23-2 A Magmatic Mystery: Origins and Volcanic History of deposits along U.S. Route 285 near Saguache, Colorado.

Session: Distributed volcanic fields of the southwestern U.S. (Posters)

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The Oligocene-Pliocene San Juan Volcanic Field (SJVF) is an extensive complex known for its calderas and associated ash flows.  Its origin is linked to the waning stages of compressional tectonism of the Laramide Orogeny followed by slab roll-back and extension that formed the Rio Grande Rift. This study reports on the physical volcanology and geochemistry of an outcrop that boarders these two magmatic provinces with the main objectives being to decipher its formation and magmatic affinity.  The outcrop is located on U.S. Route 285, ca. 4km south of Saguache, Colorado, and consists of three units described from bottom to top; 1) a ca. 13m thick, monomict clast-supported breccia with rhyolitic lava (ca. 15% phenocrysts of K-feldspar and biotite) fragments that range in diameter from <0.1-1.0m overlain by 2) a ca. 3-4m thick tephra with two distinct layers, a lower darker crystal-poor (ca. 3% plagioclase and pyroxene) lithic lapilli ash composed mostly of lithics rich in plagioclase, pyroxene, and amphibole and an upper lighter lithic lapilli tuff comprised of ash-size, crystal-poor (ca. 3% plagioclase and biotite) pumice, overlain by 3) a ca. 12m thick, finely porphyritic dacitic lava with phenocrysts of plagioclase and pyroxene. Units range in composition from trachyandesite to trachydacite to rhyolite and display trends in major and trace element concentrations that indicate an association by differentiation, likely from a single magma source. This along with the sequence of lithologies and absence of depositional breaks suggests that the outcrop is an eroded remnant of a dome complex.  Regionally, the compositions are more potassic and are higher in phosphorus relative to SJVF and they display a positive trend when comparing potassium and silica, opposite of the compositional trends displayed by SJVF magmatism. Trends of increasing potassium with silica as well as high concentration in potassium and phosphorous are observed in other felsic to intermediate early-rift volcanics in northern New Mexico.  Although the deposits exposed in the Route 285 outcrop have been previously mapped as being part of the SJVF (i.e. lower slopes of the Tracy Volcano), our study indicates a closer compositional affinity with volcanism associated with Rio Grande Rift.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4, 2026

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