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 TiO2 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.