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Bedrock and surficial geology of the southern half of the Montezuma 7.5' quadrangle, central Colorado Front Range, Colorado, USA
Bora, Erick T.
Bora, Erick T.
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2024
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Abstract
The purpose of this study was to create a 1:24,000 scale geologic map of the southern half of the Montezuma 7.5’ quadrangle in the central Colorado Front Range, and to investigate the Paleo- and Mesoproterozoic history of the area. The Paleoproterozoic geologic history in the southwestern United States is characterized by the Yavapai (~1.71-1.68 Ga) and Mazatzal (~1.65-1.60 Ga) orogenies, which both involved terrane accretion along the southeastern margin of Laurentia. Within the last two decades, evidence for a Mesoproterozoic orogeny, the ~1.48 -1.35 Ga orogeny has been recognized in northern New Mexico and Arizona, and more recently in Colorado. Mesoproterozoic deformation and metamorphism based on metamorphic monazite growth between ~1.43 Ga and ~1.42 Ga has previously been recognized in some areas of the Montezuma quadrangle and the Mount Blue Sky quadrangle east of it. Bedrock geologic mapping, structural analysis, and U-Pb LA-ICP-MS zircon geochronology was used to interpret the structural history of the southern half of the Montezuma quadrangle further. In addition, younger brittle structures and Quaternary surficial deposits in the area were mapped and analyzed.
The main metamorphic lithologies in the study area are Paleoproterozoic biotite-sillimanite gneiss/schist, biotite-quartz gneiss/schist, and hornblende gneiss. These are intruded by Mesoproterozoic granitoid plutonic rocks and local satellite intrusions related to the ~39.7 Ga Montezuma stock. The Paleoproterozoic rocks are deformed by isoclinal F1 folds in various orientations. These are overprinted by D2 structures, which include tight to open NW-trending folds that are best observed in the western half of the field area. West-trending tight to open F3 folds are best recognized in the eastern half of the field area. F3 folds are consistent with W- and E-trending folds in the Mount Blue Sky quadrangle, the Wet Mountains in southern Colorado, and the Picuris Mountains in northern New Mexico.
D2, and possibly D1 structures are interpreted as younger than ~1.75 Ga based on the maximum depositional age of a previously dated quartzite in the Montezuma quadrangle. More specifically, D2 deformation is interpreted as ~1.68 Ga based on previously analyzed in situ metamorphic monazite grains. U-Pb zircon of a granitic dike deformed by an F2 fold in the southern half of the Montezuma quadrangle, analyzed in this study, yielded an imprecise Paleoproterozoic age, but is generally within uncertainty of the ~1.68 Ga monazite ages. These ages are also consistent with the age of D1 deformation near the Idaho Springs-Ralston Shear Zone, ~40 km north of the Montezuma quadrangle, and D2 deformation in the Mount Blue Sky quadrangle. D3 deformation is interpreted as ~1.43-1.42 Ga, based on previously analyzed in situ metamorphic monazite grains aligned with F3 fold axial planes in the Montezuma quadrangle.
U-Pb LA-ICP-MS zircon analysis of a granitoid in the southeast corner of the Montezuma quadrangle indicates that it is a phase of the magnesian ~1.443 Ga Mount Blue Sky batholith, and not part of the ~1.1 Ga Pikes Peak batholith or the ~1.7 Ga Routt plutonic suite, as previously interpreted. NW-dipping tectonic foliations in the batholith suggest that NW-directed shortening occurred during or after ~1.443 Ga, probably as a result of the Picuris orogeny, because no ductile deformation is known to have occurred in the central Colorado Front Range after that time. The Montezuma stock yielded a ~39.8 Ma crystallization age with Proterozoic inherited zircons. Glacial deposits of the Pinedale glaciation are present throughout the southern half of the Montezuma quadrangle. Late middle Pleistocene Bull Lake-age glacial signatures are preserved on higher interfluve ridges as bedrock-etched trimlines and glaciofluvial benches, but deposits have been mainly reworked and deposited into lower topographic positions by late Pleistocene glaciation as Pinedale till.
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