An isotopic characterisation of rocks from the Stavely region, western Victoria:a zircon perspective

The Stavely region has been described as a continental-margin arc system that developed on the eastern margin of Australia in the mid-Cambrian (c. 510490 Ma). The joint Geoscience AustraliaGeological Survey of Victoria Stavely Project, investigating the regional geology and mineral systems of the Stavely region, resulted in 14 stratigraphic holes being drilled testing regional geological models and recovering material from potentially prospective basement rocks under cover to characterise the subsurface geology. Drill core and field site samples were taken for mineral separation to extract zircon for U-Pb, Lu-Hf, O isotope and trace element geochemical analyses. The analysed samples reported here include rocks from the Mount Stavely Volcanic Complex (MSVC), including mineralised dactic porphyries, and the Bushy Creek Igneous Complex (BCIC).
The U-Pb data provide age constraints on the rocks; the Lu-Hf data indicates the relative `maturity of the rocks; the O isotopes indicated the degrees of continental crustal input/recycling into the parent melt; the trace element data provides an indication of the crystallisation environment, including providing evidence of mixing of magmas and subsequent compositional changes.
U-Pb data indicate that the ages of many of the igneous rocks in the Stavely region are c. 510500 Ma. Available data indicate that the MSVC was emplaced both during and after deposition of the turbiditic Nargoon Group sediments. Mineralised dacitic porphyries were syn-eruptive with the MSVC, and rocks of the BCIC were emplaced at the same time and after both the MSVC and porphyries. New U-Pb dating of zircon from rocks of the BCIC also suggests they may be syn- to post-deformation, rather than purely post-deformation.
O isotope data yield d18O values less than that of the normal mantle  through to values similar to and greater thennormal mantle (values), indicating possible evolution of the arc and increase of crustal material that assimilated into later stage magmas. The samples that yield lower than `normal mantle d18O values are interpreted to have been derived from hydrothermally altered, subducted rocks that were remelted and did not significantly interact with the overlying continental crust at the time of emplacement.
The mineralised porphyries display eHf values indicative of depleted mantle suggesting the melt is juvenile (i.e. do not display significant amounts of crustal reworking), while the eHf values of the MSVC rocks are more evolved. This suggests that there were multiple, distinct source magmas that had a different history (i.e. porphyries little to no crustal input, while MSVC rocks had a source with some crustal input). The plutonic rocks of the BCIC are interpreted to have had a greater degree of crustal input again.
The combination of isotopic and trace element geochemical data from zircon allows for discrimination of the differences of rock units based on timing, maturity and degree of crustal reworking of their parent melts. This provides a powerful tool in assisting in unravelling the tectonic environment of the continental margin arc setting of the Stavely region.

Tags: Earth Sciences, External Publication, Published_Internal
Organization: Geoscience Australia Data