GEOLOGIC MAP OF THE MOUNT EVANS 7.5 MINUTE QUADRANGLE, CENTRAL COLORADO FRONT RANGE

The purpose of this project was to produce a 1:24,000 scale geologic map of the Mount Evans 7.5 minute quadrangle of Clear Creek and Park Counties, Colorado. The oldest units are a hornblende-plagioclase gneiss with amphibolite and biotite schist, a metasedimentary package composed of interbedded quartzite, calc-silicate gneiss, felsic gneiss, amphibolite, sillimanite schist and a quartz monzonite gneiss. The presence of sillimanite in the schist and extensive migmatite in both the schist and hornblende-plagioclase gneiss indicate upper amphibolite facies metamorphism. Based on regional correlations, the ages of these units are most likely ~1.8 Ga to ~1.7 Ga. Four structural domains can be recognized. Domain 1 lies in the north and south parts of the map area and displays isoclinal folds overprinted by open to isoclinal, upright to inclined folds plunging approximately 35/355. Domain 2 is located in the northwest corner of the map with an open, upright, km-scale fold with hinge line plunging 06/279. Domain 3 is located approximately 1km to the south of Domain 2, displaying open, upright folds plunging 55/225. Domain 4, defined by vertical, tight to isoclinal folds plunging 80/205 is observed on the western margin of the mapping area, 1km south of Domain 3. The 1,442 ± 2 Ma Mount Evans batholith comprises most of the northern half of the quadrangle. Its composition varies between granodiorite and monzogranite, and can be divided into three phases. The orientations of the foliation in the granodiorite are consistent with the regional schistosity and is interpreted to be at least in part a tectonic foliation, and possibly in part a flow foliation. Boudinaged pegmatite in the batholith shows top-to-the-south movement, also indicating minor ductile deformation. Brittle structures are 000-060-trending strike-slip faults with 10-250m of sinistral offset, most likely Cretaceous or younger in age based on the regional tectonic history. The southern half of the quadrangle exhibits well-defined glacial deposits associated with Bull Lake (170-120ka) and Pinedale (30-12ka) glaciations. These deposits and associated landforms are mapped and differentiated based on preservation of original depositional morphology, geomorphic and topographic position, deposit weathering and pedogenic properties.