Carbonatite Metasomatism in the Southeastern Australian Lithosphere

Abstract

New mineralogical and geochemical data from a suite of glass ± apatite ± amphibole ± phlogopite ± carbonate-bearing spinel wehrlite, lherzolite and harzburgite xenoliths from the Newer Volcanics, southeastern Australia, are consistent with metasomatic interactions between harzburgitic or refractory lherzolitic lithosphere, and penetrative sodic dolomitic carbonatite melts. Metasomatism occurred when ascending dolomitic carbonatites crossed the reaction enstatite + dolomite = forsterite + diopside + CO<inf>2</inf> at ∼1.5–2.0 GPa, resulting in partial to complete replacement of primary orthopyroxene by sodic clinopyroxene, together with crystallization of apatite, amphibole and phlogopite, and release of CO<inf>2</inf>-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 wehrlite compositions require 6–12% carbonatite addition, the lherzolites require ∼8% or less, and the harzburgites require minimal addition of carbonatite. However, metasomatism probably also involved an open system component, during which residual and metasomatic phase compositions were determined by partitioning relationships with the reacting carbonatite, resulting in loss from the metasomatized volume of a fugitive, siliceous, aluminous, alkali- and LILE-enriched silicate melt.