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Reexamining the Proterozoic orogenic history of the Colorado Front Range: the southern half of the Grays Peak 7.5-minute quadrangle
Shirey, J. Mitchell
Shirey, J. Mitchell
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2024
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Abstract
The central Colorado Front Range has been subjected to three Proterozoic orogenic episodes. They are the Paleoproterozoic Yavapai (~1.71–1.68 Ga) and Mazatzal (~1.65–1.60 Ga) orogenies and the recently recognized Mesoproterozoic Picuris orogeny (~1.4 Ga). Evidence for the Picuris orogeny was first discovered in New Mexico, Arizona, and southern Colorado. Recent studies have suggested that the effects of the orogeny are widespread, as far north as central Colorado. However, at the same time, a broad belt of ~1.50–1.34 Ga mostly ferroan granitoid plutonic rocks was emplaced in the southwestern United States and midcontinent, which has been interpreted as intraplate magmatism. The purpose of this project was to investigate the Proterozoic deformation, metamorphic and magmatic history of the southern half of the Grays Peak 7.5-minute quadrangle, in order to determine whether the latest penetrative deformation in the central Colorado Front Range was a product of the Mesoproterozoic Picuris orogeny, and to investigate the spatial and temporal relationship of this deformation with the ferroan Calymmian-Ectasian granitoid belt. Mapping of the southern half of the Grays Peak 7.5-minute quadrangle was conducted at a 1:24000 scale, with a primary focus on Proterozoic bedrock, structures and metamorphic mineral assemblages, but also including brittle structures and surficial deposits.
The primary metamorphic lithologies in the study area are migmatitic gneiss, biotite gneiss, biotite-sillimanite schist, biotite-sillimanite-garnet schist, calcareous quartzite, and quartzite. Lenses of amphibolite are most common in regions of the study area dominated by biotite-sillimanite schist. The Paleo- (to Meso-?)Proterozoic metamorphic basement is intruded by granitic gneiss and a monzogranitic gneiss in the southeastern corner of the half quadrangle. The 1426 ± 6 Ma Silver Plume granite intrudes the metamorphic basement across the northern half of the study area. The ~38 Ma Montezuma Stock monzogranite intruded the basement rock in the southwestern corner of the map. Three generations of structures were recognized. Isoclinal F1 folds that vary in orientation throughout the map area generally affect the least competent map units and are overprinted by NE- to NW-tending, open to tight F2 folds. Open to gentle, W- to WSW-trending F3 folds cause map-scale variations in the orientations of F2 folds.
New whole-rock major and trace element geochemistry of three samples of the 1426 ± 6 Ma Silver Plume granite indicate that the batholith is of ferroan, peraluminous, calc-alkaline composition, which is interpreted as the result of crustal assimilation of mantle differentiates. NE- to SW-striking foliation in the pluton is parallel to metamorphic foliation in the host rock, and crosscuts the contact at high angles, suggesting a NW-directed shortening during or after ~1426 ± 6 Ma crystallization.
U-Pb LA-ICP-MS of zircon from four quartzites in the field area yielded age populations of ~3.3 Ga, ~2.5 Ga, ~1.8 Ga in all samples, and a four-grain population of ~1.4 Ga in one of the samples. The populations of ~1.8 Ga and older are interpreted to be detrital, sourced from metamorphic and igneous rocks throughout Laurentia to the north. D2, and possibly D1 deformation is interpreted to have taken place after ~1.75 Ga based on a maximum depositional age from a previously analyzed quartzite in the Montezuma quadrangle, directly south of the Grays Peak quadrangle. Previously analyzed in-situ monazite from schist of the Montezuma quadrangle yielded U-Pb ages of ~1.68 Ga, suggesting that D2, and likely D1 deformation occurred at ~1.68 Ga. Three of the four quartzites from this study yielded maximum depositional ages of ~1.7 Ga and may be correlated to the quartzite of the Montezuma quadrangle. D2, and possibly D1 deformation in the Grays Peak quadrangle is also interpreted as ~1.68 Ga, and a result of the Yavapai orogeny. The youngest ~1402-1371 Ma zircon population is interpreted to be metamorphic based on morphology, Th/U ratios, ∑REEs, and Ti-in-zircon temperatures. The age of the population is similar to ~1.43–1.42 Ga and ~1.39–1.33 Ga monazite ages from the Montezuma quadrangle and Mount Blue Sky quadrangle to the southeast, respectively, all providing evidence for widespread deformation and metamorphism in the central Colorado Front Range during the Picuris orogeny.
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