Hawaiite of High Pressure Origin from North-eastern New South Wales

Abstract

ALTHOUGH it is now widely accepted that the source of “primary” basaltic magmas is located within the Earth's upper mantle, only rarely has a high pressure origin been proposed for the more evolved members of alkali volcanic lineages^1,2 and then partly to overcome the problems posed by the relative abundances of mafic and felsic lavas in some volcanic provinces. Current petrological opinion mostly attributes the origin of the series (alkali basalt)→hawaiite→mugearite→benmoreite→alkali trachyte (compare the Hebridean and Hawaiian sodic alkali successions) to low pressure fractionation in shallow level magma chambers^3,4. Certainly this hypothesis finds support in the chemistry of felsic veins and schlieren in differentiated alkaline mafic iritrusives, and of vitric groundmasses in rapidly cooled alkali basaltic lavas. The compositions of these “residua” indicate that low pressure fractionation of alkali basaltic liquids can produce mugearitic5, ben-moreitic⁶, alkali trachytic and phonolitic differentiates^7,8. We report here, however, the discovery of a hawaiite containing lherzolite xenoliths, and an abundance of large pyroxene crystals (“megacrysts”). The latter are considered to be high pressure cognate phases of the hawaiite, and the occurrence of both megacrysts and xenoliths indicates that comparatively “evolved” alkalic liquids may also be generated at high pressures.