REEXAMINING THE OROGENIC HISTORY OF THE CENTRAL COLORADO FRONT RANGE: BEDROCK AND SURFICIAL GEOLOGICAL MAPPING OF THE SOUTHERN HALF OF THE GRAYS PEAK 7.5’ QUADRANGLE

A 1:24,000 bedrock and surficial geologic map of the southern half of the 7.5’ Grays Peak quadrangle was created as part of a USGS EDMAP project. The area, straddling the Clear Creek and Summit County boundary, Colorado, contains the continental divide trail and numerous decommissioned 20^th century, Cu-Au-Ag mines and prospects. The main purpose of the project was to investigate the Paleo- and Mesoproterozoic structural history associated with the ~1.8-1.6 Ga. Yavapai/Mazatzal and ~1.4 Ga Picuris orogenies. The main rock types are Paleoproterozoic (and Mesoproterozoic?) migmatitic biotite-sillimanite gneiss, with minor amphibole-biotite gneiss, hornblende gneiss, quartzite, and calc-silicate gneiss. The Mesoproterozoic (1424 ± 6 Ma) Silver Plume granite occupies most of the central and NW regions of the quadrangle. Glacial deposits represented by till, moraines, and rock glaciers of the Bull Lake (170-120 Ka) and Pinedale (30-12 Ka) glaciations define much of the Quaternary influence in the region. Foliations in the basement rock typically dip steeply to moderately NW. Isoclinal folds in gneiss, schist, and migmatitic gneiss have varied hinge orientations, while gentle to tight s- and z-fold hinges generally plunge NE or SW. Locally, in the SW and NW parts of the area, fold hinges plunge moderately NNW and NNE, and in the center, they plunge moderately to shallowly ENE and WNW. Mineral and intersection lineations generally plunge steeply to moderately NE or NW with locally SE plunging mineral alignment near Grizzly Peak. The Silver Plume granite locally displays a weak foliation that is parallel to sub-parallel to the foliation in adjacent units, and otherwise contains flow foliation or is undeformed. The northern part of the ~39.7 Ma Montezuma Stock monzogranite, associated with the Colorado Mineral Belt, is exposed in the SW part of the area. Joint orientations are mostly random; however, subvertical to steeply NE-dipping, moderately SE-dipping and moderately NW-dipping sets can be distinguished. Pegmatitic and are common. The pegmatites crosscut structures in the Proterozoic and Mesoproterozoic host rocks and are undeformed or gently folded. Porphyritic dikes are undeformed.