Extension-related volcanism in the Middle to Late Devonian of the Lachlan Orogen: geochemistry of mafic rocks in the Comerong Volcanics

Abstract

The Comerong Volcanics are a Middle to Late Devonian bimodal sequence located in the southeastern Lachlan Orogen of NSW, Australia. Magmatism related to subduction was ongoing and located to the east of the continent during this period placing the Comerong Volcanics in a continental back arc setting. Mafic rocks in the Comerong Volcanics occur in three stratigraphically distinct units and range in composition from tholeiitic andesite to basalt. An inverse relationship between volume of erupted lava and degree of fractionation of the magma is evident from the base of the volcanic complex to its top. The lowermost mafic unit is the least voluminous and most fractionated and consists of flows with low Mg#, Ni and Cr and high incompatible trace element abundances. The overlying unit comprises both moderately and extremely fractionated basalt and is characterised by flows with a relatively high TiO 2 content. Lavas in this unit can be divided into higher and lower-Ti types. The uppermost unit is the most voluminous and the least fractionated with relatively high MgO, Ni and Cr and low incompatible trace element abundances. Compositions are similar to the low-Ti lavas in the unit below. Trace element characteristics of all lavas suggest they were derived from a heterogeneously enriched source in the subcontinental lithospheric mantle. Enrichment of the source in LILE and depletion in Ti and Nb likely reflect earlier subduction in the orogen. The mafic rocks were erupted in a continental within-plate setting; a setting consistent with the field relationships.

The Narooma Terrane offshore: a new model for the southeastern Lachlan Orogen using data from rocks dredged from the New South Wales continental slope

“…They have been interpreted to be a consequence of continental rifting (Dadd, 2011) and overlie Ordovician rocks and phases of the Bega Batholith, underlying the Upper Devonian Merrimbula Group west of Batemans Bay in the Budawang Synclinorium and around Eden. While these lithologies are commonly folded, they are generally only mildly deformed and usually present weak cleavage or no cleavage (Glen, 1994); they are mapped as the Comerong Volcanics in the north and the Boyd Volcanic Complex in the south.…”

Provenance and tectonic setting of the Early Permian sedimentary succession in the southern edge of the Sydney Basin, eastern Australia

“…The potential source rock candidates for the other formations include the Ordovician, high‐K calc‐alkaline, and shoshonitic volcanic, and less common plutonic rocks in the Macquarie Arc (Figure 1; Crawford, Meffre, Squire, Barron, & Falloon, 2007 and references therein), and the Silurian–Devonian felsic and mafic volcanic rocks of the Comerong Volcanics that formed in intra‐continental rift settings (Atton, 2013; Dadd, 2011), because the SSB samples show similarities to these rocks (e.g., LREE‐ and Th‐enriched, and Nb‐depleted patterns). However, the Ordovician volcanic rocks are dominantly mafic, have lower Th contents, and lack well‐developed sub‐continental lithosphere in contrast to the SSB deposits and thus are unlikely to provide most of the detritus for the SSB.…”

“…However, the Ordovician volcanic rocks are dominantly mafic, have lower Th contents, and lack well‐developed sub‐continental lithosphere in contrast to the SSB deposits and thus are unlikely to provide most of the detritus for the SSB. It is more likely that the Silurian–Devonian volcanic rocks and volcanogenic sequences have been the dominant contributors because they are moderately widespread in the eastern Lachlan orogeny and are dominantly felsic and exhibit within plate enrichment (Atton, 2013; Dadd, 2011). However, granite boulders have been observed in the Early Permian successions indicating that Silurian and Devonian granites in the Lachlan orogen have contributed detritus (Gostin, 1968).…”