MONAZITE AND GARNET U-PB DATING REVEALS AMPHIBOLITE FACIES METAMORPHISM IN CENTRAL COLORADO AS MESOPROTEROZOIC

Amphibolite-facies metamorphism of quartzites and schists at Blue Ridge, N of Cañon City, CO has been interpreted to be of Yavapai (1710-1680) or Mazatzal age (1650-1600 Ma), not long after erosion from ca. 1730 Ma granitic basement. This was based on U-Pb zircon data (e.g., Jones et al. 2009; NICE working group). We provide a new look at this interpretation using field relationships and laser ablation ICP-MS U-Pb dates of monazite, garnet and apatite. McCoy et al. (2005) already stressed the importance of ca. 1400 Ma metamorphism and deformation associated with the Precambrian structure of the Colorado Mineral Belt. We propose that the Blue Ridge area experienced its main metamorphism and ductile deformation at this time, during the Picuris orogeny. This has implications for the interpretation of which major orogenic event affected the Precambrian sequences in the Rocky Mountains and how to approach dating them.

The field evidence includes ductile deformation of cross-cutting pegmatites, in turn crosscut by an undeformed qtz-diorite leucosome. U-Pb dating of monazite in the micaschists, quartzites as well as the adjacent granitic gneisses yields dates of 1420-1400 Ma, consistent with garnet U-Pb dates. U-Pb dating of apatite in the post-tectonic qtz-diorite leucosome and a newly discovered mafic dike crosscutting the Blue Ridge sequence yields Pikes Peak intrusive event, and further extends the period of magmatic influence in the area.

We conclude that dating zircon in mid-amphibolite facies metasedimentary rocks does not always yield unequivocal evidence for timing of metamorphism. Zircon may lack discrete metamorphic overgrowths due to little dissolution-reprecipitation potential in the micaschists at 600ºC. Zircon alone cannot solve all complex metamorphic and tectonic histories.