TIMING OF PROTEROZOIC THERMAL EVENTS IN THE POUDRE CANYON, COLORADO FRONT RANGE: CONSTRAINTS FROM MICROPROBE MONAZITE U-TH-PB DATING

We have examined and analyzed a suite of samples of granulite-facies pelitic gneisses from a transect along the canyon of the Cache la Poudre River, northern Colorado Front Range, between Cameron Pass and Fort Collins. Preliminary thermobarometry of these samples indicates P of about 0.5-0.9 GPa and T from 650 to 800 C. Monazites from nine samples have been analyzed for Th, U, Pb and Y and ages calculated. Most of the chemically mapped and analyzed monazites are strongly zoned in Y, Th and U and show multiple age zones. All analyzed monazites were mapped to guide locations of analytical traverses. Analyses were done at 15 kV and 150 nA using 800 s counting on peaks and 400 s on each background. Backgrounds for Pb Ma measurements were linearly modeled. Ages from traverses were separated into areally and chronologically consistent groups using both elemental maps and probability density plots and these were then statistically analyzed using the weighted average function of Isoplot (K. Ludwig, 2003). The dominant age fraction found in all monazites was in the 1720-1770 Ma range; individual samples yielded (2-sigma at 95% confidence in parentheses) 1736 (7), 1768 (7), 1756 (7), 1723 (8), 1760 (9), 1740 (7), 1760 (7), 1721 (9), 1725 (8). Weighted average of the entire suite of data in this age range (>250 analyses) is 1746+/-4 Ma. This age is a bit younger than, but generally consistent with, the proposed Medicine Bow Orogeny in SE Wyoming (Chamberlain, 1998) and the time of accretion of the Colorado province onto the Wyoming province. In addition to this dominant ca. 1.75 Ga age, numerous monazite grains have consistent core zones that yield ages of >1.85-1.9 Ga, suggesting either detrital origin or an earlier, and as yet unknown, thermal event prior to the Colorado-Wyoming convergence. Some smaller monazite grains and rims on larger ones also yield a significant fraction of ages of ca. 1.68 Ga, although our data at this point do not constrain the age of this thermal event with precision of the ca. 1.75 Ga monazite. In addition, there are some monazite rims of ca. 1.4 Ga age that appear to represent some age resetting at the time of intrusion of ca. 1.4 Ga granitic plutons (Berthoud Suite). Monazites thus appear to retain a complex history of thermal events in the northern Front Range.