Peralkaline silicic melts of island arcs, active continental margins, and intraplate continental settings: Evidence from the investigation of melt inclusions in minerals and quenched glasses of rocks

Based on the analysis of data on the composition of melt inclusions in minerals and quenched glasses of igneous rocks, we considered the problems of the formation of peralkaline silicic magmas (i.e., whose agpaitic index, the molar ratio AI = (Na 2 O + K 2 O)/Al 2 O 3 , is higher than one). The mean compositions of peralkaline silicic melts are reported for island arcs and active continental margins and compared with the compositions of melts from other settings, primarily, intraplate continental areas. Peralkaline silicic rocks are rather common in the latter. Such rocks are rare in island arcs and active continental margins, but agpaitic melts were observed in inclusions in phenocrysts of plagioclase, quartz, pyroxene, and other minerals. Plagioclase fractionation from an alkali-rich melt with AI < 1 is considered as a possible mechanism for the formation of peralkaline silicic melts (Bowen’s plagioclase effect). However, the analysis of available experimental data on plagioclase-melt equilibria showed that natural peralkaline melts are almost never in equilibrium with plagioclase. For the same reason, the melting of the majority of crustal rocks, which usually contain plagioclase, does not produce peralkaline melts. The existence of peralkaline silicic melt inclusions in plagioclase phenocrysts suggests that plagioclase can crystallize from peralkaline melts, and the plagioclase effect may play a certain role. Another mechanism for the formation of peralkaline silicic magmas is the melting of alkali-rich basic and intermediate rocks, including the spilitized varieties of subalkali basalts.