Tectonic settings and magma associations

The largest volumes of peralkaline silicic rocks are found in areas of epeirogenic uplift and rift formation on the continents, but they may also form in several other tectonic settings, such as the oceanic islands, during the later stages of orogenic cycles, as isolated occurrences in active mobile belts, and in areas of extensional tectonics at or near continental plate margins. Their emplacement is effected during periods of dominantly tensional stresses. The peralkaline silicic rocks are typically members of the transitional, mildly aikaline basalt-trachyte association. Associated salic rocks are commonly feld-spathoidal and/or anorthite-normative, as well as peralkaline. Two main types of association are distinguished: firstly, provinces where the peralkaline rocks are mainly of comenditic type, where the salic rocks are in relatively low abundance compared to basic, and where the intermediate (Daly) composition gap is not present; secondly, provinces of pantelleritic or mixed pantelleritic-comenditic volcanism, where the basic : acid volume relationships are reversed and where the Daly gap is usually markedly developed.