Mineralogy, Textures and P-T Relationships of a Suite of Xenoliths from the Monaro Volcanic Province, New South Wales, Australia

Intraplate basalts of the Eocene–Oligocene Monaro VolcanicProvince (MVP), in southeastern New South Wales, include lower-crustaland refractory to weakly metasomatized upper-mantle xenoliths.Lower-crustal-derived xenoliths appear to be all two-pyroxeneplagioclase granulites (Cpx_{Fe:Mg:Ca 0·17–0·56:0·63–0·77:0·28–0·89}Opx_{Fe:Mg:Ca 0·39–0·52:1·37–1·47:0·02}An_72–86 and An_48–50) but may also include garnetpyroxenites at depth. Mantle-derived xenoliths are principallyspinel-bearing lherzolites (Fo_89·8–90·6Cpx_{Fe:Mg:Ca 0·07–0·45:0·70–1·70:0·01–0·94}Opx_{Fe:Mg:Ca 0·16–0·19:1·62–1·75:0·01–0·10})but also include amphibole ± spinel-bearing lherzolite(Fo_88·7–89·1 Cpx_{Fe:Mg:Ca 0·09–0·21:0·61–0·91:0·73–0·93}Opx_{Fe:Mg:Ca 0·09–0·31:0·70–1·54:0·03–0·91}),spinel-bearing harzburgite (Fo_90·5–90·7Cpx_{Fe:Mg:Ca 0·08:0·91–0·93:0·74–0·84}Opx_{Fe:Mg:Ca 0·16–0·18:1·73–1·79:0·00–0·02}),wehrlite, pyroxenite (Cpx_{Fe:Mg:Ca 0·08–0·10:0·84–0·90:0·80–0·85}Opx_{Fe:Mg:Ca 0·16–0·33:1·51–1·73:0·02–0·03})and rare garnet pyroxenite (Gt_{Fe:Mg:Ca 0·83–0·95:1·60–1·70:0·45–0·48}Cpx_{Fe:Mg:Ca 0·14–0·21:0·69–0·77:0·78–0·86}Opx _{Fe:Mg:Ca 0·31–0·42:1·43–1·56:0·02–0·03})and amphibole–apatite composites. Xenolith textures aregenerally weakly to moderately foliated, a few are mosaic-porphyroblasticand rare samples are veined or highly strained. MVP xenolithsappear to have equilibrated under similar pressure–temperature(P–T) conditions to other southeastern Australian xenolithsequivalent to the South Eastern Australia (SEA) palaeogeotherm.P–T estimates for the MVP suite of xenoliths reveal aheterogeneous lower crust and upper mantle that is thickly underplatedto c. 1·8 GPa or c. 50 km depth. MVP xenolith P–Tdata are compared with those used to derive the SEA palaeogeotherm,which is shown to be in need of revision using more modern geothermometersand geobarometers and new xenolith coexisting mineral data. KEY WORDS: xenolith; petrography; texture; geotherm; Monaro; eastern Australia