PALEOPROTEROZOIC DETRITAL ZIRCON PATTERNS DURING CRUST GENERATION WITHIN THE NORTHERN COLORADO POUDRE BASIN AND ACROSS COLORADO: MORE SIMILARITIES THAN DIFFERENCES

During the Paleoproterozoic, the western U.S. experienced a long period of continental crust generation. Tectonic models typically explain this event by the accretion of mostly juvenile arcs and extensional back-arc basins onto the Wyoming Province. The Poudre Basin in the northern Colorado Front Range is one such extensional back-arc basin and it is subdivided by four approximately NE-SW trending shear zones. From north to south, the shear zones are the Skin Gulch, Buckhorn Creek, Moose Mtn., and Idaho Springs-Ralston shear zones. The tectonic blocks between the shear zones from north to south are the Red Feather Lakes, Poudre Canyon, Big Thompson Canyon, Boulder Canyon, and Denver blocks. This work integrates new and published detrital zircon analyses from the northern three block to investigate relationships. Poudre Basin detrital zircons were also compared more regionally to identify broader crustal relationships.

One new sample from the northern Big Thompson Canyon block provides a max depositional age of 1733±24 Ma (²⁰⁷Pb/²⁰⁶Pb weighted mean of 5 analyses). Pooling our data with published data from the three northern blocks reveal that all these blocks have similar max depositional ages ranging from ~1780 to 1740 Ma and that most detrital zircons likely originate from the ~1790-1750 Ma Green Mtn. and Denver arcs that flank the basin to the north and south, respectively, and from the ~1950-1790 Ma southern Trans-Hudson orogen and the Wyoming Province that provides zircons older than 2300 Ma. Minor differences from north to south include a systematic decrease in the proportion of zircons sourced from the Trans-Hudson orogen and the Wyoming Province. These results do not suggest that any of the studied blocks are far travelled relative to one another.

The Poudre Basin samples were collectively compared to similarly aged detrital zircon samples from the Denver Arc and the newly identified Gunnison Block in west-central Colorado. All three areas are strikingly similar both in range of max depositional ages and relative proportion of zircons from the Green Mtn.-Denver arcs (or other similarly aged local arcs), from the Trans-Hudson orogen, and from the Wyoming Province. This result emphasizes the proximity and similarity in origin for these rocks across central Colorado and to northern Colorado shortly prior to accretion.