Multiple dykes situated along the southeastern Australian coastline from south of Kiama to Gerroa were examined in detail. The results of geochemical analysis indicate the dyke’s composition varies from sub-alkaline to alkaline. The dykes fall under four separate geochemical groups, with the majority characterized as foidites followed by alkali basalts, basalts and andesitic basalts. Petrographic analysis and geochemistry suggests that four dyke-forming events occurred with each geochemical group resulting from one event. Phenocrysts of olivine, clinopyroxene and plagioclase are encapsulated in the fine-grained groundmass of the dykes. Numerous dyke textures such as glomeroporphyritic, porphyritic, aphanitic, intergranular and amygdaloidal were found. The dykes primarily strike NW-SE, however four dykes strike NE-SW and two have an E-W strike. Only one dyke was found to contain xenoliths. These comprise of multiple intrusive rocktypes, including granitic and gabbroic types, with the gabbroic types including cumulate-textured, biotite-rich and alkaline varieties. NW-SE, E-W, N-S and NE-SW striking joints are present in the study area, with the NW-SE set being the most prevalent, followed by NE-SW then less common, N-S and E-W striking joints. The N-S striking joints may be linked to the N-S trending Camden Syncline while the NE-SW set are most likely associated with the Hunter Thrust, which strikes WNW-ESE . Joints striking NW-SE may be related to the Lachlan River Lineament’s reactivated basement fractures. The orientation of the dykes are similar to that of the dominant joint sets, indicating that the dykes took advantage of long-standing structural weaknesses in the country rocks.
Samar Usamah Abu Shamma (1), Ian Graham (1), Paul Lennox (1) & Glen Bann (2)
PANGEA Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales Sydney, Sydney, Australia (1); School of Earth, Atmosphere and Life Sciences, University of Wollongong, Wollongong, Australia (2)