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Journal of Petrology | Volume 45 | Number 4 | Pages 739-758 | 2004
Journal of Petrology 45(4) © Oxford University Press 2004; all rights reserved.

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

IAN C. ROACH*

CRC LEME, DEPARTMENT OF EARTH AND MARINE SCIENCES, AUSTRALIAN NATIONAL UNIVERSITY, A.C.T. 0200, AUSTRALIA

* Telephone: +61 2 6125 0030. Fax: +61 2 6125 5544. E-mail: Ian.Roach{at}anu.edu.au

Intraplate basalts of the Eocene–Oligocene Monaro Volcanic Province (MVP), in southeastern New South Wales, include lower-crustal and refractory to weakly metasomatized upper-mantle xenoliths. Lower-crustal-derived xenoliths appear to be all two-pyroxene plagioclase granulites (CpxFe:Mg:Ca 0·17–0·56:0·63–0·77:0·28–0·89 OpxFe:Mg:Ca 0·39–0·52:1·37–1·47:0·02 An72–86 and An48–50) but may also include garnet pyroxenites at depth. Mantle-derived xenoliths are principally spinel-bearing lherzolites (Fo89·8–90·6 CpxFe:Mg:Ca 0·07–0·45:0·70–1·70:0·01–0·94 OpxFe:Mg:Ca 0·16–0·19:1·62–1·75:0·01–0·10) but also include amphibole ± spinel-bearing lherzolite (Fo88·7–89·1 CpxFe:Mg:Ca 0·09–0·21:0·61–0·91:0·73–0·93 OpxFe:Mg:Ca 0·09–0·31:0·70–1·54:0·03–0·91), spinel-bearing harzburgite (Fo90·5–90·7 CpxFe:Mg:Ca 0·08:0·91–0·93:0·74–0·84 OpxFe:Mg:Ca 0·16–0·18:1·73–1·79:0·00–0·02), wehrlite, pyroxenite (CpxFe:Mg:Ca 0·08–0·10:0·84–0·90:0·80–0·85 OpxFe:Mg:Ca 0·16–0·33:1·51–1·73:0·02–0·03) and rare garnet pyroxenite (GtFe:Mg:Ca 0·83–0·95:1·60–1·70:0·45–0·48 CpxFe: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 are generally weakly to moderately foliated, a few are mosaic-porphyroblastic and rare samples are veined or highly strained. MVP xenoliths appear to have equilibrated under similar pressure–temperature (P–T) conditions to other southeastern Australian xenoliths equivalent to the South Eastern Australia (SEA) palaeogeotherm. P–T estimates for the MVP suite of xenoliths reveal a heterogeneous lower crust and upper mantle that is thickly underplated to c. 1·8 GPa or c. 50 km depth. MVP xenolith P–T data are compared with those used to derive the SEA palaeogeotherm, which is shown to be in need of revision using more modern geothermometers and geobarometers and new xenolith coexisting mineral data.

KEY WORDS: xenolith; petrography; texture; geotherm; Monaro; eastern Australia


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