Volcanic evolution of a long-lived Ordovician island-arc province in the Parkes region of the Lachlan Fold Belt, southeastern Australia

Abstract

The Ordovician mafic volcanic rocks in the Parkes region of New South Wales occur as three distinct packages of volcaniclastic and coherent volcanic rocks and minor limestone that formed part of an oceanic island arc succession. The oldest package is the Early Ordovician Nelungaloo Volcanics and overlying Yarrimbah Formation. These formations consist of volcanic siltstone, sandstone, polymictic breccia, conglomerate facies interpreted as moderately deep-water turbidites and coarser grained debris-flow deposits emplaced in the medial to distal part of a subaqueous volcaniclastic apron flanking an active volcanic centre(s). Broadly conformable massive to brecciated andesites in the apron deposits are interpreted as synsedimentary sills and/or lava flows. A hiatus in volcanism occurred between the Bendigonian and early Darriwilian (ca 476 – 466 Ma). Deposition of the second package, which produced the Middle to Late Ordovician Goonumbla Volcanics, Billabong Creek Limestone and Gunningbland Formation, commenced with shallow-water limestones and minor volcaniclastic rocks. During an approximately 15 million years period, a thick sequence of bedded volcanic sandstone, limestone and minor siltstone and volcanic breccia were deposited in very shallow to moderate water depths. The top of this package is marked by thick volcanic conglomerate and sandstone mass-flow deposits and approximately coeval basaltic andesite lavas and sills sourced from a nearby volcano. The upper age limit of this package is constrained as approximately 450 Ma by Ea3/4 fossils and monzodiorite that intrudes the Goonumbla Volcanics. The lower limit of the third package, which constitutes the Wombin Volcanics, is poorly constrained and the duration of the hiatus that separates the Goonumbla and Wombin Volcanics is unknown but may be as long as 10 million years. The Wombin Volcanics record development of a thick, proximal volcaniclastic apron flanking a compositionally more evolved volcanic edifice in the immediate Parkes area. Thick crystal-rich turbiditic sandstones of mafic provenance are intercalated with polymictic volcanic breccias and megablock breccias that are interpreted as proximal subaqueous debris-flow and debris-avalanche deposits, respectively. The sequence also includes numerous trachyandesite bodies, many of which were emplaced within the volcaniclastic apron as synsedimentary sills. No evidence was found at Parkes to support the existence of a previously proposed 22 km diameter collapse caldera and the source volcanoes for the Ordovician are envisaged as complex stratovolcanoes.