INSIGHTS INTO THE TECTONIC DEVELOPMENT OF THE WET MOUNTAINS, SOUTHERN COLORADO: INTERPRETING PROTOLITHS AND TEXTURES.

Accretion of island arcs during the Yavapai, Mazatzal, and Picuris orogenies in the Proterozoic contributed to building the southern margin of Laurentia. The Wet Mountains, in southern Colorado developed through this complex history of regional metamorphism, tectonism and magmatism. Bimodal (mafic/felsic) meta-volcanic sequences have been described from the northern Wet Mountains, with several authors suggesting the lack of meta-andesites rules out active subduction. This project evaluates amphibolites and amphibole biotite gneisses in the southern part of the Wet Mountains for insights into understanding the regional tectonic setting.

This study performed a detailed petrographic and textural analysis of amphibolites and amphibole biotite gneisses from the southern Wet Mountains. The amphibolites are typically well foliated and composed of hornblende and plagioclase with minor biotite and quartz, representing metabasites. The amphibolite biotite gneisses are strongly foliated, with well-developed quartz-rich domains and biotite-, amphibole- and plagioclase-rich domains. As the amphibolite and biotite are in apparent equilibrium, the mineralogy indicates an intermediate igneous protolith. All samples indicate deformation with undulose extinction and deformation twinning. One amphibole biotite gneiss sample contains a vein, which in 2D, is non-continuous, partially cutting some grains and fully cutting others. It is mostly infilled with quartz and biotite and where it contacts amphibole, these grains are partially or wholly replaced by biotite. All together, these data provide evidence for active subduction during formation of the protoliths, likely ~1.7 Ga, followed by burial and peak metamorphism during the Picuris Orogeny (~1.4 Ga). The textures indicate that peak deformation postdated peak metamorphic conditions. The Wet Mountains provide insights into the complex tectonic history during the development of Laurentia.