FIELD RELATIONS AND GEOCHEMISTRY OF PERALKALIC RHYOLITE DOMES AND FLOWS AND ASSOCIATED MAFIC LAVA, SW CERRO CASTELLON 71/2 MINUTE QUADRANGLE, BIG BEND NATIONAL PARK, TEXAS

Burro Mesa Rhyolite Member (~29 Ma) of the South Rim Formation forms prominent domes, flows, and associated tuff cones in the Cerro Castellan Quadrangle. Our geologic mapping has identified ten eruptive and intrusive centers of Burro Mesa activity: typically, each center would erupt explosively, forming tuff cones composed of pyroclastic flows, surges and debris avalanche deposits in which block craters occur proximally to vents. Centers then issued domes and flows. Four of these occur along a northwest trend and once formed a chain of overlapping domes and flows. The largest of these, Hill 3166, erupted a small ignimbrite, which locally is 40 m thick. Two other centers are represented today only by dikes, which are chemically and petrographically identical with erupted rhyolite; yet another center, Cerro Castellan, is an eroded dome with feeder dike exposed.

Burro Mesa domes and flows overlie mafic lava (trachybasalt and trachyandesite) of Bee Mountain Basalt; Bee Mountain Basalt may represent a shield volcano through which and upon which the Burro Mesa centers were erupted. Pillow lava in upper Bee Mountain Basalt suggests eruption of some units within lakes. Similar mafic lavas overlie Burro Mesa units in at least two localities, one on Burro Mesa proper outside of the mapped area. Alamo Creek Basalt, a significantly older Tertiary unit, also contains similar mafic units.

Burro Mesa Rhyolite within the study area shows very restricted major element variation, but more variation in incompatible trace elements (Rb, Th, Zr, Nb, REE and others). Parallelism of REE plots (all with strong negative Eu anomalies) and Spider Diagrams supports crystal fractionation as the chief petrogenetic mechanism among the rhyolites. Trachyte enclaves and rhyolite appear cogenetic. If Bee Mountain Basalt was parental to the rhyolite, some crustal assimilation may have been necessary.