WHERE DID THE TIME GO? USING ZIRCON (U-TH)/HE DATING TO UNDERSTAND THE GREAT UNCONFORMITY SURFACE IN THE MINNESOTA RIVER VALLEY

The Great Unconformity is an erosive surface that represents over a billion years of lost time, however the origins of this surface are poorly understood. This surface is partially exposed in the cratonic interior of the upper Midwest, and more specifically in the Minnesota River Valley. In this study, we examine the low-temperature (<300 °C) thermal history of zircon grains (n=23) from the Morton Gneiss (~3.5 Ga), Montevideo Gneiss (~3.5 Ga), and the Sacred Heart Granite (~2.6 Ga). Our results show dates that range from 100 to 950 Ma over a span of 80 to 1150 ppm effective Uranium (eU). We observe a prominent negative trend between zircon date and eU content for zircon grains with < 500 eU ppm, and a date-eU pediment of consistent ~200 Ma grains between 400 and 800 ppm eU. In order to understand the thermal history of our grains, we modeled our data using QTQt to reconstruct the geologic history of the rocks in terms of burial and exhumation, and more specifically during the time of the formation of the Great Unconformity surface. However, inverse thermal modeling of the data shows a relatively unresolved thermal history prior to 600 Ma. These results suggest that ZHe alone is unable to address questions related to the Precambrian thermal history at this location, and that additional 40Ar/39Ar data from potassium feldspars and biotites will improve our resolution, which we are actively collecting. In contrast, the date-eU pediment resolves the Phanerozoic thermal history for our samples and QTQt models a reheating and cooling trend between 600 and 100 Ma. This result is consistent with other sampling locations across the cratonic platform, which suggests a regional reheating event during the Phanerozoic in the cratonic platform.