ARCHEAN-PALEOPROTEROZOIC TRANSITION IN THE NORTHERN UNITED STATES

Geon 17 Paleoproterozoic (PP) crust (Yavapai orogen) is juxtaposed against the Archean (AR) Wyoming craton (Wyoming shear zone) in Colorado whereas geon 18 PP crust (Penokean orogen) is juxtaposed against the AR Superior Province in northern Wisconsin (Niagara Fault Zone). In both cases the transition from AR to PP crust is abrupt, suggesting that the southern margins of the AR cratons formed by rifting about 2.2 to 1.9 Ga. Rifting also produced the Hudsonian sea, which closed with formation of subduction-related juvenile crust of the Trans-Hudson orogen during geon 18, contemporaneous with Penokean orogenesis. The younger Wyoming shear zone truncates the Trans-Hudson orogen at a high angle. Interpretation of the AR-PP boundary between WY-CO and WI-MI is complicated by (a) needing to account for a 100 m.y. difference in age of the PP crust at each end, (b) low-resolution geophysical data for the subsurface of the northern midcontinent region, (c) sparse drill-hole control, and (d) even more sparse geochronologic control in the same region. This problem is being re-examined as a result of new geophysical data for the southern Lake Superior region, availability of additional drill core samples, and new geochronology that can work with much smaller samples. A key link in the boundary interpretation is a sharp aeromagnetic discontinuity known as the Spirit Lake Trend in northwest IA and southeast SD, which marks the southwest corner of the Superior Province. Metagabbro from a new drill hole near Elk Point, SD yields a preliminary U-Pb zircon age of 1.74 Ga, suggesting correlation with Yavapai rocks of Nebraska and WY-CO to the west, instead of correlation with Penokean rocks to the east in WI. This is further supported by a previous age of 1.77 Ga on granite from northern Nebraska. These results suggest that the CO-WY boundary between AR and PP rocks extends eastward from the Wyoming shear zone, truncates the southern end of the Penokean orogen, joins up with the Spirit Lake Trend in IA, and continues eastward across the younger Midcontinent Rift as the Trempealeau aeromagnetic discontinuity in southern WI. Geon 17 rocks north of this boundary, and a north-dipping crustal reflection across it, are consist with intervals of north-directed subduction during Yavapai accretion.