METAMORPHIC RECORD OF YAVAPAI AND MAZATZAL ACCRETION IN THE UPPER GREAT LAKES REGION, U.S. AND CANADA

The Penokean orogenic crust (1880-1830 Ma) experienced metamorphism related to subsequent Yavapai (1800-1750 Ma) and Mazatzal (ca. 1650 Ma) accretionary episodes which impinged on the Great Lakes region as Laurentia grew southward. Yavapai amphibolite-facies metamorphism and associated magmatism is recorded along the regions of greatest Penokean crustal thickening: the tectonically buried Archean-Proterozoic continental margin. To the south, Penokean-interval crust in northern Wisconsin was metamorphosed to upper greenschist facies. To the east, in the northern Lake Huron region, Yavapai metamorphism was in the dominantly upper greenschist-facies realm, with amphibolite-facies conditions having occurred only adjacent to associated geon 17 intrusions. Metamorphic effects of Mazatzal accretion are absent in the northern Lake Huron region and are restricted largely to the region south of the main Penokean suture in the Lake Superior region. Mazatzal metamorphism reached upper greenschist-facies conditions throughout most of Wisconsin and amphibolite-facies throughout Iowa and southernmost Wisconsin (Waterloo quartzite). Strikingly, published K/Ar ages from the Archean basement of the Minnesota River Valley promontory in southern Minnesota cluster around 2200-2400 Ma. The sharp contrast in thermochronometric ages across the Archean-Proterozoic boundary there (Spirit Lake discontinuity) suggests that the Archean crust of the promontory was a persistent stronghold in the region that remained unaffected by subsequent accretionary effects largely since its formation. The regional pre-geon 14 metamorphic facies record and age pattern variation broadly mimics that documented for the western United States and supports a tectonic history of protracted convergence associated with episodic accretionary events. As a final caveat, we recognize that this depiction of only two significant post-Penokean accretionary events is almost certainly a vast simplification for a large region which likely experienced a rich and very complex geologic history. However, given the limited crustal exposure in the midcontinent (compared to the western U.S.), such a first-order, broad-brush approach may be most appropriate, and perhaps even the best we can resolve.