NEW EVIDENCE FOR ~1.62 GA AND ~1.43 GA DEFORMATION AND METAMORPHISM IN PROTEROZOIC METASEDIMENTARY ROCKS IN THE MOUNT EVANS AREA, CENTRAL COLORADO FRONT RANGE

Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb analyses were conducted on zircon in one quartzite, and monazite in four schist samples in the southern half of the Mount Evans 7.5’ quadrangle of the central Colorado Front Range to help constrain the ages of deposition, deformation and metamorphism. Data can be interpreted in the context of the widespread ~1.8-1.6 Ga Yavapai-Mazatzal orogenies, and the less well known ~1.4 Ga Picuris orogeny. The Yavapai-Mazatzal orogenies involved accretion of primarily juvenile arc rocks. The Picuris orogeny is a ~1.4 Ga deformation and metamorphic event that has become increasingly well recognized in the past decade, especially in New Mexico, central Arizona and southern Colorado. It was preceded by deposition of ~1.49-1.45 Ga sedimentary rocks in New Mexico and Arizona. In the central Colorado Front Range, Paleoproterozoic shear zones were reactivated and some folding occurred at ~1.4 Ga, but no evidence for Mesoproterozoic sedimentary rocks has been found to date.

Zircon in the quartzite sample yielded ~1.70 Ga and ~1.43 Ga age populations. Cathodoluminescence images showing concentric zoning suggest igneous origins within both populations. Th/U ratios are <1.3 in the ~1.70 Ga population and <0.1 in the ~1.43 Ga population. Monazite along foliation planes from four refolded biotite schist samples yielded ~1.62 Ga and ~1.44 Ga ages. The data are somewhat enigmatic. The quartzite may be post-~1.7 Ga and pre-~1.43 Ga, and the ~1.43 Ga zircon may have formed during a metamorphic event that is also recorded by the monazite ages. Alternatively, the ~1.43 Ga zircon may be detrital. If true, then the quartzite was deposited at the onset of ~1.44-1.43 Ga metamorphism and deformation, and immediately after intrusion of the ~1.44 Ga Mount Evans batholith in the area. Regardless of the age of the quartzite, deformation and metamorphism occurred in the central Colorado Front Range at ~1.44-1.43 Ga, and is consistent with the Picuris orogenic event.