TERTIARY MAGMATISM IN THE SILVER CLIFF, ROSITA, AND DEER PEAK VOLCANIC CENTERS, WET MOUNTAINS: COLORADO-GEOCHEMISTRY, GEOCHRONOLOGY, AND STRUCTURAL EVOLUTION

The Tertiary igneous rocks in the Silver Cliff, Rosita, and Deer Peak volcanic centers formed in a large, long-lived and complex area of magmatic activity that existed from the late Eocene to middle Oligocene in the southeastern part of the central Colorado volcanic field. The earliest volcanism (>38 Ma), which began near Silver Cliff, consists of topaz rhyolite emplaced as ash-flows, air-fall tuffs, and lake sediments in a deep 2.5 km by 19 km northwest-trending graben. Concurrently, distal volcano-fluvial deposits (Devil’s Hole Fm) were distributed southward for a minimum of 30 km. Late topaz rhyolite domes, flows, and ash-flow tuffs formed along the northeastern margin of the graben at about 35 Ma.

Alkalic and alkali-calcic, intermediate-composition flows and lahars were then emplaced, between 31 and 34 Ma, along the crest and flanks of the central Wet Mountains and in the evolving adjacent Wet Mountain valley and were subsequently buried under extensive basin fill. The vent for these intermediate-composition volcanic rocks was likely in the Rosita-Deer Peak area. At Rosita, these volcanic rocks were intruded by three small alkalic and alkali-calcic stocks (31-32 Ma); subsidence in a small west-northwest-trending graben in the intermediate-composition volcanic rocks accompanied the latest intrusion. A coeval quartz latite plug was emplaced in similar volcanic rocks near Deer Peak.

Renewed alkali rhyolite volcanism (31 Ma) resulted in an extensive flow dome complex on the west side of the Rosita mafic to intermediate-composition volcano-plutonic complex. A late pulse of alkali rhyolite magmatism (29 Ma), manifested as small plugs along an arcuate fracture system near the southern and eastern margins of the Rosita volcanic center and in a breccia pipe northeast of Deer Peak, is the last phase of this rhyolitic volcanism. Northwest-trending lamprophyre dikes and plugs intruded the Rosita and Silver Cliff volcanic rocks at about 28 Ma.

The entire sequence of volcanic rocks erupted over a 10 Ma interval. These rocks are similar in age and chemistry to felsic granitic rocks at the Climax and Henderson molybdenum deposits, to intermediate-composition alkali-calcic rocks in the Thirtynine mile volcanic field, and to alkali-calcic, felsic ash-flow tuffs from Oligocene calderas in the northeastern San Juan Mountains.