THE OTTER CREEK IGNEOUS COMPLEX, NW IOWA – 2.7 GA KOMATIITIC MAGMATISM ON THE SOUTHERN MARGIN OF THE SUPERIOR CRATON

The Otter Creek Igneous Complex is a buried late Archean layered ultramafic-mafic intrusion in NW Iowa. It was discovered in the 1960s by mineral exploration drilling into one of a series of positive magnetic anomalies found just north of the Spirit Lake Tectonic Zone that separates the southern margin of the Archean Superior craton from younger Paleoproterozoic accretions. The Otter Creek Igneous Complex is highly altered with a lower ultramafic series (cyclic dunite, pyroxenite, and massive dunite) and an upper mafic series (gabbros, gabbronorites, and anorthosites), and it is dated at 2713 ± 8 Ma (unpublished industry U-Pb zircon age). The focus of this study is to use whole rock composition (ICP-MS and XRF) and chromite analyses (electron microprobe) to infer the nature of the parental magma.

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.