Geochemical evidence for provenance of Ordovician cherts in southeastern Australia

Abstract

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.

KEY WORDS:

• Chert
• Ordovician
• Lachlan Orogen
• New England Orogen
• Narooma terrane
• geochemistry
• REEs

ACKNOWLEDGEMENTS

Alan Owen (University of Glasgow) precipitated our interest in analysing cherts of the Lachlan Orogen by drawing to our attention his studies (with Howard Armstrong and John Floyd) of the geochemistry of Ordovician cherts in the southern uplands of Scotland. The majority of the chert samples analysed herein were originally collected as part of the regional mapping program of the Geological Survey of NSW. Major-, trace- and rare-earth element analyses were undertaken at the University of Queensland by Massimo Gasparon. Cheryl Hormann is thanked for cartographic services. Incisive reviews by Chris Fergusson and Dick Glen significantly improved the final publication. Published with permission of the Director, Geological Survey of NSW, Resources and Energy Division, NSW Trade & Investment. This is a contribution to IGCP 591: The Early to Middle Paleozoic Revolution.

Supplementary Papers

Appendix—locality and biostratigraphic information for analysed chert samples.