Petrographic, geochemical and age characterisation of crustal xenoliths from Coliban Dam, central Victoria, with implications for the early evolution of southeastern Australia, pre Lachlan Orogen

Xenoliths brought up by igneous activity provide insight into underlying geology which may otherwise be inaccessible. In this study whole rock major and trace element geochemistry, zircon U-Pb geochronology and Sr-Nd-Hf isotope data are presented for xenoliths hosted by the Newer Volcanics Spring Hill Complex (~ 3 Ma) at Upper Coliban Dam. These xenoliths provide insight into underlying basement known as the Selwyn Block. The xenolith suite includes two dominant groups: a mafic group with gabbroic texture and composition; and a felsic gneiss with granitic composition and glass bands interlayed between quartz-feldspar rich bands. Rarer xenoliths include trachyandesites, a massive quartz rich meta-sedimentary rock and basaltic icelandites from nearby Stewart Hill. Such diversity reflects the complex nature of the crust and mantle within the region. Zircons separated from the felsic gneiss were examined by backscattered imaging and found to be of the magmatic type, with narrow metamorphic overgrowths. They have a Neoproterozoic crystallisation age of 553 Ma with Hf isotope compositions representative of a mixed source with juvenile and crustal input. Geochemical data support an upper crustal signature. These findings extend on limited knowledge of the Selwyn Block, including a history of formation and deformation. The data also unleashes a potential source for the extensive Lachlan Orogen granites and regional sedimentary terrains. Five zircons from the mafic group xenoliths were also analysed and they provide a timeline of magmatic events within the region. The mafic and intermediate xenoliths share Sr isotope composition with the Newer Volcanic Province but heterogeneous trace element signatures suggest that they are not related.