The basanite tuffs of Bullenmerri and Gnotuk maars, Victoria, enclose abundant xenoliths of spinel lherzolites, many of which contain amphibole ± apatite ± phlogopite. The xenolith suite also includes cumulate wehrlites, spinel metapyroxenites and garnet metapyroxenites. All xenolith types contain abundant large CO2-rich fluid inclusions. Microstructural evidence for the exsolution of spinel, orthopyroxene, garnet and rare plagioclase from complex clinopyroxenes suggests that all of the metapyroxenites have formed from clinopyroxene (± spinel ± orthopyroxene) cumulates by exsolution and recrystallization during cooling to the ambient geotherm. Pyroxene chemistry implies that a range of parental magma types was involved. Garnet pyroxenites show a series of reactions to successively finer-grained, lower-P mineral assemblages, which imply a relatively slow initial upward transport of the xenoliths in the magma, prior to explosive eruption. The same process has allowed crystallization of phenocrysts from small patches of interstitial melt within xenoliths of lherzolite, wehrlite and metapyroxenite.Critically selected P-T estimates for 16 garnet websterites are consistent with published experimental studies of the spinel/garnet pyroxenite transition, and define a geotherm from 900 °C, 11 kb to 1100 °C, 16 kb. Other published data extend the curve down to c. 7 kb and up to 25 kb. This elevated geotherm suggests that the high regional heat flow is related to convective heat transfer by dike injection accompanying the vulcanism. T estimates for the lherzolites range from 850-1050 °C; comparison with the derived geotherm implies that the spinel lherzolites are derived from depths of 30-55 km. This zone has low seismic velocities (Vp = 6.8-7.8 km/sec) and has thus previously been regarded as a thick, largely mafic lower crust. The xenolith data show that this Mower crust' is dominantly ultramafic, with layers, dikes and some large bodies of pyroxenites and mafic granulites. The anomalously low Vp may be due to the high T, the high proportion of fluid-filled pore volume, and the magnesian composition of the lherzolites. The seismically defined Moho (Vp >8.0 km/sec) coincides with the experimentally determined position of the spinel lherzolite-garnet lherzolite transition.