COMPARISON OF ARCHEAN GNEISSES IN NORTHERN WISCONSIN ALONG THE SOUTHERN MARGIN OF THE SUPERIOR CRATON

The geologic setting in northwest Wisconsin is a conglomeration of events including original deposition and tectonic episodes. Some of those tectonic forces occurred along the Great Lakes Tectonic Zone which included the Niagara Fault Zone and had a significant effect during the Penokean Orogeny. These tectonic forces contributed to the metamorphism found in the Archean-aged gneisses found along the southern margin of the Superior Craton. The focus of this research is to analyze rocks collected from two sites in the study area to determine the level of metamorphism as indicated by the mineralogy. One of the sites was a paragneiss and the other an orthogneiss. Hand sample analysis revealed that the orthogneiss was more uniform. Quartz, orthoclase, biotite, and albite crystals were all present in hand sample analysis. Samples that were collected of the paragneiss were far less uniform in mineral composition but especially in mineral grain sizes. Moving from one area in the outcrop to another, there was variance as well. The presence or absence of micaceous minerals was evident through hand sample analysis. There were multiple samples analyzed at the site containing the paragneiss. Quartz, muscovite, and albite were present in one area whereas quartz, biotite, kyanite, and garnet were present at another. Another method of analysis that was used was x-ray diffraction (XRD). One orthogneiss sample was analyzed and four samples from the paragneiss were analyzed using this method. Initial interpretation of the diffraction patterns was supportive of the primary minerals observed in hand sample. The presence of kyanite and garnet in several samples is consistent with the current interpretation for this area of a moderate to high level of regional metamorphism.