Ordovician to earliest Silurian history of the Macquarie Arc, Lachlan Orogen, New South Wales

摘要

Subduction-related volcanic and associated sedimentary successions of Ordovician age in the Junee-Narromine, Molong, Rockley-Gulgong and Kiandra Volcanic Belts are correlated using the most comprehensive biostratigraphic framework yet compiled for the Macquarie Arc. Key features of the correlation include confirmation and dating of four arc-wide magmatic phases: in the Early Ordovician (Lancefieldian-Bendigonian), late Middle to early Late Ordovician (Darriwilian to Gisbornian), Late Ordovician and Late Ordovician to earliest Silurian. Emplacement ages of intrusions that form part of the four magmatic phases complement the biostratigraphic correlations, A widespread hiatus of about 9 million years separates the last-known Bendigonian deposits associated with Phase I from initiation of the second magmatic phase in early Darriwilian time, The third magmatic phase comprises the geochemically distinctive Copper Hill Suite of intrusive rocks. The fourth magmatic phase, recognised by its evolved shoshontic signature, follows an extensive volcanic hiatus marked by development of Eastonian-age shallow-marine autochthonous limestones and overlying deeper water fine-grained clastic rocks in the Junee-Narromine and western Molong Volcanic Belts, During this interval in the eastern Molong Volcanic Belt and Rockley-Gulgong Volcanic Belt, volcanism appears to have been nearly continuous with the changeover from Phase 2 to Phase 4 lavas discernible only from petrographic and geochemical criteria, Isotopic dating indicates that Phase 3 magmatism spans the Eastonian volcanic hiatus, suggesting that this intrusive episode may have been responsible for regional uplift resulting in limestone deposition. A widespread depositional and volcanic break, ranging across the Ordovician-Silurian boundary, reflects variable uplift and cessation of arc volcanism, followed by intrusion of Phase 4 porphyries in the early Llandovery that constitute the last gasp of arc activity,