CONSTRAINT ON TIMING AND HISTORY OF LATE MESOZOIC TO CENOZOIC PLUTONS IN THE SAN MIGUEL MOUNTAINS, SOUTHWESTERN COLORADO

The geology and landscape of the western San Juan Mountains was influenced by episodes of magmatism and mountain building in the last 80 million years. The San Miguel Mountains on the western edge of the San Juan Mountains expose numerous intermediate to felsic plutons. Previous mapping revealed the spatial distribution and relative intrusive relationships of different masses, but their ages were not constrained. New U-Pb zircon ages combined with existing age constraints reveal that the San Miguel complex is composed mostly of ~25 Ma intrusive rocks along with lesser intermediate to mafic dikes and sills emplaced at ~68 Ma and ~7 Ma. Some of the ~68 Ma dikes were emplaced along northwest-trending faults, but there is no evidence for structural control on intrusion of the younger plutons.

The ~25 Ma plutons in the San Miguel Mountains are similar in age and composition to a swarm of 26-25 Ma plutonic rocks that extend westward from Silverton to Ophir on the margins of the 29-27 Ma San Juan-Silverton caldera complex. These post-caldera intrusive rocks are spatially related to epithermal precious- and base-metal deposits. Fluids and heat from the magmas were a main driving force for the hydrothermal systems that were established in the area. The timing of mineralization and pluton emplacement in the region, however, are not constrained.

South of the San Miguel Mountains, in the Rico Mountains, there are ~68 Ma and ~4 Ma intermediate to felsic intrusive rocks, but a noticeable lack of ~25 Ma plutons. This argues for some sort of difference in crustal-scale control on magma production over a relatively short distance.

Emplacement of the San Miguel intrusive complex at ~25 Ma caused localized uplift of over several thousand feet in a span of less than one million years. This rapid uplift created a mountainous landscape that influenced the formation of ice caps during glacial events in the past 200,000 years. Glacial erosion shaped the 14,000-foot summits in the San Miguel Mountains, and created the headwaters for the Dolores River, West Dolores River, and San Miguel River.