GEOLOGY OF THE MONTEZUMA 7.5 MINUTE QUADRANGLE, CENTRAL COLORADO FRONT RANGE

A 1:24,000 scale bedrock and surficial geologic map of the Montezuma 7.5-minute quadrangle in Clear Creek, Summit, and Park Counties of the central Colorado Front Range was produced as part of a USGS EDMAP project. The primary goals were to investigate the Paleo- and Mesoproterozoic structural histories associated with the Yavapai/Mazatzal and Picuris orogenies, respectively. High-resolution Lidar data and satellite imagery were used to delineate various geologic features prior to field mapping. The study area is composed of Proterozoic biotite gneiss, biotite-sillimanite schist, quartz-biotite gneiss, hornblende gneiss, and local calc-silicate gneiss and quartzite in a large, heterogeneous sequence of metasedimentary and metavolcanic rocks. Regional correlations and a ~1.75 Ga youngest detrital zircon age population in a previously dated quartzite in the map area suggest that this sequence is probably ~1.8-1.7 Ga. Local migmatite and sillimanite indicate that these rocks reached upper amphibolite facies metamorphism. Based on prior monazite U-Pb analysis, regional metamorphism took place at ~1.68 Ga and ~1.43 Ga. Multiple fold generations and structural domains are apparent within the Paleoproterozoic units. Interpreted F₁ folds are isoclinal with varying orientations. F₂ isoclinal to open folds show hinge lines trending NE in the northern section of the map and NW in the southern section. F₃ folds are tight to open and plunge shallowly to the W and E. Two Mesoproterozoic granites are present in the SE section of the quadrangle. The ~1.42 Ga Silver Plume granite displays a prominent NW-dipping tectonic foliation. A second, coarser-grained porphyritic granite with large potassium feldspar phenocrysts may be correlative with the 1.06 Ga Pikes Peak batholith, or a phase of the ~1.44 Ga Mount Evans batholith. The Montezuma Stock, a ~38.8 Ma monzogranite porphyry, occupies the NW part of the area and occurs as minor intrusions throughout the field area. Within the Montezuma stock, two distinct joint sets dip steeply to the NW and shallowly to the S. Based on preservation of original depositional morphology, pedology, and geomorphic position, two series of glacial till and outwash, correlated with the Bull Lake (170-120 ka) and Pinedale (30-12 ka) glaciations, comprise the most significant surficial deposits.