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Penokean deformation in central Wisconsin

By
R. S. MAASS
R. S. MAASS
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L. G. MEDARIS, JR.
L. G. MEDARIS, JR.
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W. R. VAN SCHMUS
W. R. VAN SCHMUS
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Published:
January 01, 1980

An Archean, early Proterozoic, and middle Proterozoic sequence of metamorphic and plutonic rocks is exposed along and near the Wisconsin River in central Wisconsin, between Stevens Point and Wisconsin Rapids. Major lithologic units, from oldest to youngest, consist of (1) banded tonalitic gneiss with inter-layered amphibolite, (2) 2,800-m.y.-old migmatite, (3) diorite gneiss, (4) 1,840-m.y.-old foliated tonalite, (5) a series of 1,820-m.y.-old lineated tonalites, (6) granitic aplites and pegmatites, (7) amphibolite dikes, and (8) diabase dikes. All units except the diabase dikes have been metamorphosed under amphibolite-facies conditions and possess a steeply plunging, penetrative mineral lineation.

Deformation during the Penokean orogeny produced three steeply plunging sets of folds ranging from isoclinal to open in style. F1 folding is interpreted as culminating prior to 1,840 m.y. ago, but also extending beyond this time. Foliated tonalite appears to have been emplaced during the waning stages of F1 deformation. F2 and F3 folding occurred after the intrusion of 1,820-m.y.-old lineated tonalite, but younger limits on the age of folding are not yet available. All three fold sets are coaxial and parallel to the penetrative mineral lineation. Although not conclusive, there is evidence that suggests that an older deformation, perhaps Archean in age, affected the terrane prior to the Penokean orogeny.

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GSA Special Papers

Selected Studies of Archean Gneisses and Lower Proterozoic Rocks, Southern Canadian Shield

G. B. Morey
G. B. Morey
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Gilbert N. Hanson
Gilbert N. Hanson
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Geological Society of America
Volume
182
ISBN print:
9780813721828
Publication date:
January 01, 1980

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