MESOPROTEROZOIC METAMORPHISM OF ROCKS OF THE WET MOUNTAINS, COLORADO

Rocks were added to North America during the Proterozoic eon via multiple orogenic events, including the ~1.8-1.7 Ga Yavapai orogeny, the ~1.7-1.6 Ga Mazatzal orogeny, and the ~1.3-0.9 Ga Grenville orogeny. The documentation of ~1.4 Ga rocks across North America suggests that there was another orogeny in the Mesoproterozoic. These rocks were most likely formed as the result of a convergent tectonic event termed the Picuris orogeny. This orogeny is newly recognized, and still under investigation.

Rocks that are ~1.4 Ga in age crop out in the Wet Mountains of the Colorado Rockies. The study area for this project is in and around the southernmost part of the Wet Mountains. The country rock in the area is migmatite, which is rich in k-feldspar, biotite, and quartz. Amphibolite, schist, and a large granitic pluton, the San Isabel batholith, can also be found in the study area. Fourteen rock samples were collected during field research and cut into thin sections. Three of these samples were studied in detail. The three samples that were analyzed consisted of a calc-silicate, a granite from the San Isabel batholith, and a migmatite. Minerals and textures in the thin section are consistent with the presence of partial melt and high pressure during the time of rock formation. The texture of quartz as well as the presence of sillimanite in the granite sample indicates high temperatures and pressure. The presence of myrmekite, cordierite, and sillimanite in the migmatite sample also indicates both high temperature and high pressure at the time of metamorphism. Evidence for high temperatures and pressures at the time of metamorphism is consistent across the Wet Mountains. There is evidence of high-grade metamorphism in both the country rock and the San Isabel Batholith.