GEOCHEMISTRY AND PETROGRAPHY OF THE OSBORNE CORE (NORTHEAST IOWA INTRUSIVE COMPLEX)

The Northeast Iowa Intrusive Complex (NEIIC) is a 16,000 km² mafic complex buried about one kilometer beneath the surface of northeast Iowa that has been revealed through aeromagnetic and gravity surveys. The age of the complex is unknown at present, but is of key interest due to its potential relationship to the Keweenawan (~1.1 Ga) Midcontinent Rift System (MRS). Many MRS-related intrusions, such as the Duluth Complex, host economically significant copper-nickel sulfide and platinum group element (PGE) deposits. The only samples available for study from the NEIIC are from the Osborne core, which was drilled in 1963 into a dike extending northeast from the complex. Petrographic study of this 225 m core has revealed a layered structure with a ~110 m thick olivine-rich outer margin and an abrupt transition into a troctolite (plagioclase-olivine) unit. Both units contain abundant FeTi-oxides. Several coarser-grained feldspar+apatite-rich layers were found in the lower part of the core. A handheld portable x-ray fluorescence (pXRF) instrument was used to determine compositional variations along the core at an average spacing of 2 m. One of the feldspar-rich horizons was significantly enriched in incompatible elements (~500 ppm Zr and ~3.5% K) compared to the rest of the core (50 ppm Zr and 0.2% K) and likely represents crystallization of the final residual liquid. This feldspar-rich residual liquid horizon contains plagioclase + k-feldspar and clinopyroxene, with biotite, ilmenite±magnetite, and apatite. Importantly for geochronology, it also contains trace baddeleyite and zirconolite and work is ongoing to date these phases. Electron microprobe X-ray element mapping of large (1" x 3") polished core segments is being used to determine phase assemblages and textural relationships in representative core samples of each lithological type. Our preliminary observations suggest that the Osborne core material represents the in-situ crystallization of a single magma batch within a dike-like structure. This model will be evaluated using whole-rock elemental analysis by ICP-MS and mineral composition data (olivine, plagioclase, FeTi-oxides, apatite) by electron microprobe on a representative sample suite.