Peralkaline Phonolites: Genesis, Magmatic Evolution, and Crustal Storage History

ABSTRACT Peralkaline phonolites are the most evolved products of the evolution of Si-undersaturated magmatic suites. They can be important components of extrusive sequences globally, occurring in all major tectonic settings. Here we review aspects of their petrogenesis and evolution, including their ultimate mantle sources and polybaric liquid-lines-of-descent. There are three main compositional lineages: (i) mafic nephelinite–phonolite, (ii) basanite–phonolite, and (iii) alkali basalt–phonolite. In the majority of cases, the dominant differentiation mechanism has been fractional crystallization. There is no strong evidence for phonolite formation by partial melting of alkali basalts deep in the crust, or for significant crustal contamination. Attention is drawn to the extreme compositions generated by protracted fractionation, with Peralkalinity Indices ≤18. Several natural systems are used to describe the range of high-level magma reservoirs and the internal differentiation processes. Volcanic hazards and the potential environmental impact of eruptions are assessed and the economic potential of phonolites introduced. Keywords: peralkaline phonolites; tectonic settings; magmatic lineages; extreme compositions; volcanic hazards; economic potential