LARGE-VOLUME, ALKALIC RHYOLITE SYSTEMS, NORTHEASTERN DAVIS AND BARRILLA MOUNTAINS, TEXAS, U.S.A
Extensive silicic lavas formed the shields of the Star Mountain Formation (SMF) (37.2 Ma K/Ar; 36.84 Ma 39Ar/40Ar), and the Adobe Canyon Formation (ACF) (37.1 Ma K/Ar; 36.51 Ma 39Ar/40Ar). The Gomez Tuff (GT) (36.6 Ma K/Ar; 36.74 39Ar/40Ar) blanketed a large region surrounding the Buckhorn Caldera, forming deposits up to 500 m thick within the caldera. The GT eruption was preceded by peralkalic rhyolite domes (36.87, 36.91 39Ar/40Ar), some of which blocked movement of SMF flows. Following collapse, the Buckhorn caldera was filled by trachyte lava. ACF lavas then covered much of the region.
SMF (~220 km3) is composed of multiple flows ranging from quartz trachyte to mildly peralkalic rhyolite; three major types have been delineated that form at least six flows. Variations among flows can be modeled by fractionation of alkali feldspar-dominated phenocryst assemblages. ACF (~125 km3) contains fewer flows, some up to 180 m thick, of chemically homogeneous, mildly peralkalic comendite, extending up to 40 km. SMF and ACF flows characteristically exhibit brecciated bases and tops, flow banding, and vesiculated zones. Glasses lack vitroclastic textures.
GT (~ 220 km3) may represent the largest known pantellerite. It and the slightly younger Paisano Volcano (36.33 39Ar/40Ar) in the southern Davis Mountains appear to represent developments of the earlier, less peralkalic SMF-type magmatism. GT is typically less than 100 m thick in extra-caldera sections; these show pyroclastic bases and tops, although the interiors are commonly rheomorphic, containing flow banding and ramp structures. Most sections show one cooling unit; two sections reveal a smaller, upper cooling unit. Chemically, the tuff is fairly homogeneous, but is more evolved than older pantelleritic domes, from which it may be derived by about 23 wt. perecent fractionation of phenocryst assemblages dominated by alkali feldspar and quartz. Overall, the pantellerites appear to have evolved from trachyte magma similar to that erupted in post-caldera lavas.