Field and laboratory investigations suggest that a granophyre-gabbro complex in northern Wisconsin is similar to other acidic and basic complexes so common throughout the world. The Wisconsin complex consists of two distinct belts of gabbro and granophyre split by a narrow belt of basic extrusives, Keweenawan in age, with flows the oldest, gabbro next, and granophyre the youngest. Still younger are basic dikes that show preferred orientation. The complex is on the steeply dipping south limb of the Lake Superior geosyncline.
The gabbro was intruded between flow units along planes of shear that resulted during formation of the geosyncline. Some gabbroic magma followed thrust faults. Two structures in the gabbro—fluxion structure and rhythmic layering—appear to be unrelated in origin. Fluxion structure is believed to be the result of shear or flow occurring parallel to the walls of the intrusion as the mass crystallized, whereas the layering appears to be the result of crystal settling. A study of mineral compositions in the gabbro reveals simultaneous iron enrichment in coexisting pyroxenes and olivines, and soda enrichment in the plagioclase.
The granophyre is believed to be the result of crystallization of a granitic melt which was intruded following solidification of the gabbro. Criteria for a replacement (metasomatic) origin of the granophyre are inconclusive. It is postulated that the granitic melt originated at depth through either (1) partial remelting of the sial during downwarping of the Lake Superior geosyncline or (2) fractional crystallization. This melt was then squeezed into its present position during further deformation of the geosyncline and followed essentially the same zones of weakness taken by the gabbro magma. The minerals of the granophyre could be the residua of fractional crystallization or the first liquid formed in remelting.
The type of granophyric texture in the red rock (granophyre) is largely dependent upon the anorthite content of the microperthite. In addition, the granophyric texture cannot be the result of eutectic crystallization, for this type of crystallization implies that the melt crystallizes with no resulting change in composition; however, in the red rock, a change in composition is reflected in the anorthite content of the microperthite intergrown with quartz. Hence, the granophyric texture must be due either to simultaneous crystallization without a eutectic or to some secondary process.
Between the gabbro and the granophyre (red rock) is a narrow zone (maximum 200 feet) of “intermediate rock” which exhibits the peculiar mineralogical and textural characters of an alteration zone. Geochemical culminations of FeO, TiO2, MnO, and P2O5 exist in this zone between gabbro and granophyre. In addition, the intermediate zone has been enriched in the O18 relative to the O16 isotope. The peculiar chemical, mineralogical, and textural features of the “intermediate rock” are the result of contact metasomatism caused by the action of a granophyric melt on gabbro. Also the alteration of the basic flows in the area probably resulted from solutions emanating from the granophyre.