Mapping of ∼3,400 km2 in the Sierra del Gallego area, 110 km north of Ciudad Chihuahua, Mexico, has revealed about 1,000 m of volcanic rocks unconformably overlying Lower Cretaceous limestone of the Chihuahua Tectonic Belt. The regional structure is dominated by Basin and Range faults that trend north-northwest and cut all volcanic units.

Whole-rock and feldspar K-Ar age determinations, combined with whole-rock chemical analyses, show that subalkalic volcanism took place 45 to 29 m.y. ago, except for a conspicuous hiatus between 36 and 29.5 m.y. Between 45 and 39 m.y. ago, the area was covered by thin widespread blankets of rhyolitic ash-flow tuff derived from distant sources. These ash flows appear to correlate with units associated with a major uranium deposit about 50 km to the south, in the Sierra Peña Blanca. At Sierra del Gallego, these ash flows were followed by a 300-m thickness of extensive andesitic lava flows formed by magma mixing, and about 38 m.y. ago by a rhyolitic lava flow unit approximately 150 m thick. Around 36 m.y. ago, numerous local rhyolite flows and domes were emplaced. Approximately 29 m.y. ago, a renewed surge of volcanic activity produced a widespread 125-m thick group of basalt flows accompanied by a thin rhyolitic ash flow.

Volcanism at Sierra del Gallego is broadly contemporaneous with and is gradational in composition between voluminous calc-alkalic activity in the Sierra Madre Occidental to the west and alkalic volcanism to the east in Trans-Pecos Texas. Tertiary rhyolitic volcanism throughout this region was controlled by tectonic patterns that preceded the profound episode of crustal extension so obvious in the present regional topography.

*Present addresses: (Keller) Gemological Institute of America, 1660 Stewart Street, Santa Monica, California 90404; (Bockoven) Exxon Minerals Company, P.O. Box 120, Denver, Colorado 80201

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