THE EARLY PROTEROZOIC POUDRE BASIN, AN IMPORTANT CONSTRAINT OF 1.77—1.73-GA TECTONIC EVENTS IN NORTHERN COLORADO

The ensimatic Proterozoic Poudre basin extends north from the latitude of Denver to the Wyoming state line and occupies the width of the northern Front Range of Colorado. The significance of this basin has been largely unrecognized. We utilized SHRIMP dating of plutonic rocks that form the basin basement and detrital zircons from the basin fill, and geochemistry of the voluminous metasedimentary and minor mafic metavolcanic rocks to model the basin. The basin basement at its northern and southern margins consists of tectonized 1.77-Ga sodic granite, which intruded a mostly pelitic protolith. Calcic tholeiite flows overlie the basement and are succeeded upward by psammite and wacke protoliths in the north whereas calc-alkalic basalts are overlain by wacke and pelite in the south. A large, east-striking, mafic dike swarm cuts the basement granites at the northern end of the basin, near Poudre Canyon and appears to have locally fed the overlying basalts.

The basin rocks were regionally metamorphosed and deformed along east-striking folds. Many of the beds were overturned to the south prior to 1.72 Ga, when the trondhjemite of Palisade Mountain intruded the greenschist-grade schist as sills and plugs. Metamorphic grade of the country rock was increased at the contact with the plutonic rocks to amphibolite-grade hornfels. Farther south, folds in the basin rocks are cut by the 1.72-Ga Ma Boulder Creek pluton.

Calcic tholeiite flows throughout the basin, such as those at Buckhorn Creek, have minor-element ratios consistent with formation in an oceanic spreading center. Detrital zircons from wacke and psammite have restricted concordant U-Pb ages of 1.75-1.76 Ga, and morphologies and U concentrations that are similar to zircon in the immediately underlying sodic granites.

The basin basement rocks west of Denver are interpreted to have been rifted from the Green Mountain arc near the Wyoming border soon after their formation above a steeply north-dipping subduction zone at 1.77 Ga. Distinctive rock units in the south, such as the Swandyke metagabbro-granite dated as 1.77 Ga, are identical to the coeval Pratt Creek metagabbro-granite near the Wyoming border. The western border of the basin appears to be defined by large crustal blocks of north-striking mafic rocks and metasedimentary rocks and may be, in part, a transform structure.