BEYOND ZIRCON: DATING METAMORPHISM AND DEFORMATION BY U-PB PETROCHRONOLOGY OF MONAZITE, APATITE AND GARNET IN A MEDIUM-GRADE METAMORPHIC SETTING

Quartzites and schists at the well-known Blue Ridge locality north of Canon City CO have been interpreted as the product of Mazatzal age (ca. 1650 Ma) amphibolite-facies metamorphism, after erosion from ca. 1730 Ma granitic basement. The sequence was then crosscut by pegmatites at ca. 1440 Ma, i.e. presumably long after metamorphism and ductile deformation. These interpretations are all based on U-Pb zircon data (e.g. Jones et al. 2009; NICE working group).

The recent discovery of crucial field relationships, including ductile deformation of the pegmatites crosscut by an undeformed qtz-diorites leucosome (zircon-free), reopens the discussion on the timing of metamorphism and deformation. McCoy et al. (2005) for example pointed out the importance of ca. 1400 Ma metamorphism and deformation associated with the Colorado Mineral Belt, and we hypothesize that the Blue Ridge area was metamorphosed and ductilely deformed at this time. A previously unknown mafic dike crosscutting the Blue Ridge sequence was dated by U-Pb (apatite) as approximately age-equivalent to the Pikes Peak intrusive event, and extends the period of magmatic influence in the area.

U-Pb dating of garnet, the small and rare monazite in the micaschists, and the abundant apatite in the qtz-diorite leucosome and mafic dike, combined with petrography and trace element analyses will be used to conclusively bracket the ages of regional metamorphism, deformation and post-tectonic melting. Dating zircon from the metasedimentary rocks did not yield unequivocal evidence for timing of metamorphism due to lack of discrete metamorphic zircon overgrowths, with little dissolution-reprecipitation potential in the micaschists at 600ºC. Zircon alone can not always be relied upon to solve a complex metamorphic, intrusive and tectonic history.