Figure from Miller, 1994 in The Geology of Alaska

The only large-scale magmatic arc identified in the Brooks Range formed in Devonian to Early Mississippian time, when the Arctic CT was still a part of the margin of North America. Rocks of this arc are mainly granites with some felsic volcanics (ages 375-390 Ma). The magmatism in this belt was produced by eastward subduction under the North American margin.

Plutons with emplacement ages ranging from Proterozoic to mid-Paleozoic locally intrude the rocks of the Brooks Range. After intrusion, these plutons were affected by regional metamorphism, as was much of the southern Arctic CT. K-Ar analyses indicates Cretaceous isotopic apparent ages, while limited U-Pb and Pb-Pb dating gives emplacement ages of around 750 Ma. More detailed information on these rocks is given on the geochemistry page.