Temporal and Geochemical Insights Related to Volcanic and Plutonic Activity within Big Bend National Park, Texas

Big Bend National Park located in the southeastern part of the Trans-Pecos Magmatic Province in west Texas contains a diverse suite of volcanic and intrusive rocks. Ongoing geologic mapping, geochronology, and geochemical studies have focused on the timing and geological relationships associated with these volcanic and intrusive rocks. Igneous activity within Big Bend National Park occurred from 46.5 to 22.0 Ma. Precise ⁴⁰Ar/³⁹Ar and U-Pb zircon geochronology has resolved the timing of this activity into five discrete episodes: (1) eruption and emplacement of both intrusive and volcanic mafic rocks at 46.5 Ma followed by ∼42.0 Ma mafic volcanic pulse; (2) eruption of mafic to felsic rocks at 33.8–33.6 Ma; (3) eruption and emplacement of peralkaline rocks at 32.5 Ma; (4) intermediate to felsic volcanic and intrusive rocks from 30.3 to 28.0 Ma; and (5) mafic to intermediate intrusive activity at 25.0 Ma followed by eruptions of lavas flows at ∼22.0 Ma along the eastern margin of the Park. The largest magmatic event in the Park occurred at 32.5 Ma. This event has been associated with the Pine Canyon caldera and related intrusive rocks followed by the eruption of the Burro Mesa Rhyolite at 29.4 Ma. Intrusive and extrusive activity occurred coevally. ⁴⁰ Ar/³⁹Ar data has revealed extensive eruptions of intermediate compositional lavas at 30.3 Ma.

Previous tectonic models for Trans-Pecos igneous rocks have suggested that a shift from compressional related magmatism to extensional magmatic activity occurred at ∼31 Ma. The most recent tectonic model suggests that igneous activity had occurred in an extensional setting since 47 Ma. This study uses various major and trace element discrimination plots to determine the tectonic environment of mafic to intermediate compositional lavas. The data suggests that the earliest magmas generated in Big Bend National Park at 46.5 Ma have no geochemical affinities to subduction magmas and that they were generated in a continental setting. However, the 47 to 33 Ma igneous rocks in Big Bend National Park may represent a transitional period between subduction and extension. Igneous activity between 38–32 Ma may be genetically linked to the early stages of Rio Grande rifting. After 22 Ma, no igneous activity is present in Big Bend National Park.

New geochemical data and ⁴⁰Ar/³⁹Ar age determinations have aided our understanding of the volcanic stratigraphy in the western part of BBNP. This data not only shows the complexity of the volcanic stratigraphy, but the variability that exists for the intermediate compositional lavas found throughout most of the western half of the Park. Detailed field observations from this study and those of other researchers have identified several dozen mafic volcanic centers throughout much of the western half of the Park.