U-PB BADDELEYITE AGE LINKS THE OSBORNE ANOMALY (NE IOWA) TO THE EARLY (1,150 MA) PHASE OF MIDCONTINENT RIFT ACTIVITY

Recent geophysical surveys over portions of the entirely concealed Northeast Iowa Intrusive Complex (NEIIC) have provided a clearer picture of the region’s Precambrian basement geology. The proximity and orientation of the NEIIC relative to the ~1,100 Ma Midcontinent Rift System (MRS), along with large amplitude magnetic and gravity anomalies, suggest that this terrane may be akin to other economically rich mineral deposits in the Lake Superior region (i.e. Duluth and Coldwell complexes). A core drilled into the northeast-trending Osborne Anomaly in Clayton County, Iowa provides the only available samples in the vicinity of the NEIIC. Obtaining a reliable age from the Osborne core has been paramount to making the argument that the NEIIC is Keweenawan and thus temporally related to other magmatic intrusive terranes in the Lake Superior region. The Osborne core encountered 722 feet (220 m) of mafic-ultramafic rocks that have been previously described as olivine-plagioclase cumulates. Screening of the core using a handheld XRF instrument revealed several discrete zones of late stage melt with elevated concentrations of zirconium and potassium. Electron probe micro-analyzer imaging showed the presence of potentially datable minerals, baddeleyite and zirconolite.

Baddeleyite crystals obtained from 2,416.3 feet (736 m) depth were analyzed using LA-ICP-MS at the University of New Brunswick. A U-Pb crystallization age of 1,148 ± 14 Ma provides the first reliable date to come from the NEIIC region. This age is similar to other intrusions outboard of the MCR, such as the Corson Diabase in eastern South Dakota (1,149 ± 7 Ma), the Great Abitibi dike (1,141 ± 2 Ma), and the Inspiration diabase (1,159 ± 33 Ma). The general age of these intrusions has been interpreted as an early stage of magmatism (~1,150 Ma) potentially related to the onset of the MCR. The latest geophysical survey over the majority of the southern portion of the NEIIC shows that the Osborne Anomaly is cut by NEIIC intrusions, thus providing a maximum emplacement age of ~1,150 Ma. The age, orientation, and geochemistry of the Osborne Anomaly makes a strong case that this early precursor phase of magmatism, prior to the main episode of MCR magmatic activity, may have occurred over a much wider area than previously thought.