STUDIES OF THE PRECAMBRIAN GEOLOGY OF IOWA:
PART 1. THE OTTER CREEK LAYERED IGNEOUS COMPLEX
K.E. Windom, K.E. Seifert, and R.R. Anderson
Journal of the Iowa Academy of Science, 1991, v. 98, no. 4, p. 170-177
Rocks from a buried igneous body, herein called the Otter Creek layered igneous complex, were recovered by drilling in Archean rocks in northwestern Iowa. This complex consists of layers of ultramafic and mafic cumulate rocks, including brozitite, harzburgite, dunite, gabbro, anorththosite. These rocks have been subjected to low-grade metamorphism with a paragenesis including serpentine, chlorite, talc, uralitic amphiboles, magnetite, albite, epidote, sericite, and minor quartz and calcite. During its intrusion, the magma which gave rise to the layered body engulfed a large block of rock consisting of banded iron formation and thin lamprophyre dikes. Both the iron formation and the lamprophyre show evidence of high-temperature metamorphism followed by a retrograde event. Chemical compositions of the layered rocks and the lamprophyre are indicative of both having been derived from a primitive to slightly depleted mantle source. Comparison with similar rocks in the Superior Province indicate that the Otter Creek complex is part of a greenstone belt and was probably generated near the terminal stages of its development.