Proterozoic history of the southern half of the Mount Evans 7.5-minute quadrangle: evidence for a CA. 1.4 Ga orogenic event in the central Front Range, Colorado, The

Abstract

The Proterozoic history of the southeastern margin of Laurentia, especially during the Mesoproterozoic, is not well defined. Several tectonic events occurred during this time. The first two events were the Paleoproterozoic Yavapai (~1.71-1.68 Ga) and Mazatzal (~1.65-1.60 Ga) orogenies. The third is the more recently recognized Mesoproterozoic Picuris orogeny (~1.4 Ga). Evidence for the Picuris orogeny has been found in northern New Mexico, Arizona, and Colorado; however, the extent of the orogen is unclear. In Colorado, previously recognized effects of the Picuris orogeny are primarily reactivations along shear zones. The purpose of this study was to investigate the Proterozoic deformation history in the southern half of the Mt. Evans 7.5-minute quadrangle, in order to test whether pervasive folding is a result of the Mesoproterozoic Picuris orogeny and/or of earlier Paleoproterozoic orogenies. The area was selected, because of the exposure of Proterozoic ductile structures away from localized shear zones and from younger overprinting structures. Field mapping revealed evidence for four deformation events. The first (D1) consists of isoclinal folds. These are overprinted by D2 isoclinal to open folds with northerly plunging fold hinge lines. Poles to F2 axial planes plot along a great circle suggesting a third generation of folds (D3) plunging to the NNE. D4 includes non-pervasive open upright E-trending folds. These folds are only located towards the very north and south of the mapping area. Detrital zircon from one quartzite was selected for U-Pb laser ablation inductively coupled mass spectrometry (LA-ICP-MS) analysis, in order to test whether some of the metasedimentary rocks may be younger than their interpreted Paleoproterozoic age, and perhaps correlative with Mesoproterozoic metasedimentary rocks in northern New Mexico and Arizona that are associated with the Picuris orogeny. The zircon yielded ~1.55 Ga (n=4) and ~1.44 Ga (n=26) age populations and a spread of ages between ~1.81 Ga and ~1.61 Ga. The ~1.55 Ga age population may represent a true 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, or a mixing age between the older and younger populations. It is unclear if all the zircon from the quartzite is detrital, or whether some grew during metamorphism in the quartzite. In general, Th/U ratios for ~1.44 Ga zircon is <0.1, possibly suggesting metamorphic growth, while for ~1.81-1.55 zircon they are both <0.1% and >0.1%, suggesting a detrital origin. However, zircon in both age groups exhibit a variety of textures and shapes. Some ~1.44 Ga zircon grains are euhedral and exhibit oscillatory zoning, some display narrow overgrowths, and others are anhedral with no zoning. The variety of textures and morphologies of ~1.44 Ga zircon suggest that this population is detrital, and that the quartzite was deposited and metamorphosed after. In-situ LA-ICP-MS U-Pb analysis was carried out on monazite from four biotite schist samples to constrain the metamorphic history. The monazite yielded ~1.73 Ga and ~1.42 Ga age populations, and separate populations that show ~1.67-1.48 Ga and ~1.39-1.34 Ga age spreads. The ~1.73 Ga and ~1.67-1.48 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 ~1.39-1.34 Ga populations are most likely metamorphic for two reasons. First, if the biotite schist experienced metamorphism after the Mesoproterozoic there would be evidence for a younger metamorphic event. Second, a monazite inclusion in garnet yielded 1416 ± 85 Ma and 1355 ± 86 Ma ages, indicating that garnet grade metamorphism occurred at or after ~1.4 Ga. Deformation and metamorphism at ~1.42-1.34 Ga is consistent with the age of the Picuris orogeny. Thus, based on the likely <~1.44 Ga deposition of sedimentary rocks in the southern half of the Mt. Evans 7.5-minute quadrangle, and on the ~1.42-1.34 Ga ages of folding and metamorphism it is concluded that the Picuris orogeny caused penetrative deformation and metamorphism in this part of Colorado.