abundant in a second peak at ca 417 Ma, although the Jindabyne pluton from the Kosciuszko Batholith is slightly older, at 424 Ma. A broader survey of S-type granite throughout the eastern Lachlan Orogen shows that the 432 Ma event is ubiquitous. There is no temporal overlap between S- and I-type granites in the Kosciuszko and Berridale Batholiths, which suggests that factors other than variations in degree of crustal contamination (which may include variation in tectonic setting, heat-flow, mass transfer across the crust–mantle boundary and/or availability in source materials) contribute to the diversity in granite types. The S-type granitic rocks occupy an aerial extent of greater than 28 000 km², and geochronological constraints suggest that the crystallisation of these granites took place over a relatively small interval, probably less than 10 m.y. This implies a magmatic flux of over 64 km³/Ma per km strike length, comparable to other high-flux granitic belts. Previous work has linked the Benambran Orogeny to the generation of the S-type granites, and so the age of these granites constrains the age of Benambran Orogenesis

KEY WORDS:

• Berridale
• Kosciuszko
• granite
• S-type
• I-type
• geochronology
• zircon
• U–Pb

Acknowledgements

This work was funded by the Australian Research Council grant DP0559604. RBI was supported by an Endeavour International Postgraduate Research Scholarship awarded by the ANU and a postgraduate scholarship provided by the Natural Sciences and Engineering Research Council of Canada. Marc Norman kindly supplied sample MN99-6. Bruce Chappell is thanked for helping to initiate the project, guiding the sampling strategy, and for discussion. Lloyd White is thanked for comments on an earlier version of the manuscript. Reviews by Tony Kemp and an anonymous reviewer helped to temper the arguments and clarify the writing.

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Notes on contributors

R. B Ickert

Present address: Canadian Centre for Isotopic Microanalysis, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada T5K 159.