ARCHEAN CRUST NEAR BROKEN-HILL

A layered quartzo-feldspathic amphibolite facies gneiss from the Willy ama Supergroup in the southern part of the Broken Hili block (New Sout h Wales) contains predominantly 2670-2550 Ma old zircons and a lesser number of cc 2400 Ma, 2100 Ma and rare ca 1950 Ma, cc 1820 Ma grains. The gneiss could be an Archaean basement rock, affected by several Pro terozoic tectonothermal events. An alternative interpretation is that all its zircons are inherited and it is a Palaeoproterozoic supracrust al rock. If the latter, this sample is anomalous, because it is missin g 1800-1730 Ma zircons, the most important age component in the zircon populations of all other Willyama Supergroup supracrustal rocks and t he commonest age for inherited zircons in Broken Hili area felsic igne ous rocks. Whichever interpretation of the rock is chosen, the predomi nance of Archaean zircons strongly suggests that Archaean crust in som e form is present in the area, As the Broken Hili Block is near the ea stern margin of Australian Rodinia, this finding has ramifications for the identification of once adjacent parts of Rodinia (perhaps contain ing Broken Hill-type massive Pb-Zn-Ag sulfide mineralisation), which c ould now be in north America or southern China.