Source and significance of the felsic magmatism in the Paleoproterozoic to Mesoproterozoic Broken Hill Block, New South Wales

Abstract

Major, trace, rare-earth elements and isotopic (Sm – Nd and Rb – Sr) data from the ca 1704 – 1685 Ma Alma, Farmcote, Rasp Ridge and Hores-Potosi felsic magmatic gneisses and the ca 1600 Ma syn-orogenic felsic intrusions of the Willyama Supergroup in the Broken Hill Block of western New South Wales are documented. The ca 1704 – 1685 Ma felsic melts were generated by anatexis of the Willyama Supergroup metasediments with variable degrees of mixing of a juvenile, mantle-derived component represented by coeval high Fe – Ti metatholeiitic rocks. This is consistent with previous interpretations for bimodal magmatism occurring in an extensional environment with an elevated geothermal gradient driven by lithospheric thinning and mafic magmatism. Interpretations involving the presence of a mafic underplate as a source for the ca 1704 – 1685 Ma felsic melts are not supported by these data. The ca 1600 Ma syn-orogenic felsic intrusions are a direct product of partial melting of the sedimentary sequences of the Willyama Supergroup as a response to the high-temperature, low-pressure amphibolite- to granulite-facies metamorphic event accompanying the Olarian Orogeny. The presence of an Archean basement for the Willyama Supergroup remains unclear, although if this basement exists it was not sampled during the period of felsic magmatic activity that took place in the Broken Hill Block between ca 1710 and 1600 Ma.

Key Words:

• Alma Gneiss
• Broken Hill Block
• Farmcote Leucogneiss
• felsic intrusions
• geochemistry
• Hores-Potosi Gneiss
• Proterozoic
• Rasp Ridge Gneiss
• Willyama Supergroup

ACKNOWLEDGEMENTS

We thank Lachlan Rutherford and Bill Collins for thoughtful and thorough reviews that greatly improved the manuscript. Kurt Stüwe's constructive review of an early version of the manuscript is also acknowledged. We would also like to thank Ian Nicholls, Tony Crawford, Bruce Schaefer and Roberto Weinberg for many discussions and much constructive criticism. Thanks also to David Bruce, from the radiogenic isotope laboratory of the University of Adelaide, for his invaluable assistance during the isotope work. We thank BHP Billiton for financially supporting this study, and in particular Jean des Rivières, Darren Stephens and Sudipta Nag for discussing their ideas. We thank Tony Cockbain for his editorial advice.