THE GOMEZ TUFF REVISTED: THE WORLD'S LARGEST PANTELLERITIC IGNIMBRITE

The ~37 Ma Gomez Tuff (~220 km³) was erupted from the 18 x 24 km Buckhorn Caldera in the Davis Mountains Volcanic Field of Trans-Pecos Texas as a pyroclastic component of peralkaline, and dominantly comenditic flood rhyolite lava. The ignimbrite covered an area of at least 5,700 km² in what is now mostly preserved as ~20 m thick, densely-welded tuff; within its caldera, the tuff has a maximum thickness >450 m. The tuff experienced secondary flowage, creating recumbent folds and ramp structures, especially in upper parts of thicker sections. Breccia is conspicuously absent.

Phenocrysts, in decreasing order, of alkali feldspar, quartz, ferrohedenbergite, ilmenite, apatite, and, in a few samples, fayalite, are set in groundmass ranging from glassy, to cryptocrystalline to granophyric. Alkali feldspar phenocrysts range from Or_36-39; groundmass laths Or_42-52. Granophyric mafic minerals include sodian clinopyroxene, arfvedsonite, and aenigmatite. Whole-rock Peralkaline Indices (P.I. = mol. Na + K/Al) are typically 1.1 to 1.3; glass and calculated groundmass compositions range up to 1.41, approaching the high values typical of the type rock from Pantelleria.

Ignimbrite eruption was preceded by emplacement of trachyte to peralkaline rhyolite domes and flows and was followed by eruption of trachyte lava within the caldera. The Gomez system involved development of a peralkaline silicic cap above a reservoir of trachytic magma, which was tapped in pre-Gomez silicic domes and shallow intrusions, and then the Gomez eruption. Modelling suggests that ~70 wt.% crystallization of trachyte could generate Gomez magma. The ignimbrite itself is relatively homogeneous, showing <10 wt.% variation in incompatible elements.