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  • The San Juan volcanic field, located in southwestern Colorado, contains two distinct phases of volcanism. The earlier volcanism is Oligocene in age, and consists of largely intermediate composition lavas and breccias. A few ash flow tuffs also constitute this phase of volcanism. Later volcanism is Miocene–Pliocene in age, and is basaltic in composition. The Oligocene volcanism probably was related to subduction along the western margin of North America, and the Miocene-Pliocene volcanism is best interpreted as a partial melt of the lower crust that was erupted onto the surface. Wikipedia