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Intermediate to mafic intrusive rocks are exposed in the Mount Sneffels stock in the western San Juan Mountains, Colorado. Debate over the timing of pluton emplacement in previous studies impeded an understanding of how this pluton fit into the regional magmatic history. Detailed field studies revealed that the pluton intruded adjacent Oligocene volcanic rocks of the Southern Rocky Mountain volcanic field. However, a previous K/Ar and fission track...