Eruptive history of the Pine Canyon Caldera, Big Bend National Park, Texas

Ogley, David Scott
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The Chisos Mountains in the central part of Big Bend National Park are a deeply-eroded volcanic-intrusive complex of Oligocene age. Intrusive and extrusive units of the complex are subalkaline to midly peralkaline, and belong to the Trans-Pecos magmatic province, forming the southernmost expression of that province in the United States. At least two calderas are present in the Chisos Complex, both of which were active during the mid-Oligocene. The younger of the two known calderas, the Pine Canyon Caldera, was the source vent for the four major ash-flow units of the South Rim Formation. Volcanic and sedimentary rocks of the Chisos Formation, which unconformably underlies the South Rim Formation, were deformed by pre-eruption inflation of the Pine Canyon magma chamber. During eruption of the lower members of the South Rim Formation, the caldera collapsed by sagging rather than by faulting. Eruption of the youngest member of the South Rim Formation was accompanied by ring fracturing and fault-controlled collapse. Whole-rock major-element chemistry demonstrates a petrogenetic relationship between the intrusive and extrusive members of the caldera complex. The volcanic members were derived by progressive tapping of a shallow magma chamber in which alkali feldspar and later quartz were fractionating. Compositions of successive members of the eruptive sequence project progressively closer toward the minimum in the system albite - orthoclase - quartz at 500 bars water pressure. A late ring dike is cumulus material from the magma chamber; its composition plots slightly more quartz-rich than the temperature minimum. The distribution of the South Rim Formation and the related intrusions can be explained by a multi-stage eruptive history. Two similar ash-flow cycles are represented by the four members of the South Rim Formation. All four members spread southwest from the caldera, and their distribution was restricted by topography of the caldera rim and the volcanic flank. Caldera collapse at the end of ash-flow activity resulted in uneven subsidence and allowed emplacement of the ring dike. Intrusion of granitic bodies west of the caldera, framing The Basin on the north and west, was the final stage of igneous activity