**6. Peralkaline affinity of Kone silicic volcanics**

Kone silicic eruptives are characterized by the phenocryst assemblage of alkali feldspar, quartz, hedenbergite, aenigmatite and fayalite. This assemblage is accompanied by rare ilmenite and apatite. The appearance of aenigmatite in silicic volcanics has been attributed to reflect a peralkaline affinity [17]. Peralkaline magmas likely form when fractional crystallization removes a high proportion of plagioclase relative to mafic minerals [18]. The coexistence of aenigmatite and hednbergite is thought to be a typical feature of the mildly peralkaline silicic rocks such as comendite and trachycomendite [19].

The presence of quartz as a major phenocryst phase in Kone silicic volcanics indicates an advanced stage of fractionation (i.e., the more SiO2-rich magma). Furthermore, the SiO2 content of the glass matrix (69.3 wt.%, **Table 4**) shows a rhyolite composition. Thus, the phenocryst mineralogies reflect the SiO2 content of the magma from which they crystallized. On this basis, the alkali feldspar + hedenbergite + aenigmatite + fayalite assemblage only crystallizes in the most evolved magmas (rhyolites) with the appearance of quartz as a phenocryst phase.

On the basis of the mineral assemblage, the Kone silicic volcanics can be regarded as more mildly peralkaline rhyolite (comendite) as evidenced by the appearance of hedenbergite, which appears to cease crystallizing in strongly peralkaline rhyolite (pantellerite), generally coinciding with the crystallization of aegirine or aegirineaugite [19]. This is probably related to the low fO2, at or close to FMQ, at which the magmas evolve [20]. We conclude that the alkali feldspar + quartz + hedenbergite + aenigmatite + fayalite assemblage is essentially restricted to the mildly peralkaline rhyolite (comendite), but close to the comendite-pantellerite boundary as crystallization of aenigmatite is restricted to pantellerite [16].
