Volcanic activity, the result of crustal differentiation during the Hercynian orogeny, generated eight explosive eruptions in the Vilancos region of the Spanish Pyrenees. The volcanic products comprise the Erill Castell Volcanic Formation of Stephanian age, which crops out as a 20 km long, WNW-trending strip < 2 km wide dipping steeply to the south. The Vilancos region represents a small fragment of an originally extensive regional terrain of silicic centres.

The explosive eruptions mainly generated strongly peraluminous and phenocrystal garnet-bearing subaerial ignimbrite facies. Proximal intra-formational breccias represent a substantial volume of the preserved erupted product and one phreatoplinian deposit is exposed. Mass-flow deposits are common, and small-volume basalt, andesite and rhyolite lava flows, minor tuffs and palaesols also occur.

Electron microprobe data show that each garnet-bearing member of the Vilancos region has a distinct garnet composition. This is used as geochemical fingerprinting tool to aid mapping and correlation between proximal intra-formational breccias and ignimbrite of the same eruption. Within one debris-flow deposit (the Vilancos Breccia Member) at least three garnet populations occur. Two of these are derived from pyroclastic members within the mapped region, the other comes from an unexposed rhyolite lava source.