High pressure cognate inclusions in the Newer Volcanics of Victoria

High pressure pyroxene- and amphibole-rich inclusions are found in a number of Victorian Newer Volcanics volcanoes. The host lavas range from nepheline basanite to nepheline hawaiite and nepheline mugearite. The wide variation in chemistry and mineralogy of the inclusions is explained by crystallization from basaltic magmas under varying P-T and P_H2_O conditions at depth. At moderate pressure wehrlite inclusions (ol+cpx) form, whereas at higher pressures pyroxenites (opx+cpx) and genetically related megacrysts form. Under relatively anhydrous conditions the clinopyroxene megacrysts show a trend of Ca enrichment whereas under hydrous conditions, when amphibole is also stable, the pyroxene shows a trend to greater iron enrichment. The trend nepheline basanite to nepheline mugearite has developed by extensive fractionation of amphibole at elevated pressures under hydrous conditions. Under less hydrous conditions where clinopyroxene assumes the dominant role during crystal fractionation, derivative liquids display a trend of increasing K₂O/Na₂O ratio, with little modification of their level of undersaturation. Olivine plays a decreasing role in crystal fractionation processes with increasing pressure. The available evidence indicates that the only magma which could have been parental to the observed basanites was a more picritic basanite.