School of Earth & Environmental Sciences
Alcorn, Courtney J., Zircon geochronology and geochemistry of xenoliths in a Jurassic dyke at Bombo, NSW, Australia: Evidence of deep crustal structure under the Sydney Basin, BSci Hons, School of Earth & Environmental Sciences, University of Wollongong, 2016.
Mafic and ultramafic xenoliths and xenocrysts are abundant in alkali volcanic and hyperbyssal systems intruded within the Permo-Triassic Sydney Basin of eastern Australia. The locality of one of these dykes from Bombo Headland, along the New South Wales coastline was studied in detail to provide information on the crust and mantle below Sydney Basin. The host rock of the dyke is a Permian latite flow interpreted to be related to back-arc activity behind an arc to the east. The dykes contain abundant xenoliths of gabbro and dunite with minor amounts of eclogite, metasedimentary rocks and granitoids.
Xenoliths of gabbro are coarse-grained olivine-bearing with well preserved plutonic igneous textures. SEM-EDS analyses integrated with petrographic studies recorded three types of olivines with different #Mg. Most magnesian are olivine xenocrysts as aggregates, which show textural evidence of reaction with dyke magma. These are interpreted as small fragments of mantle dunite. Intermediate #Mg olivines occur in the gabbro clasts in the dyke. Lowest #Mg olivines are euhedral phenocrysts within the dyke that are in textural equilibrium with their matrix.
A separated ovoid inclusion of gabbro yielded abundant zircons with a Jurassic age of 204.6 ± 4.6 Ma. This is interpreted as giving the maximum age of the dyke. A random sample of the dyke with abundant compositionally different gabbro clasts and some metasedimentary clasts yielded several zircon ages. Small pitted zircons with ages ≥ 300 Ma are interpreted as derived from Lachlan Orogen basement. However most of the zircons are large euhedral magmatic grains with a Carboniferous age of 303.8 ± 5.5 Ma. These have high Th/U ratios and are interpreted as giving the age of the gabbro xenoliths. The Carboniferous gabbro xenoliths provide the first definitive evidence supporting the existence of a mafic gabbroic pluton underlying the Sydney Basin.