DETRITAL ZIRCON GEOCHRONOLOGY OF THE LATE PALEOPROTEROZOIC POUDRE BASIN IN THE NORTHERN COLORADO FRONT RANGE: INSIGHTS INTO TECTONIC PROCESSES AND CRUST GENERATION

Large amounts of predominately juvenile continental crust were added to Laurentia during the late Paleoproterozoic as part of the Yavapai and Mazatzal provinces. Arc accretion is an important process that produced and added these provinces to Laurentia. However, there is debate regarding the importance and commonality of extensional backarc basins forming intermittently with arc accretion, a process called tectonic switching. The existence of extensional backarc basins and tectonic switching can be tested with U-Pb analysis of detrital zircon (DZ) as the maximum depositional age (MDA) of an extensional backarc basin must be younger than the first magmatism in the adjacent arcs.

Detrital zircon geochronology has been applied to the late Paleoproterozoic quartz-rich metasedimentary rocks within the Poudre basin of northern Colorado. The Poudre basin is flanked by the Denver arc, found west of Denver, CO, and the Green Mtn. arc, found along the CO-WY border. First magmatism in both arcs is about 1780 Ma. Published DZ data from four samples within the inferred extent of the Poudre Basin provide MDAs of 1776-1739 Ma. We provide laser ablation-inductively coupled plasma-mass spectroscopy data from three additional samples to better constrain timing of deposition within the Poudre basin.

Sample sites range from within the Poudre Canyon to just north of the Big Thompson Canyon in the northern Colorado Front Range. Maximum depositional ages were calculated from the weighted mean of the youngest, concordant, and most overlapping analyses. Analysis locations used to calculate the MDA were assessed via cathodoluminescence to ensure that the dates were of detrital zircon. From north to south, these samples provided MDAs of 1760 +/- 17 Ma (n=11), 1754 +/- 10 Ma (n=8), and 1761 +/- 16 Ma (n=6). Because the Poudre basin was flanked by active arcs during sedimentation, we interpret these MDAs as essentially the timing of sedimentation.

These results further support that Poudre Basin is younger than the flanking arcs and that the crust of the northern Colorado Front Range formed and was accreted to Laurentia by tectonic switching processes. Tectonic switching is additionally compatible with the age and character of magmatism, deformation, and metamorphism within the Poudre basin.