The Complex History of a Caldera-Forming Magma: the Abrigo Ignimbrite, Tenerife, Canary Islands

The 0.196 Ma Abrigo Member of the Diego Hernandez Formation is a widely-distributed, lithic-rich, dominantly phonolitic ignimbrite produced during the most recent caldera-forming eruption on Tenerife. It had a minimum original extent of ~2,000 km². It represents the climax of a ~40 k.y. magmatic cycle recorded in 12 underlying plinian pumice deposits that have strong chemical affinities to the Abrigo. The juvenile component of these plinian deposits consists solely of aphyric to weakly porphyritic phonolitic pumice, with no mixed or petrographically distinct clasts. In contrast, nine distinct types of fresh juvenile clasts occur in the Abrigo: light green aphyric pumice, dark green pumice with large tabular alkali feldspars, dark green crystal mush (>90% phenocrysts), nepheline syenite blocks with minor interstitial glass, black aphyric and porphyritic pumice (some bearing olivine phenocrysts), mafic crystal cumulates, grey hybrid pumice, and coarsely to finely banded pumices. Bulk compositions range from primitive (9.5% MgO) basanite to highly evolved phonolite. Hence, the complete range of compositions and crystallinities occurs among the Abrigo ejecta. Chemical variations among the phonolitic component indicate derivation from at least two distinct magma sources, and/or a role for assimilation within the Tenerife edifice. Our model for the Abrigo magma system invokes a crystallizing body of phonolitic magma, consisting of crystal mush beneath crystal-poor liquid that produced several eruptions over a 40,000 year period. Subsequently, the crystal mush was invaded by mafic magma which itself cooled and partly crystallized while heating, remobilizing and stirring the mush and supernatant liquid (and perhaps melting some wall-rock) to produce mingled, mixed and hybrid phonolite-intermediate-basanite magmas with a large range in crystallinity. Large-scale caldera collapse may be responsible for the seemingly near-complete evisceration of the magma system during the eruption.