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Abstract
The Eagle Mountains are a northwest trending block fault mountain range in southwestern Trans-Pecos, Texas. The central portion of this range is formed by Tertiary igneous rocks. The magmatic rocks represent intra-caldera facies of two eruptive cycles. Each cycle produced rhyolitic ash flow tuffs followed by trachytic volcanism. The Carpenter Canyon Tuff and Frenchman Canyon Trachyte represent the first cycle while the High Mill Tuff and Eagle Peak Syenite represent the second. A caldera was formed during each eruptive cycle.
The Carpenter Canyon Tuff is composed mainly of rhyolitic ash flow tuffs with minor amounts of air fall tuffs and tuffaceous sandstones. The Frenchman Canyon Trachyte overlies the Carpenter Canyon Tuff and consists predominantly of trachyte porphyry lava flows and minor thin trachytic ash flow tuffs. The High Mill Tuff overlies the Frenchman Canyon Trachyte and consists of rhyolitic ash flow tuffs. The Eagle Peak Syenite, a syenite porphyry, intrudes the High Mill Tuff.
The rocks of both eruptive cycles show a similar trend of compositional variation. The Carpenter Canyon Tuff and High Mill Tuff are richer in silica (72% average) than the Frenchman Canyon Trachyte and Eagle Peak Syenite (66% average). Rocks intermediate in composition are rare.
Evidence indicates that the Eagle Mountains magmatic rocks represent volcanism from two zoned magma chambers which were filled by the same source.
Caldera subsidence occurred during both periods of eruptive activity but evidence of resurgent doming is not apparent. The excellent exposure of intra-caldera facies is caused by Basin and Range tectonism and differential erosion.
