PETROLOGY & GEOCHEMISTRY OF THE OSBORNE MAFIC INTRUSION, NW IOWA

Aeromagnetic and gravity surveys have revealed the existence of the North-East Iowa Intrusive Complex (NEIIC), a large (~16,000 km²) buried mafic/ultramafic complex that is possibly related to the ~1.1 Ga Mid-continent rift system, and potentially analogous to the Duluth Complex. An industry core from 1963 drilled the outer 225 m of an inclined NE-trending dike (near Osborne, Clayton Co, Iowa) that is inferred to be related to the main NEIIC complex. Petrographic examination of this core show that it is a layered mafic intrusion, with a ~110 m olivine-rich outer margin that transitions sharply into troctolite. Fe-Ti oxides are abundant (~30%) throughout the core, and several coarser-grained, plagioclase-rich layers are found sporadically in the lower half of the core. A systematic survey of compositional variations was done using a handheld pXRF, with replicate analyses made on individual cores pieces, at an average sampling interval of 2 m along the core. These data largely confirm the petrographic division into two dominant lithologies (dunite and troctolite). 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. Electron microprobe X-ray element mapping of large (1" x 3") polished slabs is being carried out to determine phase assemblages and textural relationships in representative core samples of each lithological type. The feldspar-rich residual liquid horizon contains plagioclase + k-feldspar and clinopyroxene, with biotite, ilmenite±magnetite, and apatite, together with trace baddelyite and zirconolite. A systematic survey of mineral compositions through the Osborne intrusion is currently in progress, and preliminary data on a troctolite sample show that the olivine is Fo60 and the plagioclase is An55.