Pyroclastic rocks and calderas associated with strongly peralkaline magmatism

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Abstract
Strongly peralkaline volcanic complexes tend to be shieldlike due to coalescence of low‐viscosity lavas from numerous vent areas and blanketing by pyroclastic flows and falls. Dense welding and rheomorphism commonly mask the nature of pyroclastic units; welded pumice falls are common. Relatively low eruptive columns produce poorly sorted pumice falls that are difficult to distinguish from topography mantling pyroclastic flows. Welded fall deposits are characterized by multiple, fine‐scale, welding reversals, welding zones that correspond with stratification, and by flattening of fiamme parallel to underlying slopes, whereas welded topography‐mantling ignimbrites may have fine‐grained basal layers, lithic lenses, imbricate fiamme, and variations in crystal content and fiamme size with topographic position. Calderas are relatively small, typically 3–9 km in diameter, with little evidence for single‐stage subsidence in excess of a few hundred meters. Many strongly peralkaline volcanic centers show evidence of two or more nested calderas, and partial reactivation of older ring fractures is common. Replenishment of magma reservoirs beneath calderas can promote dilatant reactivation of older subsidence structures, resulting in multicyclic collapses that reuse these structures. In several centers, the caldera‐forming unit is zoned from pantellerite to more crystal‐rich trachyte. Following collapse, trachyte lavas commonly erupt from a central vent on the caldera floor, building a cone that nearly fills the caldera. Reestablishment of isostatic equilibrium may be accomplished both by eruption of these lavas and by flow of trachytic and more‐mafic magma into the root zone beneath the volcano. Resurgent doming sensu strictu has not been documented in strongly peralkaline systems; at Pantelleria and Mount Suswa, however, the central portions of the cauldron blocks were uplifted and tilted. Magma reservoirs that feed explosive eruptions generally are shallow, as indicated by pyroclastic units that contain lithic fragments of the volcanic edifice (commonly including cognate syenite) but lack subjacent crustal lithologies. In addition, the overwhelming predominance of alkali feldspar as a phenocryst phase and the scarcity of quartz are consistent with pantellerite liquidus relations only at pressures <100 MPa. Pantellerite and trachyte shields may represent the mature stage of continental rift volcanism; once the flux of mantle‐derived magmas has been sufficient to generate a large mass of felsic liquid that can act as a density filter, mafic magmas rarely reach the surface.