Archaean crust near Broken Hill?

Abstract

A layered quartzo‐feldspathic amphibolite facies gneiss from the Willyama Supergroup in the southern part of the Broken Hill block (New South Wales) contains predominantly 2670–2550 Ma old zircons and a lesser number of ca 2400 Ma, 2100 Ma and rare ca 1950 Ma, ca 1820 Ma grains. The gneiss could be an Archaean basement rock, affected by several Proterozoic tectonothermal events. An alternative interpretation is that all its zircons are inherited and it is a Palaeoproterozoic supracrustal rock. If the latter, this sample is anomalous, because it is missing 1800–1730 Ma zircons, the most important age component in the zircon populations of all other Willyama Supergroup supracrusfal rocks and the commonest age for inherited zircons in Broken Hill area felsic igneous rocks. Whichever interpretation of the rock is chosen, the predominance of Archaean zircons strongly suggests that Archaean crust in some form is present in the area. As the Broken Hill Block is near the eastern margin of Australian Rodinia, this finding has ramifications for the identification of once adjacent parts of Rodinia (perhaps containing Broken Hill‐type massive Pb‐Zn‐Ag sulfide mineralisation), which could now be in north America or southern China.

Key words:

• Archaean
• Broken Hill
• radiometric dating
• Rodinia
• zircons.