THE OTTER CREEK IGNEOUS COMPLEX, NW IOWA – 2.7 GA KOMATIITIC MAGMATISM ON THE SOUTHERN MARGIN OF THE SUPERIOR CRATON
Dispersed chromites show compositions consistent with equilibration with trapped melt and subsolidus exchange with silicate phases. Chromites in thin chromitite layers are more robust and provide more information about the parental magma. Chromites from three chromitite layers have high Cr#’s (70 to 75) and Mg#’s (60 to 75), with sample C2-1252 being the most primitive (Cr# 74 and Mg# 75) and low TiO2 (~0.20 wt%) and Fe3+# (< 10). These chromitite analyses overlap with those found at the komatiite-related late Archean Great Dyke and Inyala intrusions in Zimbabwe. Calculated melt Al2O3/TiO2 values inferred from chromitite compositions and from trapped melt in the dunite are 15 to 25. These low values are consistent with a komatiitic (< 32) parental magma rather than a boninite (> 32). Sub-chondritic Nd isotopes and elevated Th/Nb indicates some crustal assimilation. Given the similarities with the komatiitic intrusions in the Belingwe greenstone belt of Zimbabwe and the melt Al2O3/TiO2 estimates, the Otter Creek Igneous Complex was formed from a contaminated komatiite magma that is related to one of the southernmost Neoarchean greenstone belts found in the Superior province.