Residual glasses from some alkali basaltic lavas,from New South Wales

Summary. The glassy residua from an alkali olivine-basalt and a nepheline-basanite are respectively alkali trachytic and phonolitic in composition and contrast strongly with the compositions ofthe host rocks. The analysis of the glass from the alkali olivine-basalt reveals appreciable normative corundum, which is considered a consequence of the crystallization f excessive diopsidic linopyroxene. This results in a deficiency in lime relative to alumina during the crystallization f much of the plagioclase. A consideration ofthe compositions ofthe glasses and the salic differen-tiates from alkaline basic intrusions indicates that the development of either trachyte or phonolite depends on differing degrees of undersaturation of the parental basic magmas. T HE chemistries of the intersertal glasses in igneous rocks are of particular importance in petrogenesis for they provide the best indi-cations of the compositions of liquid residua. Vincent (1950), Walker, Vincent, and Mitchell (1952), and Elliot (1956) have presented chemical data on glasses from tholeiites. However, hypocrystalline alkaline basic rocks have usually been discussed from a petrographic standpoint only. This study deals principally with the chemistry of the glasses in two alkali basic lavas (an alkali olivine-basalt and a nepheline-basanite) from the Central Volcanic Province of north-eastern New South Wales. This Province is developed mainly on the New England Tableland, where it extends as a belt from south of Armidale north to Glen Innes and west to the Inverell area. The available data indicate that the most common lava is alkali olivine-basalt. The associated volcanic rocks include nepheline-basanites and olivine-nephelinites, analcime-basanites and olivine-analcinfites, and ankaramitic types (Wilkinson, 1966). Tholeiitic lavas also occur in the Invere]l area. The Central Province is a major alkali olivine-basalt province in a continental setting, the volcanic rocks out-cropping over an area greater than 2000 square miles. The analcime-basalt (olivine-analcimite) from Spring Mount, west of Glen Innes (Wilkinson, 1962), is lower Oligocene. This age determination is based o