ADDING DETAIL TO A GEOLOGIC MAP USING GEOCHRONOLOGY AND METAMORPHIC MODELING TOOLS

Reconnaissance field maps provide an invaluable foundation for research, but these geologic maps are necessarily limited by the constraints of field investigation, including the ability to visually distinguish between map units with distinct geologic histories. Rocks with complex geologic histories and rocks from the mid to lower crust commonly contain fewer visual markers to serve as a basis for interpretation. What do we do in field areas where the rocks “all look the same?” Here we present an application of geochronology, supported by isochemical phase diagram modeling, trace element zoning, and diffusion modeling, to construct a more detailed geologic map of the Proterozoic rocks of the Wet Mountains, Colorado.

The Wet Mountains expose a set of Proterozoic rocks including gneiss, migmatite, and plutonic rocks, with smaller volumes of schist and amphibolite. Field exposures of gneiss and migmatite exhibit mineralogical differences, such as the presence or absence of garnet, muscovite, or sillimanite, but have not been systematically mapped based on these differences, some of which are only visible in thin section.

In this study, we focus on three garnet-biotite gneisses from the central and southern Wet Mountains. Sample 12W13 contains epidote, sample OC14 contains hercynite, and sample 12W6 lacks sillimanite or cordierite. Lu-Hf garnet analyses yield ages from these samples at 1497 ± 4 Ma, 1601 ± 6 Ma, and 1476 ± 4 Ma, respectively. We measured trace element profiles of garnet in thin section and constructed isochemical phase diagrams for each sample. Based on minerals, textures, trace element profiles, and inferred reaction histories, we interpret these ages to represent garnet growth during partial melting. These samples share similar mineral makeup and texture, but grew garnet during episodes of partial melting that are separated by more than 100 My. These episodes of partial melting likely represent two distinct tectonic events, the Mazatzal and Picuris orogenies.

Age and petrologic data allow us to distinguish between samples that exhibit similar outcrop appearances, which in turn allows us to separate rocks that have been previously mapped as one rock unit. Rocks from the mid to lower crust that appear homogeneous may be divided based on criteria that show contrasting tectonic histories.