Petrology of the Mitchell Mesa Rhyolite, Trans-Pecos Texas




Burt, Edward R.

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An ash-flow sheet, to which the names Mitchell Mesa Rhyolite and Brite Ignimbrite have been applied, crops out prominently in Presidio and western Brewster Counties, Texas. Because of its great areal extent it is the most important unit for correlation in the Tertiary volcanic field of southern Trans-Pecos Texas, and it should bear a single name. Priority and widespread use in published literature support the name Mitchell Mesa. The ash-flow sheet is divisible into two cooling-units. The lower, a simple cooling-unit that grades locally into a compound cooling-unit, is a vitric-crystal rhyolitic ash-flow tuff with 15 to 25 percent opalescent alkali feldspar and bipyramidal quartz phenocrysts as long as 4 mm in a light brownish gray, grayish pink, or light gray vesiculated groundmass. The lower cooling-unit ranges in thickness from about 230 feet immediately north of Pinto Canyon to 2 feet at South Lajitas Mesa. The upper, simple cooling-unit is a vitric-lithic ash-flow tuff with as much as 20 percent lithic fragments in a very light gray to brownish gray groundmass containing about 10 percent non-opalescent alkali feldspar and quartz phenocrysts. The upper unit ranges in thickness from 60 to 100 feet. Its only outcrops are overlain by Petan Basalt north and northeast of Pinto Canyon. Except in a few places, the pyroclastic texture of the lower cooling-unit was obliterated by vapor-phase crystallization. Any tridymite and cristobalite originally present were subsequently converted to quartz. Four whole-rock chemical analyses of samples from widely separated localities are similar, showing only minor variations in K₂o and Na₂o. The alkali feldspar phenocrysts are richer in Na₂O and poorer in K₂O than the whole rock. Therefore the feldspar in the groundmass is more potassic than that in the phenocrysts. Foreign inclusions are most abundant in outcrops of Mitchell Mesa Rhyolite closest to the Chinati Mountains. Immediately north of the mountains, a separate ash-flow tuff is present beneath the Mitchell Mesa Rhyolite. This and other evidence leads to the conclusion that the Chinati Mountains area was the source of the ash-flow sheet.