Contrasting styles of compositional zonation in pyroclastic deposits of the Mogán Group, Gran Canaria, Canary Islands

Many volcanic eruptions produce pyroclastic deposits that display pronounced internal gradients in composition, mineralogy, and phenocryst abundance. The volcanic succession of the Mogán Group (14.2–13.6 Ma) of Gran Canaria, Spain, provides an exceptional opportunity to study the temporal evolution of a long-lived magmatic system and the associated diverse styles of zoning preserved in the volcanic record.

The Upper Mogán Formation (13.9–13.6 Ma) contains strongly to moderately welded, peralkaline trachytic to rhyolitic ignimbrites erupted from the multicyclic Tejeda Caldera. Many ignimbrites exhibit upward increases in crystallinity and pumice size and abundance associated with decreases in welding intensity and abundance of lithic clasts. A typical characteristic of the deposits is the coexistence of crystal-poor and crystal-rich pumices along with mingled varieties within a given cooling unit. Major element variations between pumice types are commonly small yet some trace elements (Rb, Zr, Sr, Ba) may differ substantially. We focus here on ignimbrite ‘D’ (13.7 Ma, ~15 km³), which contains dominantly crystal-poor trachydacite fiamme (SiO₂ 68–69 wt%, Ba 80–180 ppm, Zr 2250–2460 ppm) in the lower parts that show increased mingling with a crystal-rich comenditic trachyte (SiO₂ 64–66 wt%, Ba 540–2090 ppm, Zr 420–930 ppm) in the upper parts of the deposit. The mineral assemblage of ‘D’ is dominated by anorthoclase and amphibole with minor clinopyroxene, plagioclase, FeTi-oxides and apatite. Phenocryst abundance increases from nearly aphyric at the base to ca. 15 vol% towards the top. The crystal-poor trachydacite is enriched in components that are incompatible with the observed phenocryst assemblage (FeO, Zr, Rb, Nb, REE). Conversely, the crystal-rich trachyte is enriched in typically compatible major and trace elements (Al₂O₃, Na₂O, P₂O₅, Ba, Sr).

We suggest that crystal-poor pumices in ignimbrite D are derived from evolved aphyric melts extracted from highly crystalline portions of an upper crustal magma reservoir and that crystal-rich pumices contain a considerable component of remobilised shallow cumulate. This interpretation is supported by strong enrichments in Ba in the crystal-rich pumices, significantly larger than expected in melts evolving along liquid lines of descent from a trachybasaltic parent. We show that extensive fractionation of a mineral assemblage dominated by anorthoclase is an important petrogenetic mechanism in the Upper Mogán Formation and not only produces the observed depletions in Ba, Sr and negative Eu anomalies in the crystal-poor pumices but also lays the foundation for remelting of a feldspar-dominated cumulate.

This type of zoning strongly contrasts with that of cooling unit ‘P1’ (14.2 Ma, ~45 km³), which is compositionally zoned from a silicic lower part to a basaltic top and the result of mixing between rhyolitic, trachytic and basaltic magma components. We propose that the observed temporal changes in zoning style and decrease in frequency of basaltic eruptions are related to progressive thermal conditioning and increasing ‘mushification’ of the upper crust, which creates a thermal and viscosity barrier suppressing basaltic eruptions.