ABSTRACT

Two different classifications for granitoids have been proposed independently in Australia (S-type and I-type) and in Japan (ilmenite-series and magnetite-series). The former classification is genetic and the latter is descriptive. We compared these classifications from a different chemical viewpoint: the feature of carbon-bearing gas species (methane, ethane, carbon dioxide) occluded in granitoids. These species were contained in mineral grains and grain boundaries and their composition might reflect environmental parameters at the crystallization of granitoids. We collected 315 granitoid samples from both classifications in Australia and Japan, and analyzed occluded gas by crushing these samples. Results indicated that magnetite-series granitoid samples generated under oxidized environmental conditions contained notably low carbon-bearing gas species; in particular, they contained no ethane. On the other hand, I-type granitoid samples were rich in carbon dioxide rather than methane compared with S-type samples. The identification of these qualitative features of carbon-bearing gas species of the S-/I-types and ilmenite-/magnetite-series correspond with data from previous work on the source rock of granitoids and the mineral assemblage of iron-oxide minerals. The carbon-bearing gas occluded in granitoids might be a useful conceptual tool for discussing the generation process and classification of granitoids.