The basalts and rhyolites of the Keweenawan of Minnesota have been invaded by the huge mass of the Duluth gabbro as well as by a great number of smaller sills, dikes, and irregular intrusives, mainly diabase. Close to the contact the flows are generally altered to fine granoblastic rocks which may not differ radically from the originals in mineral and chemical composition but are easily recognized by their texture. In the basalts the tendency is clearly to form a mineralogically simple rock consisting of plagioclase, pyroxene, and magnetite-ilmenite. The rhyolites tend to form a granular aggregate of acidic feldspar and quartz with minor amounts of other minerals. Dehydration is the important chemical process in the development of the hornfels, but above the gabbro hydrothermal conditions at places may have prevented the formation of hornfels.
The characteristics of the metamorphism here described are peculiar to the effect of basic intrusives which in general form anhydrous minerals and a rock with granular texture with little sign of stress effects.