Carbonatite Antiskarns: A Critical Appraisal

Abstract

The antiskarn hypothesis considers that ultramafic rocks, such as pyroxenites, occurring in alkaline rock-carbonatite complexes, are formed by assimilative reactions between silica-free carbonatite-forming magma and solid silica-bearing country rocks. The geological, experimental, and thermodynamic evidence presented by proponents of the antiskarn process is examined critically, and it is concluded that carbonatite-forming magmas are not deficient in silica and typically crystallize abundant primary silicate minerals. Field and mineralogical evidence for antiskarn-formed rocks in most alkaline rock complexes is either absent, incorrectly interpreted, or of only local significance. Carbonatite-forming magmas can react with country rocks to form exoskarns (fenites) and minor ultramafic selvages (endoskarns) around silicate xenoliths and veins, but these reactions cannot result in the formation of voluminous silicate rocks present in alkaline rock-carbonatite complexes. The antiskarn reaction process is evaluated in terms of the enthalpies of 16 endothermic reactions and the thermal equilibration of granitic country rock when assimilated isenthalpically by carbonatite-forming magma by consideration of the relationship between the temperatures of post-mixing assemblages and the mass fractions of granite and carbonatite-forming magma endmembers. Calculations show that the large fractions of carbonatite-forming magma required to reach the solidus temperature of granitic country rock are not supported by field relations and that crystallization of the carbonatite-forming magma will occur before assimilation. It is concluded that there is no geological or thermodynamic evidence to support the antiskarn hypothesis.