C02 as the propellant of carbonatite-kamafugite cognate pairs and the eruption of diatremic tuffisite.

Tuffisite pipes are characteristic of ultramafic rocks among which the rare association of kamafugite and carbonatite is prominent. Four main areas are known at present where this association is found: Katwe-Kikorongo and Bunyaruguru in Uganda, IUP localities in Central Italy, Mat a da Corda in Brazil and Qinling in Gansu, China. In all areas dominant volcanic forms are pipes (diatremes) with maar-tuff rings. Tuffisite lapilli-bearing breccia fills the diatremes and tuffisite lapilli tuff surges form pyroclastic aprons around the vents. Tuffisite lapilli are so named to reflect their specific structure, which has probably been generated during a deep magmatic tuffisitisation. Mantle xenoliths are typical of these rocks and the only realistic agent of acceleration to the surface is a deep-seated concentration of volatiles with a high proportion of C02. The genesis of the kamafugite- carbonatite association can be assessed both in terms of field occurrence and geochemistry. In three cases out of the four the kamafugite-carbonatite association occurs in intracratonic grabens, only Italy being controversial. Specific magma composition is germane to a specific magma propagation mechanism producing a distinctive style of intrusive and extrusive pyroclastics that ar gues cogently for a fundamentally similar tectonic environment.