THE MESOPROTEROZOIC PICURIS OROGENY IN THE CENTRAL COLORADO FRONT RANGE

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. The effects and extent of the Mesoproterozoic Picuris orogeny in Colorado are less well known. It is part of the larger ~1.50-1.35 Ga Pinware-Baraboo-Picuris orogen that extends across North America into Quebec and Labrador, Canada. In Colorado, ~1.4 Ga deformation has previously been recognized only locally, primarily as tectonic foliations in ~1.4 Ga granitoids, and as ~1.4 Ga reactivation along Paleoproterozoic shear zones. Widespread folding at that time has only been recognized in the Wet Mountains in southern Colorado. New structural, petrographic and U-Pb laser ablation inductively coupled mass spectrometry (LA-ICP-MS) monazite data show evidence for pervasive ~1.43-1.42 Ga folding in parts of the central Colorado Front Range and metamorphism as young as 1.39-1.33 Ga. A ~1.36 Ga monazite is included in garnet. The orientations of the folds vary from NE-trending along the NE-trending Idaho Springs-Ralston shear zone NE of Idaho Springs, north-trending south of Mount Evans, and west-trending in the Montezuma area. A moderately NW-dipping foliation in the ~1.44 Ga Mount Evans batholith indicates NW-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, especially southeast of the Idaho Springs-Ralston shear zone. New U-Pb LA-ICP-MS zircon data of a syn-tectonic pegmatite indicate that this transport occurred at ~1.4 Ga.

New U-Pb detrital zircon LA-ICP-MS results for a quartzite provide the first evidence for <1.43 Ga sediment deposition in Colorado. This quartzite is probably younger than Mesoproterozoic 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 remains enigmatic, the data presented here show that its effects were more widespread in the central Colorado Front Range than previously recognized.