Precambrian metamorphic units in northwestern Marathon County, Wisconsin, consist of an extrusive region composed of basaltic to felsic metavolcanics, and a gneissic region composed of granitic to dioritic gneisses. Both the metavolcanic and gneissic regions have been intruded by felsic plutonic rocks. The metavolcanic region has also been intruded by mafic units. The contact between these two terranes is a cataclastic zone which trends approximately N75°E through Athens, Wisconsin. The zone itself appears to have gradational boundaries and may be anastamosing. Structural and petrologic investigations indicate two deformational phases in the area accompanied by varying degrees of recrystallization. The first deformational phase is represented by quartzofeldspathic and mafic mineral assemblages diagnostic of amphibolite facies metamorphism. Relationships in the gneisses generally indicate textural disequilibrium although locally equilibrium textures are found. The presence of migmatites and complex folds is further evidence for regional metamorphism involving deformation at high temperatures and pressures. The first deformational phase is also represented by the metamorphism of volcanic rocks to greenschist facies and local deformation. Where the rocks are not highly deformed, primary volcanic textures and structures are well preserved. The development of similar penetrative structural trends in both terranes is further evidence for the first deformational phase. The structural features suggests a strain history in the study area such that maximum elongation (extension) occurred parallel to lineation in the plane of foliation, while shortening (flattening) occurred in a northwest-southeast direction, perpendicular to foliation. Strain results indicate the rocks were shortened up to 40 percent of their original length. The second deformational phase is represented by an assortment of cataclastic rocks derived from both terranes, the local development of a linear fabric associated with cataclasis, slickenside surfaces, and the development of two major joint systems. The major joint system trends northwest-southeast, perpendicular to foliation, while a second system trends northeast-southwest, parallel with foliation. If these joint sets are tensional features, the stress field which caused their development would be consistent with the stress field which developed the foliation during the first deformational phase. The intrusion of tonalitic to granitic plutonic rocks into the gneissic and volcanic terranes probably occurred about 1840 m.y. ago. Pervasive cataclastic textures in the plutons suggests deformation was operative after the emplacement of the intrusions. The next event in the area was the emplacement and local recrystallization to greenschist facies metamorphism of mafic rocks. These units intrude the volcanic terrane and trend subparallel to the cataclastic zone. Following this, quartz veins were introduced into these and associated units. The quartz veins and their host rocks were then locally faulted. The petrologic and structural evidence in northwestern Marathon County, Wisconsin, define a high-grade metamorphic complex which is lithologically distinct and tectonically separate from a lower-grade suprastructure. The contact between the two terranes is defined by a relatively thin boundary fault zone having a metamorphic gradient with complex shearing.
A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Elizabeth Ann Palmer in partial fulfillment of the requirements for the degree of Master of Science, December 1980.
Palmer, Elizabeth Ann.
The Structure and Petrology of Precambrian Metamorphic Rock Units, Northwestern Marathon County, Wisconsin.
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