GEOLOGY, March 2008 239 ABSTRACT We have discovered a robust microcrystalline record of the early genesis of North American lithosphere preserved in the U-Pb age and oxygen isotope zoning of zircons from a lower crustal para- gneiss in the Neoarchean Superior province. Detrital igneous zircon cores with δ 18 O values of 5.1‰–7.1‰ record creation of primitive to increasingly evolved crust from 2.85 ± 0.02 Ga to 2.67 ± 0.02 Ga. Sharp chemical unconformity between cores and higher δ 18 O (8.4‰–10.4‰) metamorphic overgrowths as old as 2.66 ± 0.01 Ga dictates a rapid sequence of arc unroofing, burial of detrital zircons in hydrosphere-altered sediment, and transport to lower crust late in upper plate assembly. The period to 2.58 ± 0.01 Ga included ~80 m.y. of high-temperature (~700–650 °C), nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root. Huronian continental rifting is recorded by the youngest zircon tip growth at 2512 ± 8 Ma (~ 600 °C) signaling magma intraplating and the onset of rigid plate behavior. This >150 m.y. microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of U, Pb, and O in zircon throughout protracted regional metamor- phism, and the consequent advances now possible in reconstructing planetary dynamics with zircon zoning. Keywords: zircon, U-Pb, oxygen isotopes, ion probe, lower crust, Kapuskasing, Archean. INTRODUCTION The integration of U-Pb and O isotope data for igneous zircons is a powerful methodology for revealing continental lithosphere–hydrosphere interaction through time (e.g., Valley et al., 1994; Rumble et al., 2002; Valley et al., 2005). For metamorphosed zircon, however, questions remain as to whether primary zircon oxygen isotopic compositions reequilibrate due to rapid, wet (P H 2 O >70 bar) volume diffusion without affecting U-Pb ratios, a scenario suggested by experimental data (Watson and Cherniak, 1997). We have investigated detrital zircons with primary mantle–like oxygen isotope compositions that underwent tens of millions of years of high-grade regional metamorphism and zircon overgrowth in an 18 O/ 16 O-enriched sedimentary matrix in the lower crust of the Neoarchean Superior province. Our aim was to test whether U-Pb age and oxygen iso- topic zoning in such zircons can be primary, and interpretable as a record of thermotectonic processes operating at length scales more than 10 orders of magnitude larger than the crystals; i.e., the genesis and evolution of the ancient core of the North American plate. TECTONIC SETTING The Superior province of the Canadian shield is the largest Archean tectosphere fragment, and peneplanation after the Paleoproterozoic Kapuskasing uplift event has exposed an oblique crustal cross section through its southern subduction-accretion margin (Fig. 1) (Percival and West, 1994). The Abitibi and Wawa subprovinces in the southern Superior province (Fig. 1, inset) were among the last to be accreted (2750–2670 Ma; Corfu and Davis, 1992) during northward subduction in the Kenoran orogeny. At the south end of the 300-km-long Kapuskasing uplift, a more or less continuous metamorphic and structural gradient representing a 15-km-thick section of accreted crust consists of a series of metaplutonic and metasupracrustal belts. The largest and most extensive of the latter is the Borden Lake belt (Fig. 1) (Bursnall et al., 1994), a 5 km × 25 km east-striking synformal structure consisting mostly of multiply deformed mafic gneiss interlayered with narrow, discontinuous units of garnetifer- ous paragneiss (metawacke and metaconglomerate). The Borden Lake belt strikes at a high angle across the amphibolite-granulite transition and has a maximum age of 2667 ± 2 Ma for conglomerate deposition (Krogh, 1993). Here we report zircon data for a sample of metawacke at the high- grade end of the Borden Lake belt (site 32, Fig. 1). Metamorphism The metamorphic ages of zircon in the high-grade end of the Borden Lake belt have not been previously investigated. The age of peak meta- morphism of surrounding gneisses has been estimated as ca. 2.66 Ga based on the age of widespread zircon growth in granulite facies meta- basalts (Krogh and Moser, 1994). Deep crustal metamorphism followed polyphase compressional folding at all levels and was broadly coeval with orogen-parallel ductile flow in the middle and lower crust between 2660 Ma and 2630 Ma. This is a Superior province–wide event that, in the Kapuskasing cross section, appears to young with depth (Krogh, 1993; Moser et al., 1996). Paleopressures gradually increase to the east across the granulite facies domain, reaching maximum values of 1.1 GPa and peak temperatures of ≥850 °C (Mäder et al., 1994; Pattison, 2003). Water activity during metamorphism has been estimated as between 0.1 and 0.5 (Mäder et al., 1994). Granulite facies gneisses are cut locally by meter-scale tonalitic melt pods and granitic pegmatite dikes dated as 2640 ± 2 Ma and 2584 ± 2 Ma, respectively (Krogh, 1993). Geology, March 2008; v. 36; no. 3; p. 239–242; doi: 10.1130/G24416A.1; 3 figures; Data Repository item 2008059. © 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Creation of a continent recorded in zircon zoning Desmond E. Moser Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada John R. Bowman Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA Joseph Wooden U.S. Geological Survey, Menlo Park, California 94305, USA John W. Valley Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA Frank Mazdab U.S. Geological Survey, Menlo Park, California 94305, USA Noriko Kita Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA Figure 1. Inset: Geology of southern Superior province of the Cana- dian shield; Kapuskasing uplift is indicated. Bedrock geology map of the Borden Lake belt in southern Kapuskasing uplift is modified after Percival and West (1994) and shows amphibolite-granulite transition (dashed line), paleopressures, and zircon sample location site 32.