Barium contents of granites: key to understanding crustal architecture in the southern Lachlan Fold Belt?

Abstract

Victorian granites containing more than 750 ppm Ba are almost entirely confined to the region between a line from Geelong to Swan Hill and the Wonnangatta Fault Zone. Granite Ba contents normalised to 70% SiO₂ range from 620 to 733 ppm in western Victoria, 719 to 1560 ppm in central Victoria and 493 to 689 ppm in eastern Victoria. Melting of Ba-rich (meta)sedimentary rocks in the lower – middle crust is implicated in the petrogenesis of central Victorian S types, at least. Thus, granite geochemistry supports the concept of some sort of (largely concealed) Ba-rich Selwyn Block beneath central Victoria, although the boundaries that have been proposed for the block are modified here. There is a strong possibility that the Selwyn Block is an exotic terrane emplaced by northwest-directed strike-slip movement during the Bindian Orogeny. Such movement appears to have been controlled by the previously postulated Baragwanath Transform and another fundamental fault here called the Ulrich Transform. Asthenospheric upwelling related to movement on the Ulrich and Baragwanath Transforms may be the explanation for the twin belts of 400 Ma plutonism occurring to the west of the former and to the east of the latter. The southern extension of the Ulrich Transform may be the Tamar – Tiers structure in Tasmania. Plate-tectonic models suggesting Ordovician – Silurian subduction in Victoria need to be carefully revisited given the possibility of Siluro-Devonian exotic terrane emplacement.

Key Words:

• Baragwanath Transform
• barium
• Devonian
• exotic terranes
• granite
• Selwyn Block
• Silurian
• Ulrich Transform
• Victoria

Acknowledgements

We would like to acknowledge Ian McCabe and Jorg Metz for their assistance with the analytical work on which this paper is based. We thank Allan White, Peter Fleming, Roland Maas and an anonymous reviewer for their constructive reviews of the manuscript. Ian Widdowson drafted the figures. This research was funded in its initial stages by a La Trobe University Postgraduate Scholarship to AGR.

Notes

∗‘Selwyn Block’ (with quotation marks) refers to our definition of the block which has different boundaries to the Selwyn Block (without quotation marks) as defined by VandenBerg et al. (2000) and Cayley et al. (2002).