Contrasting interactions of two magmas with the same water-saturated hosts, Big Bend National Park, Texas

Rhyolitic and mafic dikes cut lower Tertiary sediments, tuffs and mafic lavas of the Eocene–Oligocene Chisos Formation in Big Bend National Park, Texas. Two contrasting magmas injected into the same hosts under the same conditions provide a “controlled” experiment on magma–host interaction. Peralkaline rhyolite dikes are extensive and stand in bold topographic relief. On contact surfaces against mudstones, close-packed spheroids of chilled rhyolite indicate vigorous movement of magma within envelopes of fluid. Against sandstones, the rhyolite dike contacts are more irregular and locally protrude as blunt fingers into the wall rock. Against mafic lava flows, dike contacts are planar, and the rhyolite is less altered. Mafic dikes cut the rhyolite dikes. Some contacts of these mafic dikes are peperitic, indicating intrusion into soft wet sediment and tuff. The same hosts of the older rhyolitic dikes must also have been water-saturated and unconsolidated. Higher viscosity and lower temperature prevented rhyolite magma from forming detached bodies, or engulfing its host, to form peperites. ted pressures and contact temperatures approximate or exceed the critical point of water, indicating that the fluid envelopes at rhyolite dike contacts were very unstable, because even small temperature fluctuations would have produced wide variations in fluid density around the critical point. The resulting volume instability triggered rapid pulsations in the fluid envelope, deforming the contacts. Where permeability of the host was low, hot fluid accumulated at contacts, but where permeability was high, fluid could escape before reaching high temperature.