CRATONS, KEELS, AND EARLY CRUSTAL GROWTH IN NORTH AMERICA

North America, like many modern continents, was assembled around a core of Archean cratons. At least in part, these core cratons served as nuclei for continent formation because of their extensive keels, which appear to have played a significant role in limiting the reworking of this continental crust by later orogenies. The origin of these crust-keel structures remains a matter of debate, generally between those favoring an origin in an Archean plume-like system and those favoring a subduction regime. In the northern Wyoming craton, rock ages to 3.5 Ga and detrital zircon ages to 4.0 Ga provide an opportunity to track the evolution of one of North America’s core cratons from its inception to the time of cratonic stabilization. Insight into the earliest phase of crustal genesis comes from the combination of U-Pb ages (SHRIMP-RG) and Lu-Hf systematics (LA-ICP-MS) measured in 3.5-4.0 Ga old detrital zircons from high-grade metasedimentary rocks. Extensive trace element analyses in conjunction with concordance in the U-Pb system suggest that reliable initial εHf can be determined from these zircons. The resulting pattern of initial εHf values over this period has two important features: 1) there is a continuum of ages without pronounced age-peaks as typically seen in younger detrital suites and 2) the lowest initial values conform to a pattern of continually decreasing values indicative of evolution in an environment characterized by a crustal Lu/Hf ratio, which suggests extensive recycling of that crust. This pattern ended at ~3.5 Ga and subsequent crustal growth was episodic, with major episodes at ~3.3 and 2.8 Ga that incorporated much higher proportions of juvenile material. Although these younger episodes had higher contributions from the depleted mantle, they also contained distinctive isotopic signatures derived from the oldest crust, particularly for Pb. Integrating these observations suggests a two-stage model for crust-keel formation that began with the generation of the earliest crust over a mantle upwelling (anhydrous melting) followed by subduction-based crustal growth (hydrous melting). Cratonization and keel formation is tied to the sequential interplay of these processes and helps explain the lack of craton-keel structures associated with Proterozoic and younger episodes of crustal genesis.