Evidence for the ∼ 1.4 Ga Picuris orogeny in the central Colorado Front Range

Abstract

We present the first evidence for sedimentation and new evidence for penetrative deformation and metamorphism in the central Colorado Front Range associated with the ∼ 1.48–1.35 Ga Picuris orogeny. This orogeny has recently been recognized in New Mexico, Arizona and southern Colorado and may be part of a larger active accretionary margin that includes the ∼ 1.51–1.46 Ga Pinware and Baraboo events, in eastern Canada and central US respectively, that preceded the amalgamation of the Rodinian supercontinent. We demonstrate that in addition to ∼ 1.4 Ga reactivation of northeast-trending Paleoproterozoic shear zones, regional folding occurred in an area south of Mt. Evans, away from these shear zones.Detrital zircon from one quartzite yielded U–Pb laser ablation inductively coupled mass spectrometry (LA-ICPMS) major age populations of ∼ 1.81–1.61 Ga and ∼ 1.49–1.38 Ga, and minor ones of ∼ 1.90 Ga and ∼ 1.56 Ga. The Paleoproterozoic and ∼ 1.49–1.38 Ga populations have numerous local and regional sources. The ∼ 1.56 Ga age population may represent a minor exotic population as recognized in Defiance, Arizona the Yankee Joe and Blackjack Formations in Arizona, the Four Peaks area in Arizona, and the Tusas and Picuris Mountains in New Mexico. Alternatively it may be a result of mixing between zircon age domains reflecting the older and younger populations, or Pb loss from 1.81 to 1.61 Ga zircon.In-situ LA-ICPMS U–Pb analysis on monazite from four biotite schist samples yielded ∼ 1.74 Ga and ∼ 1.42 Ga age populations, and separate populations that show ∼ 1.68–1.47 Ga and ∼ 1.39–1.33 Ga age spreads. The ∼ 1.74 Ga and ∼ 1.68–1.47 Ga populations may be detrital or metamorphic. Monazite ages between ∼ 1.6 Ga and ∼ 1.5 Ga may be due to the mixing of age domains or Pb loss, because metamorphism during that time has not been recognized in Laurentia. The ∼ 1.42 Ga and ∼ 1.39–1.33 Ga populations are most likely metamorphic and consistent with the age of the ∼ 1.48–1.35 Ga Picuris orogeny. The evidence for ∼ 1.4 Ga sedimentation, and especially regional folding and metamorphism in the central Colorado Front Range indicate that the impact and extent of the Picuris orogeny in the southwestern U.S. are larger than previously thought.