Above the large gabbro mass in the city of Duluth are several typical Keweenawan sills intrusive into flows of Keweenawan basalt and rhyolite. Three of the sills are large, and each consists of diabase in the lower portion and granite near the top. They were mapped in detail, and samples were selected for petrographic study and chemical analysis.
There is a gradation mineralogically and chemically from the most basic to the most acidic facies. There is, however, a sharp break between the mineral and chemical composition of the granite of the sills and the rhyolite in the roof. The gradational zone within the sill consists mainly of diabase with various amounts of interstitial micropegmatite.
Five possible origins for the rocks in the sill are considered. These are: (1) six separate intrusions of basic and acidic magma, (2) metamorphism of rhyolite by diabase, (3) hydrothermal alteration of the upper portion of the diabase, (4) syntexis, and (5) differentiation of the sill magma.
It is concluded that many factors had some effect, but that differentiation within each sill accounts for the major part of the segregation of the rock in each of the three sills into two distinct facies. It is difficult to see how crystallization differentiation alone could produce the rock and relations found. Probably a complex process of differentiation involving several processes especially aided by mineralizers might explain the origin of the facies in the sills.