A NEW U–PB BADDELEYITE CRYSTALLIZATION AGE OF A CORSON DIABASE SILL AT THE TYPE LOCALITY, EASTERN SOUTH DAKOTA, U.S.A. SUGGESTS WIDESPREAD, EARLY, CA. 1140 MA PHASE OF MID-CONTINENT RIFT MAGMATISM

The Corson diabase is composed of a number of small mafic dikes and sills of restricted areal extent (probably < 5,000 km²) intruding the Sioux Quartzite in eastern South Dakota. A U-Pb IDTIMS baddeleyite crystallization age of 1149.4 ± 7.6 Ma was previously reported from a Corson diabase dike core sample. A core sample from a Corson diabase sill at the type locality, ca. 30 km southeast of the previously dated intrusion, has now been dated, also by U-Pb IDTIMS baddeleyite methods. The new age is 1143 ± 15 Ma, suggesting the two intrusions represent one magmatic event. No 1140-1150 Ma magmatic event has been previously recognized in the region. However, the Corson diabase sill is within 270 km of the Midcontinent ift (MCR) that regionally underlies parts of Minnesota, Iowa, and Nebraska. Moreover, dikes have been documented within the Sioux Quartzite in Minnesota as well as cutting parts of the newly dated ca. 1170 Ma northeastern Iowa intrusive complex in Iowa, suggesting the possibility that the Corson magmatic event is much more extensive. At least as compelling, the Corson sill is similar in age to the Great Abitibi Dyke (1140.6 ± 2 Ma), located more than 1,000 km to the northeast. The proximity of the Corson diabase to the MCR, the presence of other dikes of potentially comparable age in Minnesota and Iowa, and the similarity of the Corson diabase ages to the age of the Great Abitibi Dyke suggest the Corson diabase intrusions are related to an early stage of development of the MCR, perhaps even plume-related, in the northern Great Plains.