Petrology of the leucite‐bearing lavas in New South Wales∗

Abstract

Several isolated outcrops of melanocratic leucitite lava in central New South Wales are interpreted as remnants of flows erupted in late Cainozoic time by small lava‐volcanoes. The general distribution of outcrops conforms to the regional structural trends of the Palaeozoic basement complex, characterized by a prolonged post‐orogenic stabilization history dominated by block‐faulting and multiple uplift. The lavas are generally microphyric to aphyric and essentially composed of olivine (Fa_7–21), diopsidic pyroxene, leucite, and iron‐titanium oxide minerals, with subordinate titaniferous phlogopite and amphibole (richterite), nepheline, and alkali feldspar. The Colour Index is generally close to 70. Bulk‐rock analyses and electron microprobe data on the constituent phases show the unusual chemistry of the suite and its close affinities to the alkali ultrabasic lavas of the ‘stable’ continental regions. Compositional variations are similar, in, their predominant trends, to their analogues in the alkali olivine basalt series and may be interpreted in terms of crystal‐liquid equilibria. The leucitites are thought to have crystallized mainly above about 700°C under pressures lower than 2 Kb. The source magma probably originated in the mantle at a minimum depth of 100 km and reached the surface through several independent channelways, probably assisted by the regional tectonic activity.