NEW AGE CONSTRAINTS ON MIDDLE TO LATE CENOZOIC PLUTONS IN THE WESTERN SAN JUAN MOUNTAINS, SOUTHWESTERN COLORADO: IMPLICATIONS FOR LANDSCAPE EVOLUTION

The distribution and general composition of Cenozoic intrusive rocks in the western San Juan Mountains is documented in previous studies, but the ages of many plutonic masses have been poorly constrained. New U-Pb age data from zircons further define the temporal-spatial trends of post-30 Ma intrusive rocks within the regional geologic record.

Twelve different masses of felsic to intermediate plutonic rock were sampled across the southern edge of the San Juan Mountains from Rico to Pagosa Springs. The ages for most of these rocks were not previously constrained. U-Pb analyses on zircons from these samples document a widespread pulse of plutonic activity at ~26 Ma during the waning stages of regional volcanism. The data also reveal the emplacement of hypabyssal granitic masses at ~15 Ma. A previous U-Pb zircon age reported for a granitic stock in the Needle Mountains indicates that emplacement of these shallow felsic plutons continued until ~10 Ma.

The plutonic events from 26 to 10 Ma in the western San Juan Mountains are broadly coeval with mafic to ultramafic dikes and diatreme complexes exposed in the Navajo volcanic field and on the northern margin of the San Juan Basin. The age data collectively record a long-lived period of plutonic activity from 26 to 5 million involving mantle and crustal melts. The infiltration of mantle magmas into the lithosphere likely contributed to the extended production of crustal magmas.

The landscape evolution in the western San Juan Mountains was once considered a simple two step process involving uplift from 70 to 60 Ma followed by an extended period of erosion. Over the past twenty years, new evidence has supported a more complicated story involving multiple stages of uplift, magmatism, and erosion. The new and existing age constraints on plutonic masses provide supporting evidence that: (1) Cenozoic plutonic activity in the western San Juan Mountains was long lived; and (2) emplacement of Oligocene to Miocene magmas contributed to uplift and mountain building in the area. Many of the post-30 Ma plutonic masses were also important in the formation of hydrothermal systems that gave rise to economic mineral deposits.