Precambrian amphibolite gneisses and schists at Big Falls County Park in Eau Claire County, Wisconsin, include four main lithologic units: banded amphibolite gneiss, amphibolite schist, transition gneisses and feldspathic gneiss. Structural and petrologic investigation indicate three deformational phases accompanied by recrystallization. The first deformational phase is represented by coarse garnet porphyroblasts that were shattered and flattened in the plane of younger schistosity. Rootless intrafolial folds of compositionally banded rocks are relics of this first deformational phase. Inferred medium pressures and temperatures suggest metamorphism in the amphibolite facies. The second deformational phase resulted in folding of the banded amphibolite gneiss into open folds whose isogon pattern and layer geometry suggest formation as flattened flexural slip folds that were later deformed by inhomogeneous strain. Fold axial plunges are to the north and are coincident with the plunge of the earliest definable lineation. Folds in the amphibolite schist are tight, often isoclinal, isogon patterns suggest formation by inhomogeneous compressive strain. Mineral lineations cross-cut the south plunging fold axes. Folds in both units developed between subparallel slip planes with folding occurring between the slip surfacese Faults, with apparent left-lateral displacements, developed in the banded amphibolite gneiss prior to final recrystallization. Garnet, hornblende and plagioclase were the main phases crystallized under medium temperatures and pressures in the amphibolite facies. The third deformational phase resulted in development of shear zones and cataclastic deformation. A major fault developed between the banded amphibolite gneiss either prior to, or during the third deformational phase. A second schistosity cross-cuts the schistosity formed during the second deformational phase. Crystallization of epidote, cummingtonite, and sphene as well as blue-green hornblende and plagioclase indicate low load pressures and low to medium temperatures. The third recrystallization resulted in disequilibrium textures and incomplete mineral reactions and represents retrograde metamorphism in the epidote-amphibolite facies. Late faulting and jointing were not accompanied by major recrystallization, however alteration is associated with one of three fault sets. Whole rock chemical analyses and individual phase chemistry indicate that the amphibolite schist is more magnesian than the banded amphibolite gneiss. High weight percent of Al2O3 is typical of the banded amphibolite gneiss and the feldspathic gneiss. The chemistry of the rocks indicate the protolith to be a differentiated mafic intrusive. The banded amphibolite gneiss represents a plagioclase cumulate while the amphibolite schist and feldspathic gneiss represent a gabbro grading to an anorthosite or anorthositic gabbro. Traces of vanadium, nickel and chromium in hornblende further suggest a mafic igneous protolith. Isotopic age dates on zircons from an intrusion one mile east of Big Falls were 1900 m.y. The main structural elements of the region are considered coincident with this intrusion. The 1900 m.y. date is interpreted as the age of the second deformation at Big Falls, suggesting intrusion of the Big Falls rocks into the "Chippewa Amphibolite Complex" in early to middle Precambrian time.
A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Michael Levi Cummings in partial fulfillment of the requirements for the degree of Master of Science, February 1975.
Cummings, Michael Levi.
Petrology and Structure of Precambrian Gneisses at Big Falls, Eau Claire County, Wisconsin.
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