The ~1.4 Ga Picuris orogeny in the central Colorado Front Range
Yvette Kuiper, Colorado School of Mines
Erick Bora, Colorado School of Mines
Ariel Borsook, Colorado School of Mines
Mitchell Shirey, Colorado School of Mines
James Salazar, Colorado School of Mines
Madeline Ferguson, Colorado School of Mines
Joshua Thomas Zúñiga, Colorado School of Mines
Hunter Ransom, Colorado School of Mines
Abstract
Widespread evidence exists that Proterozoic rocks of the central Front Range, Colorado, have been deformed during the Paleoproterozoic Yavapai (~1.75-1.68 Ga) and Mazatzal (~1.65-1.60 Ga) orogenies. In the past decade, the effects and extent of the ~1.4 Ga Picuris orogeny have become increasingly apparent. The Picuris orogeny is part of the larger ~1.50-1.35 Ga Pinware-Baraboo-Picuris orogen that extends across North America into Quebec and Labrador, Canada. Initially, ~1.4 Ga deformation in Colorado had been recognized only locally, primarily as tectonic foliations in ~1.4 Ga granitoids, and as ~1.4 Ga reactivation along Paleoproterozoic shear zones, while widespread folding had only been recognized in the Wet Mountains in southern Colorado. Past and ongoing mapping, structural analysis, petrography and geochronology in the central Colorado Front Range has yielded increasing evidence for the effects of the Picuris orogeny.
U-Pb detrital zircon laser ablation inductively coupled mass spectrometry (LA-ICP-MS) analysis of a quartzite provided the first evidence for <1.43 Ga sediment deposition in Colorado, about ten km south of Mount Blue Sky. To date, no other Mesoproterozoic metasedimentary rocks have been found in Colorado. Isoclinal F1 folds in Paleoproterozoic rocks are overprinted by predominately NE-, N- and NW-trending F2 folds, folded by shallowly to moderately west-, and locally east-plunging and generally NNW-dipping F3 folds. A moderately NNW-dipping foliation in the ~1.44 Ga Mount Blue Sky batholith, and locally in the ~1.42 Ga Silver Plume granite indicates NNW-directed shortening at or after ~1.44 Ga. Localized, shallowly north-dipping top-to-the-south shear/fault zones with a ten meter-scale spacing indicate south-directed transport. U-Pb LA-ICP-MS zircon data of a syn-tectonic pegmatite indicate that this transport occurred at ~1.4 Ga. Structural, petrographic and U-Pb LA-ICP-MS monazite data show evidence for pervasive ~1.43-1.42 Ga folding (F3, and possibly F2) in parts of the central Colorado Front Range and metamorphism as young as 1.39-1.33 Ga.
The <1.43 Ga quartzite is younger than quartzites in New Mexico and Arizona that were deposited prior to or early during the Picuris orogeny, and the relationships between them is not clear. The 1.39-1.33 Ga monazite dates suggest that the quartzite was buried and metamorphosed late during the Picuris orogeny. While the architecture and evolution of the Picuris orogen is still under investigation, our data show that its effects in the central Colorado Front Range were pervasive and widespread.
Session
Large-scale tectonics