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High pressure minerals found in Miocene basaltic volcanics at Ballogie include large garnets, aluminous clinopyroxenes and orthopyroxenes, olivine, kaersutitic amphibole, anorthoclase and opaque oxides; they occur together with minor amounts of biotite, corundum and zircon. The garnet, some pyroxenes and anorthoclase can be of gem quality. The minerals accompany abundant lherzolite xenoliths in the volcanics and resemble some other occurrences in SE Queensland. A magnetic survey of the site suggests a diatreme composed largely of breccia intruded by small basaltic bodies. The garnet (Mg62 66 Fe2i-24 Ca12 _i4), clinopyroxenes (Mg49_s6 Ca34-39 FelO-13, with 6.7-8.50/0 Ab03) and orthopyroxenes (Mg8i-84 Fell_iS Ca3_4, with 4.8-5.8% Ab03) probably represent xenocrysts derived from garnet pyroxenites and pegmatitic garnetites interlayered with spinellherzolite mantle. The compositions suggest that these minerals crystallized under pressuretemperature conditions around 14-15 kb and l000-1100°C. The Ti content of the kaersutites, using a new geobarometer, gives approximate pressures of crystallization mostly between 12 kb and 14 kb. The bulk of the Ballogie minerals were sampled from a volatile-bearing upper mantle, relatively rich in Ti, but poor in Cr. The model invoked for the emplacement of a composite diatreme such as the Ballogie pipe involves sudden outgassing above a rising diapir by crack propagation. The resultant updrag also provides the potential to transport very deep material from the diamond stability zone. HOLLIS, J.D., F.L. SUTHERLAND & R.E. POGSON, 1983. High pressure minerals and the origin of the Tertiary breccia pipe, Ballogie Gem Mine, near Proston, Queensland. Records of the Australian Museum 35(4): 181-194. 181 In 1981 Mr E.A. Thomson of Boondooma, Queensland asked the Department of Mineralogy and Petrology at the Australian Museum to investigate his gemstone lease at Coverty Creek, 17 km SSW of Proston (151 °31.5' E, 26° 18.0' S). The Ballogie Gem Mine, on the east side of the creek, yields gem-quality red garnet, comparable with the Garnet Gully prospect 10 km west at Brigooda (Queensland Geological Survey, 1981). The fragments come from a small alkali basalt exposure within granite country rock. (Fig. 1.) An anorthoclase megacryst from the basalt gave a K-Ar age of 16.0 ± 0.2 Myr, interpreted as the age of eruption (Sutherland & Wellman, in prep.). Other prominent minerals brought up by the basalt include olivine, pyroxenes and oxides, typical of high pressure megacryst suites found in eastern Australia (Wass & Irving, 1976). Queensland (Murphy et al., 1976). It belongs to a younger episode than the bulk of these basalts and is

High pressure minerals and the origin of the Tertiary Breccia Pipe, Ballogie Gem Mine, near Proston, Queensland