Understanding the petrogenesis of the late Archean Otter Creek Layered Igneous Complex, NW Iowa

The oldest dated rocks found in Iowa are located in the northwest of the state near the town of Matlock. These rocks are part of a geologic unit called the Otter Creek Complex which is an ancient (2.7-billion-year-old) igneous intrusion that is part of the core (craton) of North America from a period early in Earth’s history. Studies of the Otter Creek Complex have been limited because it is buried under younger rocks which makes it only available through drill holes. This project has filled in gaps of our understanding of the Otter Creek Complex and how it is related to the assembly of the North American continent.

I used the chemical composition of the rocks and minerals from the intrusion to figure out what type of magma formed the Otter Creek Complex. The rocks have been altered by over two billion years of weathering, and chromite is the only original mineral phase still present. The composition of chromite can be used to figure out the original magma type. By comparing the chromite compositions in the Otter Creek Complex to other similar well-studied intrusions elsewhere on Earth of a similar age (Stillwater Complex, Montana; Bushveld Complex, South Africa; Inyala intrusion and Great Dyke, Zimbabwe), I was able to determine that the chromites and the magma in which they formed is most like that of the Inyala intrusion and Great Dyke in Zimbabwe. The likely magma type that solidified into the Otter Creek Complex is called a komatiite. High mantle temperatures are required to form komatiite magmas, and so they are only found early in Earth's history. This is not the only komatiite found within the North American craton: other examples are found in structures called greenstone belts that occur along the joints between large jigsaw pieces of ancient crust that form the craton of North America.