THERMAL AND EXHUMATION HISTORY OF PROTEROZOIC BASEMENT AND OLIGOCENE PLUTONIC ROCKS, SOUTHWESTERN COLORADO

The Proterozoic basement of southwestern Colorado records a complicated low temperature thermal history related to Laramide exhumation, mid-Cenozoic heating, and Miocene cooling. Apatite fission-track (AFT) ages decrease from ~180 Ma (from an exhumed AFT PAZ) to 54-67 Ma to 40 Ma along an east to west traverse through Proterozoic rocks along the Gunnison River valley at the northern end of the study region. The young age on the west end of the traverse appears to have been partially reset by elevated heat flow and burial associated with the Eocene to Oligocene San Juan volcanic field (SJVF). To the south, AFT ages for Proterozoic rocks in the Needle Mountains are 8 to 14 Ma in Chicago Basin and 13 to 30 Ma in the Animas River valley; the ages correlate well with elevation. The AFT ages in the Proterozoic basement of the Needle Mountains were totally reset by SJVF activity and record primarily middle Miocene exhumation of the Animas River region. AFT and (U-Th)/He ages of Oligocene plutons across the SJVF reveal an interesting pattern of post-emplacement cooling. AFT and (U-Th)/He ages in the SE SJVF are similar, indicating relatively rapid cooling of shallowly-emplaced plutons. In contrast, the plutons in the NW SJVF have a more protracted cooling history. For example, the 26.6 Ma Sultan Mountain Stock near Silverton cooled below 110°Cat ~23 Ma and below 70°C at ~10 Ma. The Mount Sneffels pluton in the San Miguel drainage cooled below 70°C 5 to 9 Ma. Enhanced Pliocene exhumation in the western SJVF is associated with tributaries of the Colorado River system, while tributaries of the Rio Grande have only locally dissected the SE SJVF plateau. The Rio Grande of Colorado, blocked by Pliocene basalt eruptions in the Taos volcanic field, integrated with the larger Rio Grande drainage system ~440 ka and thus was ineffective in exhuming the eastern SJVF during Pliocene time.