VOLCANIC AND MAGMATIC EVOLUTION OF A SMALL TRACHYTIC VENT COMPLEX, NORTH BURRO MESA, BIG BEND NATIONAL PARK, TEXAS

Volcanic rocks exposed on the northern end of Burro Mesa in Big Bend National Park portray the evolution of an Oligocene central volcanic vent complex that produced two generations of welded block and ash deposits associated with 1) initial dome collapse and 2) subsequent central spine collapse. Peripheral to the vent complex, isolated breccia deposit exposures overlie ignimbrites, tephras, and lavas. These blocks are a few meters to several hundred meters long and 30 m high and consist of monolithic angular and welded trachytic lava clasts in finer-grained matrix. Rheomorphic structures in the breccia deposit show ductile deformation and suggest it formed while above the glass transition temperature.

A large, west-facing amphitheater composed of inward-dipping lava flows is filled by a central volcanic spine that rises 65-70 m above the surrounding terrain. On the open side of the amphitheater, isolated exposures of breccia containing clasts of trachytic lava have been significantly affected by hydrothermal alteration. At higher elevations around the flanks of the central volcanic spine, breccia blocks containing fragments of spine-type lava have frothy vesicular cores. Some exposures are strongly jointed; all are compositionally indistinguishable from the central spine.

The stratigraphically highest lavas formed a trachytic endogenous dome that produced inward-dipping lava flows when the complex experienced sector collapse. The collapse resulted in partial dome fragmentation and displacement of large blocks of hot lava down steep dome flanks and produced large isolated exposures of welded block and ash. Emplacement of a near-vertical spine of viscous lava depicts the end of volcanism associated with this complex. Younger lahar/debris avalanches were plastered on the walls of the spine and adjacent to blocky lava flows. Field relations and geochemical characteristics indicate that the ignimbrites, lava-forming dome and breccias, spine and spine breccias, and a small satellite intrusive on the northwest flank are genetically related.

Chemical and whole rock oxygen isotope variations indicate a pervasive hydrothermal system existed after the vent complex formed. Oxygen isotope values from 7.8 to 21.3 per mil indicate that alteration occurred at temperatures between 50 and 200^oC.