The Van Horn Mountains Caldera, Trans-Pecos Texas: Geology and Development of a Small (10-km2) Ash-Flow Caldera

The Van Horn Mountains caldera is a small (~10-km2) igneous center in the Trans-Pecos volcanic province. The caldera formed 37 to 38 mya during eruption of the first of two ash-flow tuffs related to the caldera. Part of the first tuff ponded within the caldera; the lower marker horizon of the Chambers Tuff in the Sierra Vieja south of the Van Horn Mountains is probably the correlative outflow tuff. Following collapse, the caldera was partly filled by a heterogeneous assemblage of air-fall tuff, tuffaceous sediment, and possible ash-flow tuff. Contemporaneous with this filling, a rhyolite porphyry was emplaced in the middle of the caldera. The porphyry is comparable to the resurgent dome of other calderas but does not uplift the intruded rocks. An intrusive-extrusive complex of basalt, trachyte, and minor rhyolite was emplaced just outside the eastern margin of the caldera; flows from this complex spilled over the rim and into the caldera. Eruption of a second major ash-flow tuff, herein named the High Lonesome Tuff, may have led to additional collapse.Collapse occurred dominantly along a single, major, nearly circular fracture zone. Embayments in the fracture zone were influenced by precaldera, largely Laramide structures. Large, commonly brecciated blocks of wall rocks slumped into the caldera and were incorporated into caldera fill. Precaldera topography channeled most of the ash-flow tuff to the south, through a low area in the caldera wall.The 10-km2 area of the caldera and the no more than 10-km total volume of erupted ash-flow tuff make it one of the smallest ash-flow calderas worldwide and more comparable to collapse features associated with stratovolcanoes. Nevertheless, it shows all the features typical of much larger volcanic centers.No known ore deposits are associated with the caldera, but the rhyolite porphyry is hydrothermally altered, exhibiting silicification and sericitization typical of many porphyry molybdenum deposits. The apparent lack of deposits may be a function of the small size of the magmatic system, lack of sufficient trace element enrichment by differentiation, or lack of exposure resulting from minimal erosion.