Granite - greenstone connection in western Victoria: an example from the Bushy Creek Igneous Complex.

The Bushy Creek Igneous Complex includes rock types ranging from greenstone through diorite, granodiorite and granite to aplite and was emplaced in the Grampians - Stavely Zone 30 km southeast of the Grampians Ranges at ca 495 Ma. Striking correlations with varying silica contents are observed in major- and trace-element abundances which, coupled with a close spatial and temporal association, suggests a strong petrogenetic link between all rock types. It is proposed that these trends reflect the operation of fractional crystallisation processes dominated by hornblende (and/or clinopyroxene) and plagioclase at lower silica levels, changing to an assemblage dominated by plagioclase and accessory phases in the felsic lithologies. However, it is apparent from Sr - Nd isotope data that some form of two-component mixing also played a key role in the evolution of these magmas prior to the final episode of fractional crystallisation. The most contaminated magma (a granodiorite) requires approximately equal contributions from each of two end-members, one similar to the greenstones and the other comparable to Adelaide Fold Belt S-type granites derived from Kanmantoo Group metasediments. Such a model also accounts for the broad major- and trace-element characteristics of the various rock types of the complex. Although supporting the view that the Adelaide Fold Belt extends to the Moyston Fault, the proposed model for the origin of the Bushy Creek magmas also has considerable bearing on the petrogenesis of Devonian granites of the Victoria Valley Complex (Mafeking Suite) in the Grampians Ranges. These were emplaced nearly 100 million years later yet share very similar chemical traits, consistent with a derivation via the partial re-melting of Bushy Creek-like lithologies. [ABSTRACT FROM AUTHOR]