Investigating detrital apatite from the Castlemaine Group, central Victoria, to unravel sources and multi-cycle sedimentation

Detrital mineral studies have been largely undertaken using zircon, a resilient mineral to weathering, that accumulates in sediments through multiple cycles of sedimentation preserving information from early cratons and provinces. The greater susceptibility of apatite to weathering offers a specific advantage to apatite geochemistry and geochronology over zircon: an insight into first-cycle sedimentation. Such insight is critical in reconstructing the provenance history of ancient sediment successions potentially sourced from a diversity of terranes, such as the Castlemaine Group in the Lachlan Fold Belt, southeastern Australia. This kilometres-thick deep marine turbidite succession was deposited along the eastern edge of Gondwana in the Ordovician. It has many potential sources throughout the supercontinent including nearby in Australia, further afield in Antarctica, and from other former Gondwana constituents including Africa. Deep diamond drill core near the Fosterville deposit in central Victoria, southeastern mainland Australia, intersects large sequences of the Ordovician turbidite succession. The combined geochemistry and U-Pb geochronology of detrital apatite grains from the Fosterville drill core by LA-ICP-MS allows for not only the classification of the source lithology through its rare earth element (REE), Sr and Y chemistry but also apatite U-Pb age. The majority of igneous and metamorphic apatite grains are aged between 580–480 Ma and are likely to be sourced from the nearby Adelaide Rift Complex, Delamerian Orogen and East Antarctica Ross Orogen. Older apatite linked to Grenvillian (1300–900 Ma), Rodinian Rifting (850–650 Ma) and Early Pan-African events (650–580 Ma) are significantly smaller populations compared to previously published U-Pb zircon data from the Castlemaine Group, indicating that these older populations were likely inherited through multiple sedimentation cycles with greater loss of apatite versus zircon. We conclude that although a direct source from East Antarctica to supply sediment of this age and older remains possible, a proximal source terrane comprised of rocks displaying inherited multi-cycle provenance mixed with first-cycle apatite derived from Delamerian/Ross Orogen igneous and metamorphic events is most likely.