Petrology and Structure of Precambrian Gneisses at Holcombe, Chippewa County, Wisconsin

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Petrology and Structure of Precambrian Gneisses at Holcombe, Chippewa County, Wisconsin

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Amphibolite, synkinematic intrusives and amphibolite schist crop out along the Chippewa and Fisher Rivers, near Holcombe, in Chippewa county, Wisconsin. These rock formations occur near the northern boundary of an Archean (?) terrane of amphibolitic gneisses and schists known as the Chippewa Amphibolite Complex. To the north of the Chippewa Complex lies the Middle Precambrian Flambeau Province, composed of intermediate to felsic volcanics and metasediments. Three major rock units are exposed near Holcombe. From oldest to youngest these are: interbanded amphibolite gneiss and tonalite at the Fisher River, quartz diorite gneiss and associated meta-igneous rocks at Holcombe Dam, and amphibolite schist along the Chippewa River. Intrusives into the quartz diorite include medium-grained granite and granite dikes, and a hypabyssal andesite intrusive. Banded gneiss at the Fisher River displays three periods of folding. During the first phase of deformation, isoclinal folds, F1, were produced in intrusive tonalite veins, as well as fold axis and mineral lineations, L1. A dominantly east-west, vertically dipping mineral foliation, S1, was also produced. During F1 a lithologic layering of tonalite with amphibolite was formed in selected zones probably during transposition of an original S0 surface of tonalite to S1 and subsequent shearing. Extensive recrystallization in the lower amphibolite facies occurred during the F1 event. Continued recrystallization after F1 produced garnets in biotite rich zones of the banded gneiss. F2 deformation produced tight to isoclinal folds which refolded L1 lineations along a great circle distribution. F2 folds trend east-northeast at an oblique angle to F1 folds and plunge moderately to steeply west. A third fold deformation, F3, produced broad, open folds along a steeply dipping north-south axis. At Holcombe Dam, S1 foliation, and F2 and F3 folds can be observed in the quartz diorite gneiss. The quartz diorite was intruded and partially metamorphosed to the lower amphibolite facies during F1. Granite intrusives and the hypabyssal andesite display faint east-northeast foliations and steep to moderate westerly plunging lineations, suggesting that these two units were intruded during the F2 event. These units were partially metamorphosed to the amphibolite facies during F2. After folding, an extensive period of shearing affected the quartz diorite gneiss and intrusives. A series of small shear zones fanned in the gneiss. These trend N70E and have moderate to steep westerly plunging lineations. The amphibolite schist displays all the same structural characteristics as small shear zones in the quartz diorite gneiss, and is therefore interpreted to represent a major shear zone which crosscuts the area. Average modal compositions of the amphibolite schist suggest that the protolith was an igneous rock of dioritic composition. Final phases of deformation produced brittle faulting throughout the area. A set of faults trending N50W and N20W offset all other structural features in the area. Structures in the area closely resemble those produced by the Penokean event in the region.


A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Stephanie Wurdinger in partial fulfillment of the requirements for the degree of Master of Science, May 1980. There is 1 supplementary file also attached to this record, which contains Plate 1 referenced in the thesis.

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Wurdinger, Stephanie. (1980). Petrology and Structure of Precambrian Gneisses at Holcombe, Chippewa County, Wisconsin. Retrieved from the University Digital Conservancy,

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