THE FARMINGTON CANYON COMPLEX: IMPLICATIONS FOR THE EVOLUTION OF THE SW LAURENTIAN MARGIN

The Farmington Canyon Complex (FCC) of northeastern Utah is located along the southwestern margin of the Archean Wyoming Province and constitutes the largest exposure of Early Paleoproterozoic crust along the Laurentian margin. It is comprised of quartzofeldspathic gneisses and migmatites intercalated with amphibolites and metasedimentary rocks. Previous work suggested an Archean age for the Farmington Canyon Complex based partly on Archean Sm-Nd model ages. While new whole rock common Pb data confirm the predominantly Archean source of the metasedimentary rocks, some younger (2.4 Ga) detrital zircons were also present indicating an Early Paleoproterozoic depositional age of the protoliths. It has also recently been discovered that zircon grains from an orthogneiss exhibit a U-Pb crystallization age of ~2.44 Ga and an age of metamorphism at ~ 1.7 Ga. These data suggest the Farmington Canyon Complex is a Paleoproterozoic rift zone along the margin of the Wyoming Province that formed on extended Archean crust. The crustal extension produced melting and the formation of igneous rocks onto which sediments were deposited. Detritus entering the rift was predominantly, although not solely, derived from the Archean Wyoming Province. The igneous rocks and metasedimentary protoliths were then deformed together during the ~1.7 Ga metamorphic event. Our new isotopic data show that the initial Archean age estimates for the FCC are problematic and that these rocks record a more complex tectonic history than previously recognized and that a correct interpretation of the history of the FCC is critical for elucidating the accretionary history of SW Laurentia. By constraining the primary depositional and crystallization ages of these rocks in conjunction with their metamorphic and cooling histories, comparisons can be made with the tectonic histories of other Paleoproterozoic rocks along the western margin of Laurentia (e.g., the Great Falls Tectonic Zone). These constraints are necessary to test models of possible Neoproterozoic conjugates, and to establish the piercing points critical to a more accurate reconstruction of Neoproterozoic global paleogeography, and the supercontinent Rodinia.