Rb-Sr isotope systematics of the Burro Mesa Rhyolite, Big Bend National Park, Texas.
Tsai, Heng (1992) Rb-Sr isotope systematics of the Burro Mesa Rhyolite, Big Bend National Park, Texas.
| Authors | Tsai, Heng |
|---|---|
| Year | 1992 |
| Title | Rb-Sr isotope systematics of the Burro Mesa Rhyolite, Big Bend National Park, Texas. |
| Document type | Thesis-or-Dissertation |
| Degree | Master's University of Texas, Austin. Austin, TX, United States. Pages |
| GeoRef | 1995-031795 |
| Keywords | absolute-age; Ar-Ar; basaltic-composition; Big-Bend-National-Park; Brewster-County-Texas; Burro-Mesa-Rhyolite; Cenozoic-; dates-; dikes-; felsic-composition; igneous-activity; igneous-rocks; intrusions-; mafic-composition; major-elements; Oligocene-; Paleogene-; Rb-Sr; rhyolites-; Sr-Sr; Tertiary-; Texas-; trace-elements; United-States; volcanic-rocks |
Abstract
Burro Mesa Rhyolite occurs in sporadic exposures along a northeast-trending, 15-km long strip in which four small eruptive centers were investigated. Sparsely porphyritic lava with flow banding is intruded by felsic dikes, and overlain in the northernmost center by abundantly porphyritic lava that lacks flow banding. The arfvedsonite-bearing Burro Mesa Rhyolite commonly contains 1- to 10-cm mafic inclusions consisting chiefly of pyroxene and plagioclase. Rb-Sr isochron ages indicate that magmatism extended over approximately a million years. It began at Trap Mountain 29.26+ or -0.04 Ma and Goat Mountain 29.5+ or -0.1 Ma, followed by eruptions in the southernmost Cerro Castellan 28.3+ or -0.1 Ma and northernmost Burro Mesa lower unit 28.40+ or -0.05 Ma centers. Rb-Sr whole-rock and feldspar data for the northern three eruptive centers are linearly well correlated, comprising apparently well-behaved isotopic systems, whereas at Cerro Castellan the feldspar is in isotopic disequilibrium with whole rock. Within 2sigma analytical error, the Rb-Sr ages agree with laser-fusion super 40 Ar- super 39 Arr ages of feldspar phenocrysts Copeland and others, 1992. There is no geographic trend of ages or of initial super 87 Sr/ super 86 Sr ratios. Isotope data and major- and trace-element data indicate that Burro Mesa Rhyolite was derived from multiple-event, small-scale crustal re-melting. The mafic inclusions are representative of basaltic magma that was genetically unrelated, but which provided the heat to melt the crustal material and trigger the eruption of the rhyolitic liquid. Various extents of crustal assimilation account for the geochemical characteristics for each eruptive center.