VOLCANIC GEOLOGY OF THE CERRO CASTELLAN AREA, BIG BEND NATIONAL PARK, TEXAS

The Cerro Castellan 71/2 minute quadrangle, western Big Bend National Park, contains a sequence of mafic lavas, trachyte flows, and peralkalic rhyolite intrusions and dome complexes. Mafic lava units range in age from ~46 to ~29 Ma, are interlayered with sedimentary rock and, in the upper part of some sections, with trachyte lava. Rhyolite dome complexes (~29 Ma) had eruptive histories dominated by crater formation, followed by eruption of tuff rings and lava, the later forming biscuit domes and flows. One center in the southwestern part of the quadrangle produced a significant local ignimbrite. Mafic lava eruption in the Cerro Castellan area became volumetrically less important with time, although mafic lava has been found throughout the volcanic section, including at least two flows that post date rhyolite domes.

Mafic units include basalt, hawaiite, mugearite, shoshonite and latite. No Daly Gap is present, but a compositional gap of ~7 wt. % SiO2 exists between trachyte and peralkaline rhyolite. Magmatic enclaves of trachyte, however, are common in some silicic domes. These suggest that trachyte and rhyolite are co-genetic. Major and trace element modeling has demonstrated that variation within peralkaline rhyolite could be explained by reasonable amounts of crystal fractionation of observed phenocrysts.

Sample/mantle ratio plots for basalts show the convex upward plot with a gentle slant downwards towards Y and Yb characteristic of Ocean Island Basalt. Evolved units (trachyte and peralkaline rhyolite) show expected increases in incompatible elements, with negative anomalies developed for elements expected to be relatively depleted by fractional crystallization of observed phenocrysts (Ti, Eu, P, Sr, Ba). Several samples show, in addition, a slight negative Nb anomaly, which may be attributed to assimilation of crustal material with such geochemical signature.

All Cerro Castellan rocks erupted within the span ~46-29 Ma are alkalic; mafic units show no detectable geochemical change during this period that may be associated with a hypothesized tectonic shift from regional compression to Basin and Range extension. Furthermore, the appearance of local peralkaline rhyolite at ~29 Ma has no regional significance, as similar peralkaline rhyolite was erupted much earlier elsewhere in west Texas.