Isotopic constraints on stratigraphy in the central and eastern Yilgarn Craton, Western Australia

Abstract

Zircon grains in rocks from the Yilgarn Craton record crust formation dating back to shortly after the formation of the Earth. However, much of the evidence is cryptic and not apparent in the mapped geology. New Lu–Hf isotopic results, combined with existing Lu–Hf and Sm–Nd isotopic data, indicate five model-age probability peaks in the central and eastern Yilgarn Craton: at ca 4200, ca 3500, ca 3100, ca 2800 Ma and ca 2700 Ma. The ca 3100 Ma, ca 2800 Ma and ca 2700 Ma model-age peaks likely correspond to crust formation events. Evidence of the earlier peaks is not seen directly in the rock record, although zircon crystals in rocks of the Southern Cross Domain of the Youanmi Terrane show a long history of reworking pointing back to mantle extraction more than 4200 million years ago. The earliest peak is not recorded in the Eastern Goldfields Superterrane, indicating that crust formation in this region post-dated the earliest development of the Yilgarn Craton. Subsequent, broadly contemporaneous, episodes of mantle extraction and crustal reworking are indicated by the datasets for both the Eastern Goldfields Superterrane and the Southern Cross Domain. Magmas in the Eastern Goldfields Superterrane had a substantial juvenile input whereas those in the Southern Cross Domain recorded major reworking of older crust. The rock records for both the Eastern Goldfields Superterrane and the Southern Cross Domain share common elements of history after ca 2960 Ma. Both regions appear to have been subjected to major heating at ca 3100 Ma and ca 2800 Ma that resulted in the generation of juvenile crust in the east and reworking of older crust in the west. The ca 3500 Ma event is not readily evident in the rock record and may reflect a mixed age. However, the ca 3100 Ma and ca 2800 Ma events are recorded by both granite suites and greenstone successions across the craton. The ca 2700 Ma event is most evident in rocks from the Eastern Goldfields Superterrane.

KEY WORDS:

• Archean
• Yilgarn Craton
• Eastern Goldfields
• Southern Cross Domain
• zircon
• hafnium
• geochronology
• model age

Acknowledgements

Zircon analyses were conducted using the SHRIMP II ion microprobes at the John de Laeter Centre for Mass Spectrometry at Curtin University, in Perth, Australia. Lu–Hf isotope analyses were conducted at the Centre for Geological Evolution and Metallogeny of Continents (GEMOC), at Macquarie University, Sydney. Geological Survey of Western Australia's Carlisle laboratory staff are thanked for their diligent efforts in mineral separation. M. Prause is thanked for assistance in drafting the figures. David Champion and an anonymous reviewer are thanked for reviews which significantly improved the paper. Kevin Cassidy also provided some valuable feedback. The authors publish with permission of the Executive Director of the Geological Survey of Western Australia.