31-3 Geology of the Northern Half of the Grays Peak 7.5’ Quadrangle, Central Colorado Front Range

Session: Best Student Geologic Map Competition (Posters)

Presenting Author:

Authors:

Abstract:

A 1:24,000-scale bedrock and surficial geologic map of the northern half of the Grays Peak 7.5’ quadrangle (Clear Creek County, central Colorado Front Range) was produced as part of the USGS EDMAP program. The primary objective was to refine the structural history of Paleoproterozoic and Mesoproterozoic rocks within the mapping area, with emphasis on deformation associated with the ~1.8–1.6 Ga Yavapai–Mazatzal and ~1.4 Ga Picuris orogenies. This work builds on previous EDMAP projects led by Kuiper. High-resolution topographic data, LiDAR, satellite imagery, and previous geologic maps were used to guide field mapping and to interpret geologic relationships in inaccessible areas. The dominant lithologies include Paleoproterozoic migmatitic biotite–sillimanite gneiss and schist of sedimentary and volcanic protoliths. These units are intruded by the ~1.4 Ga Silver Plume Batholith, which locally contains abundant xenoliths of the surrounding gneiss and schist. Isoclinal F1 folds within the gneiss and schist are overprinted by prominent NE-trending F2 folds, while faults and slickenlines are randomly oriented. Younger geologic features include porphyritic intrusions potentially associated with the Late Cretaceous–Paleogene Laramide orogeny and/or the Oligocene or younger Rio Grande Rift, along with extensive surficial deposits. The most significant of these deposits include Pinedale glacial till (30–12 ka), younger mass-wasting features, and alluvial–colluvial sediments, representing the youngest deposits in the area. U–Pb LA–ICP–MS zircon analysis yielded a ~1413 Ma crystallization age for a granite, consistent with that of the Silver Plume Granite, and ~45 Ma, ~43 Ma and ~27 Ma ages on three porphyries emplaced in Proterozoic rocks. To evaluate the hypothesis that parts of the area were thrust westward over Phanerozoic units during the Laramide orogeny, inherited zircon was analyzed from the three Paleogene plutons. If Phanerozoic inherited zircon would be present, then Phanerozoic rocks would need to exist below the Proterozoic ones. Only Proterozoic zircons were identified, leaving the hypothesis unresolved. Magmatic zircon from the ~27 Ma porphyry yielded rare earth element (REE) concentrations up to 1.7 wt.%. This unit is located along the northwestern margin of the Colorado Mineral Belt and is associated with the Urad orebody.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.