The petrology, geochemistry and tectonic setting of basic volcanics on the Stuart Shelf and in the Adelaide Geosyncline, South Australia

Abstract

In 1980, von der Borch suggested that the Adelaide Geosyncline formed as a result of a rift initiated in the Late Proterozoic. In 1984, Gunn added further to the idea, and proposed that the Roopena Volcanics represented alkaline igneous activity associated with the initial doming phase. The basaltic lavas of Depot Creek, Port Pirie, Wooltana, the Adelaide Geosyncline, and the Beda Volcanics represent tholeiitic flood basalts from a later rifting stage, with the Gairdner Dyke Swarm acting as feeder dykes to the basalts. In hand specimen the volcanics look very similar, i.e. red-brown to green-grey fine vesicular basalts, but in thin section they are quite different. The Beda Volcanics are merocrystalline with an intersertal texture, the main mineral being plagioclase with small patches of subophitic augite (maximum 10%). The Gairdner Dyke Swarm rocks consist of either fine grained, curved branching augite with coarse laths of plagioclase and pehnocrysts of olivine set in an iron rich glass, or coarser grained holocrystalline ophitic rocks. The Depot Creek volvanics have a fine grained intersertal texture, consisting of potassium feldspar and recrystallised glass. The Port Pirie Volcanics are interbedded with both Calanna Group and Emeroo Subgroup sediments. The Emeroo Volcanics are intersertal fine grained rocks containing potassium feldspar and minor pyroxene set in an iron rich glassy ground mass. The Calanna Volcanics are subophitic in texture. Geochemically all the volcanics except the Port Pirie Volcanics are very similar, with the Beda Volcanics and Gairdner Dyke Swarm being the most fractionated. Magma chamber fractionation simulation studies suggest that the Gairdner Dykes were extruded from a crustal magma chamber of much greater depth, but the similar geochemistry suggests they may have stemmed from the same magma chamber as the other volcanics. Geochemical discrimination diagrams indicate the volcanics are tholeiitic continental flood basalts, and this is reinforced by using a spidergram plot developed by Pearce (1979). Comparison of these volcanics with volcanics from the central Karoo Province and northern Utah and southeastern Idaho on spidergrams show very similar trace element patterns. The basic volcanics of Depot Creek, Wooltana, and the Beda Volcanics, along with the Gairdner Dyke Swarm represent co-magmatic tholeiitic igneous activity associated with the Spencer Gulf rift. The more enriched Port Pirie volcanics were extruded at a later stage of the reactivation of the rift.