Geochemical evidence for provenance of Ordovician cherts in southeastern Australia

Early to Middle Ordovician cherts of the Girilambone and Adaminaby groups are widespread in the Lachlan Orogen in central New South Wales. Their ages are well constrained biostratigraphically by conodonts ranging from the late Tremadocian to earliest Sandbian. Broadly contemporaneous cherts are exposed in the Narooma terrane (latest Cambrian to Darriwilian), and in allochthonous blocks (of late Middle and Late Ordovician age) of the New England Orogen at Port Macquarie. In the Kiandra-Tumut region of southern NSW Darriwilian to earliest Gisbornian cherts are interbedded with volcaniclastics of the Macquarie Volcanic Province. To determine their provenance and compare depositional settings, 60 chert samples representative of these regions were analysed for selected major, trace and rare earth elements (REE). Al2O3/TiO2 ratios enable recognition of two dominant sources from which the sediment fraction of the cherts was derived at different times in the evolution of the Tasmanides, one indicative of old continental crust and the other sourced from a juvenile continent or plateau. All cherts analysed, regardless of geological province, carry continental margin signatures demonstrated by high Al2O3/Fe2O3 ratios, LREE enrichment, small negative Ce anomalies, prominent negative Eu anomalies, low total REEs and near-chondritic Y/Ho ratios. The late Cambrian and Early Ordovician Narooma terrane cherts display a clastic component derived chiefly from a juvenile continent or plateau, whereas cherts of the Hermidale and Albury-Bega terranes contain detritus of mixed origin. During the Early to Middle Ordovician magmatic hiatus in the Macquarie Volcanic Province, all cherts regardless of tectonic affiliation incorporate terrigenous detritus solely of Gondwana origin. Upon resumption of magmatism in the Macquarie Volcanic Province, a mixed terrigenous source is recorded in all analysed cherts, before a dominant Gondwana source is re-established in the late Darriwilian for cherts of the Hermidale, Albury-Bega and Kiandra regions.