The Eau Claire River complex is a metamorphosed differentiated mafic intrusion within the Chippewa amphibolite complex of western Wisconsin. The intrusion is layered and contains ultramafic, mafic, and feldspathic rocks; however, the internal stratigraphy and size of the complex are not known. Primary igneous structures, other than compositional layering, are rarely preserved, although autoliths and possible slump structures are exposed at Big Falls County Park. Primary igneous mineral textures are not preserved.
Three metamorphic episodes are recognized and are characterized by distinct structures and mineral textures. Coarse garnet porphyroblasts, as much as 6 cm in diameter, are preserved from the mineral assemblage that formed under conditions of the upper amphibolite facies during the first metamorphism. Alternating hornblende-rich and plagioclase-rich compositional layers were isoclinally folded by associated deformation. These folded compositional layers are displaced across faults that were recrystallized and healed during the second metamorphism. During the second metamorphism, the complex was recrystallized to medium- to coarse-grained plagioclase and hornblende-bearing assemblages under conditions of the amphibolite facies. Compression perpendicular to compositional layering wrapped layering about older garnet porphyroblasts and produced zones parallel to compositional layering of rootless intrafolial folds and transposed foliation. The third metamorphism, under conditions of the green-schist to lower amphibolite facies, resulted in a partial recrystallization. A weak schistosity developed oblique to compositional layering.
The age of the complex is uncertain. A uranium-lead age determination of 1,835 m.y. for a rutile sample is believed to be the age of the second metamorphism, the Penokean orogeny. The first deformation and metamorphism and magmatic crystallization may have occurred during the late Archean or very early Proterozoic.