INTERACTIONS OF MAGMAS WITH WATER-SATURATED HOSTS, BIG BEND NATIONAL PARK, TEXAS

Two swarms of compositionally distinct dikes cut upper Cretaceous and lower Tertiary sedimentary rocks, and tuffs and mafic lavas of the Eocene-Oligocene Chisos Group, in Big Bend National Park. Two contrasting magmas injected into the same hosts under the same conditions provide a “controlled” experiment on magma – host interaction. Peralkaline rhyolite dikes are numerous and extensive, and stand in bold topographic relief. On contact surfaces against mudstones, ripple-like features and close-packed spheroids of chilled rhyolite indicate vigorous movement of magma within envelopes of fluid or fluidized wet sediment. Against sandstones, the rhyolite dike contacts are more irregular and locally protrude as stubby fingers into the wall rock. Mafic to intermediate dikes cut the rhyolite dikes. Some contacts of these aphyric mugearite to benmoreite dikes are peperitic, indicating intrusion into soft wet sediment and tuff. The same hosts of rhyolite dikes must also have been unconsolidated and water-saturated. Higher viscosity and lower temperature of rhyolite magma prevented it from forming detached bodies in sediment, engulfing its host, or causing explosions to form peperites.

Estimated pressures and contact temperatures approximate or exceed the critical end point of water, indicating that the fluid envelopes at rhyolite dike contacts were very unstable, because even small temperature fluctuations produce wide variations in fluid density. The resulting volume instability triggered pulsations in the fluid envelope, deforming the contacts. Permeability of wall rock was an important constraint in producing the features on dike contacts. Where permeability was low, hot fluid accumulated at contacts, but where permeability was high, fluid could escape before reaching high temperature.