AR-40 AR-39 CHRONOLOGY AND VOLCANOLOGY OF SILICIC VOLCANISM IN THE DAVIS MOUNTAINS, TRANS-PECOS TEXAS/

Field studies and Ar-40/Ar-39 dating reveal that silicic volcanism in the Davis Mountains part of the Trans-Pecos Texas volcanic field occur red in six episodes at 0.3 m.y. intervals between 36.8 and 35.3 Ma. Ad ditionally, 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 hows and by volume. The preponderance of voluminous sil icic 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 vo luminous (possibly >1,000 km(3)) suite of rhyolite and quartz trachyte lavas, several rhyolite domes, and a strongly rheomorphic, peralkalin e ash-flow tuff erupted from a caldera in the northern Davis Mountains . The lava suite extends well beyond the Davis Mountains. Silicic lava s 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 (similar to 200 km(3)). The 36.3 Ma episode c onsisted of rhyolite to trachyte tuffs and lavas erupted from a centra l vent volcano in the southern Davis Mountains. The 35.9 Ma episode co nsisted of a single, moderately large (similar to 50 km(3)) rhyolite l ava and a small-volume ashflow tuff erupted from a caldera in the west ern Davis Mountains. Rocks emplaced during the 35.6 Ma episode were al so rhyolites, including an enigmatic rock that may be strongly rheomor phic ash-flow tuff or a combination of tuff and lava, followed by defi nite lavas. The 35.3 Ma episode consisted of two ash-flow tuffs, one o f which is strongly rheomorphic, and additional voluminous rhyolite la vas (similar to 120 km(3)). The rheomorphic tuff erupted from a calder a in the southwestern Davis Mountains. The source of the other Buff is probably in the western Davis Mountains. Intermediate and mafic rocks are minor, except around the southeastern flank of the Davis Mountain s, where basalt is abundant, Mafic lavas erupted only during gaps in t he silicic activity and on the flanks of the Davis Mountains. Neverthe less, basaltic magma probably drove the silicic magmatism, either by d ifferentiation or by crustal melting, and was present throughout the t ime of Davis Mountains activity but could not penetrate the low-densit y silicic magma chambers until they cooled and solidified. The time re quired for cooling and solidification appears to be 0.1-0.2 m.y.