Carbonatite metasomatism in the southeastern Australian lithosphere

New mineralogical and geochemical data from a suite of glass +/- apatite +/ - amphibole +/- phlogopite +/- carbonate-bearing spinel wehrlite, lherzolit e and harzburgite xenoliths from the Newer Volcanics, southeastern Australi a, are consistent with metasomatic interactions between harzburgitic or ref ractory lherzolitic lithosphere, and penetrative sodic dolomitic carbonatit e melts. Metasomatism occurred when ascending dolomitic carbonatites crosse d the reaction enstatite + dolomite = forsterite + diopside + CO2 at simila r to 1.5-2.0 GPa, resulting in partial to complete replacement of primary o rthopyroxene by sodic clinopyroxene, together with crystallization of apati te, amphibole and phlogopite, and release of CO2-rich fluid. In the sample suite examined, the minimum amount of carbonatite melt may be estimated on the assumption that metasomatism occurred in a closed system, and that the precursor lithology was clinopyroxene-poor harzburgite. The derivative wehr lite compositions require 6-12% carbonatite addition, the lherzolites requi re similar to 8% or less, and the harzburgites require minimal addition of carbonatite. However, metasomatism probably also involved an open system co mponent, during which by partitioning relationships with the reacting carbo natite, resulting in loss from the metasomatized volume of a fugitive, sili ceous, aluminous, alkali- and LILE-enriched silicate melt.