Abstract

Field studies and 40Ar/39Ar dating reveal that silicic volcanism in the Davis Mountains part of the Trans-Pecos Texas volcanic field occurred in six episodes at 0.3 m.y. intervals between 36.8 and 35.3 Ma. Additionally, two groups of silicic intrusions were emplaced at 34.6 and 32.8 Ma. This episodicity is similar to that determined for volcanic fields dominated by ash-flow tuffs, yet voluminous, extensive silicic lavas are considerably more abundant than tuffs in the Davis Mountains, by number of flows and by volume. The preponderance of voluminous silicic lavas over tuffs most likely reflects low water contents and high temperatures of the alkalic and commonly peralkaline Davis Mountains magmas.

The earliest episode, at 36.8 Ma, included a widespread and voluminous (possibly >1,000 km3) suite of rhyolite and quartz trachyte lavas, several rhyolite domes, and a strongly rheomorphic, peralkaline ash-flow tuff erupted from a caldera in the northern Davis Mountains. The lava suite extends well beyond the Davis Mountains. Silicic lavas of all episodes probably erupted from widespread, fissure vents. The 36.5 Ma episode consisted of rhyolite to quartz trachyte lavas, also extensive and voluminous (∼200 km3). The 36.3 Ma episode consisted of rhyolite to trachyte tuffs and lavas erupted from a central vent volcano in the southern Davis Mountains. The 35.9 Ma episode consisted of a single, moderately large (∼50 km3) rhyolite lava and a small-volume ash-flow tuff erupted from a caldera in the western Davis Mountains. Rocks emplaced during the 35.6 Ma episode were also rhyolites, including an enigmatic rock that may be strongly rheomorphic ash-flow tuff or a combination of tuff and lava, followed by definite lavas. The 35.3 Ma episode consisted of two ash-flow tuffs, one of which is strongly rheomorphic, and additional voluminous rhyolite lavas (∼120 km3). The rheomorphic tuff erupted from a caldera in the southwestern Davis Mountains. The source of the other tuff is probably in the western Davis Mountains.

Intermediate and mafic rocks are minor, except around the southeastern flank of the Davis Mountains, where basalt is abundant. Mafic lavas erupted only during gaps in the silicic activity and on the flanks of the Davis Mountains. Nevertheless, basaltic magma probably drove the silicic magmatism, either by differentiation or by crustal melting, and was present throughout the time of Davis Mountains activity but could not penetrate the low-density silicic magma chambers until they cooled and solidified. The time required for cooling and solidification appears to be 0.1-0.2 m.y.

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