A mineralogical investigation of some trachytic lavas and associated pegmatoids from camel's hump and turritable falls, central Victoria∗

The texture and mineral chemistry of trachytic lavas from Camel's Hump and Turntable Falls have been studied as an extension to bulk rock geochemical studies by previous workers. Three texturally distinct varieties are recognized: trachyte flows, chilled alkali trachytes, and pegmatoidal veins. Although each assemblage has a distinctive mineralogy, remnant features and similar zoning within minerals show the varieties to be related. Variations of mineral chemistry and stability within the three types are discussed. Data on the chilled alkali trachyte and the residual pegmatoidal phases indicate the development of a peralkaline residual liquid rich in volatiles. The data also provide further information on the mineralogical changes which occur as trachytic rocks become peralkaline.