Syndeformational granite crystallisation along the Mount Magnet Greenstone Belt, Yilgarn Craton: evidence of large-scale magma-driven strain localisation during Neoarchean time

Abstract

The Neoarchean is characterised by a global-scale period of crust formation, extensive mafic to granitic magmatism, crustal reworking and intracrustal partial melting. Since a dramatic strength drop is associated with the presence of melt in crystallising or melting rocks, Neoarchean continental deformation is thought to have been largely accommodated by shearing of high-grade gneisses and by syntectonic granitic magmatism. However, direct field examples of such strain localisation in hot and partially molten granites are relatively scarce. The Archean Yilgarn Craton, which is made up of volumetrically dominant granites and granitic gneiss, represents a natural laboratory to study the effect of melting on continental deformation. This paper describes meso- and microstructural evidence of large-scale, synmagmatic deformation recorded during crystallisation of a Neoarchean granitic complex. Two main kilometre-scale synmagmatic shear zones have been identified, and they are inferred to have been active during granite crystallisation. Structural evidence suggests that the dome-shaped structure that characterises the granitic complex represents a primary feature developed during pluton crystallisation. In the Mount Magnet greenstone belt, early structures predate granite emplacement, while the kilometre-scale, Boogardie synform developed during pluton crystallisation in response to granite doming. The studied complex represents a natural example of strain localisation within a low-viscosity corridor that may have accommodated large amounts of strain during the Neoarchean, as predicted by numerical experiments and tectonic models.

KEY WORDS:

• Neoarchean
• Yilgarn Craton
• granite
• synmagmatic deformation

Acknowledgements

Detailed reviews by R. Blewett, S. Micklethwaite and R. Weinberg considerably helped improve the manuscript. M. Pawley, M. Van Kranendonk, A. Hickman, S. Wyche and T. Ivanic are gratefully thanked for discussions. M. Prause and M. Jones significantly improved the quality of some images. P. Vota and T. Zama provided invaluable assistance during fieldwork. The paper is published with permission of the Executive Director of the Geological Survey of Western Australia.