Peralkaline silicic extrusive rocks: magma genesis, evolution, plumbing systems and eruption

Abstract

Peralkaline silicic extrusive rocks are an important component of the volcanological record.Here we review several aspects of their formation and evolution, including the tectonic settings in which they occur, their main petrological and geochemical features, the magmatic lineages along which they evolve, and the parameters (T , P , f O 2 , melt water contents) that control the lineages. Particular attention is paid to the composition of the extraordinary melts formed at the lowest temperatures. Various lines of evidence are presented to explain the silica gaps in some lineages. The partial melting of continental crust and the role of crustal contamination are considered to be of relatively minor importance in their genesis. High P -T experiments aimed at quantifying the lineages are assessed. Geophysical and petrological evidence for the depth and nature of the plumbing systems are presented. Differentiation mechanisms within reservoirs and the ubiquity of the formation of compositional zonation are discussed, as are the timescales involved. Volcanic hazards and the environmental impact of eruptions are described and a brief assessment of the ore potential of the extrusives is given.

Magmatic to solid-state evolution of a shallow emplaced agpaitic tinguaite (the Suc de Sara dyke, Velay volcanic province, France): implications for peralkaline melt segregation and extraction in ascending magmas

“…The differentiated products of the Velay are represented by trachytes and phonolites with alkalinity (Na / Al-K) ranging from 0.8 to 1.3 (Fig. 2b, c;Batard, 1974;Macdonald et al, 2021, for another recent review). Batard (1974) showed that phonolites are derived from a normal crystallization sequence at depth (magnetite, ilmenite, apatite, zircon, sphene, pyroxene, plagioclase, amphibole, alkali feldspar, feldspathoid) that was superimposed by reverse paragenesis (feldspathoid, alkali feldspar, Zr/Ti silicates, alkali pyroxene) developed at lower depth.…”

Reassessment of the sulfur and halogen emissions from the Millennium Eruption of Changbaishan (Paektu) volcano

“…Comendites are dominantly produced by crystal fractionation of alkali basalt, via a trachyte stage (Macdonald et al, 2008;2021). The 10-30 km 3 of near aphyric comendite discharged with the Millennium Eruption implies efficient separation from a trachytic magma mush at depth, whose volume should be approximately 50-100 km 3 , on the basis of the fivefold increase in incompatible elements between trachyte and rhyolite (Fig.…”

The role of gas flushing on magma reservoir crystallization and its consequences for the growth of planetary crust

“…Journal Pre-proof Carlo et al, 2010;Lucic et al, 2016;Romano et al, 2018;Iddon and Edmonds, 2020), with pre-eruptive H 2 O content of 3-5 wt%, very little if any CO 2 (hence XH 2 O fluid =1), at pressures of 1-3 kb, ie occupying a position similar to those of arc-magmas. This reflects the overall incompatible behaviour of H 2 O in crystallising magmas, and the extreme degree of fractionation of parental magmas that felsic daugthers produced in ridges or hot-spot contexts require (e.g., Macdonald et al, 2021). Anyway, the very CO 2 -rich nature of high pressure fluids in ridges and hot spot settings indicates that underplating of magmas in the crust of such settings will likely favour crystallization rather than melting.…”