DETRITAL ZIRCON GEOCHRONOLOGY OF KEWEENAWAN INTERFLOW SEDIMENTS WITHIN THE NORTH SHORE VOLCANIC GROUP

During the early stages of the Mesoproterozoic Midcontinent Rift, the North Shore Volcanic Group was deposited around 1100 Ma. This group of volcanic rocks, composed of rhyolite, basalt, and andesitic basalt, are interlaid with detrital sediments whose source zircon ages do not coincide with the age of the rift system. These interflow sediments vary in composition, comprised of quartz arenite, lithic arenite, conglomerate, and conglomeratic sandstone. Collection of samples took place at two locations along the north shore of Lake Superior in Minnesota, USA. Samples were collected from ~10 m thick conglomeratic sandstone at Caribou Creek and ~1 m thick overturned lithic arenite entrained in a xenolith of the Beaver Bay Complex at milepost 61 on Highway 61. Zircon analysis using LA-ICPMS at the University of Arizona Laserchron Center, determine the provenance of both these sandstones. Detrital zircon ages were determined for both Caribou Creek and milepost 61 interflow sandstones.

Milepost 61 sample set (n=102) contains zircons with a maximum deposition age of 1081 Ma in addition to zircon ages ranging from 1073.0-1879.9 Ma. Using an age probability plot, four peak ages are identified to be 1116, 1440, 1688, 1778 Ma. Caribou Creek sample set (n=61) contains zircon ages ranging from 1051.2-3184.1 Ma, with three peak ages of 1109, 1377, and 1730 Ma. Thirteen zircons in this sample set are Archean in age, creating a new revelation about the provenance from which these interflow sediments were sourced. Prior notions that interflow sediments were sourced only from within the rift system cannot be entirely true. New data we collected suggests that some of the interflow sediment was derived from an external source outside of the Midcontinent Rift basin. Zircon ages coincide with Archean terranes to the south, and may also include the Midcontinent Granite-Rhyolite, Mazatzal and Yavapai provinces. Fluxes in high lands from reactivation of faults bounding these provinces may have uplifted these potential source areas.