Geochemical and Isotopic Evidence for Paleoproterozoic Recycling of Archean Crust and Mantle Lithosphere along the SW Laurentian Margin

The Farmington Canyon Complex (FCC) of northeastern Utah is located along the southwestern margin of the Archean Wyoming province. U-Pb zircon geochronology of metaplutonic rocks by ion microprobe indicates the FCC is at least in part Early Paleoproterozoic (2.45 Ga) and is, therefore, one of the few exposures of ~2.4 crust in the northern hemisphere. It comprises metaplutonic quartzofeldspathic gneisses and migmatites associated with amphibolites and metasedimentary rocks in an extensive metasupracrustal assemblage. Geochemical and isotopic data indicate the metasedimentary rocks represent recycling of Wyoming Province Archean crust. Zircons found within a metasupracrustal rock, for example, yield U-Pb crystallization ages of ~2.4 Ga and metamorphic ages of ~1.8 Ga. Common Pb isotopic data for these rocks range from ~15.5-17.5 for 207/204 and ~17.6-27.5 for 206/204 and plot coherently well above the Pb evolution curve for average continental crust. The amphibolites within the metasupracrustal sequence are characterized by HFSE depletion, negative Eu anomalies, and Archean (2.9-3.8 Ga) Sm-Nd model ages. These data, in conjunction with Late Archean-Early Paleoproterozoic crystallization ages for the amphibolites (U-Pb zircon), indicate they represent remobilization of Archean mantle lithosphere during the 2.4 Ga rifting event and that this lithosphere imparted its Archean, arc-like signature to the amphibolite protoliths. We interpret these data to indicate that the FCC formed in 4 stages: 1) rifting in latest Archean/earliest Proterozoic time initiated accumulation of an extensive sedimentary sequence derived from Wyoming province crust; 2) continuing extension resulted in melting of ancient sub-lithospheric mantle to produce the protoliths of the amphibolites intercalated within the sedimentary sequence; 3) extension-related heating reached crustal levels and resulted in melting of Archean crust to produce the granitic plutonic rocks; 4) Precambrian evolution of the FCC and much of the SW Laurentian margin concluded with a widespread tectonothermal event at ~1.8 Ga.