The Eulogie Park Gabbro, a layered basic intrusion from Eastern Queensland

The Eulogie Park Gabbro comprises approximately 65 saucer‐shaped layers consisting of olivine gabbros, ferrigabbros, troctolites, leucogabbros, gabbros and magnetite‐rich rocks. The intrusion is divisible into three main parts: a lower portion consisting mainly of ferrigabbros and magnetite‐rich layers, a middle portion composed of troctolites and olivine gabbros, and an upper portion consisting mainly of olivine gabbros and leucogabbros. The minimum stratigraphic thickness is 3,000 ft (900 m). Modal compositions and textures exhibit marked changes across the boundaries of the layers. Mineral compositions vary irregularly with height in the layered sequence, and small oscillations are common. Compositional data suggest that temperature increased during the first half of the crystallisation period, and then remained fairly constant. Chemical and petrographie studies of the iron‐titanium oxides indicate that the primary phase was an ulvöspinel‐magnetite solid solution. Subsolidus oxidation resulted in magnetite‐ilmenite intergrowths, which also contain exsolved pleonaste. The oxygen fugacity, which controlled the crystallisation and exsolution behaviour of the iron‐titanium oxides, is thought to have beeen affected by the fugacity of sulphur. Many features of this unusual intrusion are not obviously related to the conditions of crystallisation commonly supposed to exist in basic intrusions, and most of the anomalies have not been satisfactorily resolved. Undercooling and rhythmic concentration changes are believed to have been responsible for both the small‐scale rhythmic layering and the larger scale layering in the intrusion.