| Structural and petrochemical data from Precambrian rocks east of Eau Claire, Wisconsin, indicate that the region was part of a convergent plate margin during the Penokean Orogeny (1860-1830 Ma). The rocks are divided into two informal units: (1) the Chippewa Amphibolite Complex (CAC) containing amphibolites, feldspathic gneisses, tonalites, and trondhjemites; and (2) metasedimentary and metavolcanic (MSMV) rocks. Volcanic and plutonic rocks show cal(,c)-alkaline igneous trends and contain andesitic rocks; together they suggest a subduction-related origin. The CAC rocks were metamorphosed to amphibolite facies as many as three times; the MSMV rocks were metamorphosed to upper greenschist facies or higher, but they lack diagnostic mineral assemblages.;Structures in the area are characterized by steeply dipping foliation which is axial planar to isoclinal folds. Amphibolite xenoliths in tonalite show F(,1) isoclinal folds; these were followed by F(,2) isoclinal east-west folds in the tonalites, accompanied by intrafolial open to tight F(,3) folds. All three folding events are considered part of the cycle of deformation from the main phase of the Penokean Orogeny. Open to tight, NNE-trending F(,4) folds and small east-west trending faults suggest late D(,4) WNW-ESE compression and possible strike-slip faulting. Polyphase deformation has resulted in steeply plunging mesoscopic fold axes and mineral lineations, commonly with a non-Pumpellyan relation to the statistical B axes of structural domains. Shear zones and mylonites appear to be of minor importance in the area.;A tectonic model of rifting of the Superior craton in the late Archean or early Proterozoic and creation of an oceanic basin <1,000 km wide, followed by convergence, north-dipping subduction and collision is proposed. Rocks of the CAC are thought to represent an island arc. Subduction of basaltic crust is suggested by petrochemical similarities of tonalites (CAC) and metavolcanic rocks to Mesozoic and Cenozoic orogenic suites.;Comparison of apparent ages, modal compositions, and petrochemistry of the McGrath Gneiss and Freedhem Granodiorite from east-central Minnesota suggest that they do not correlate with CAC tonalites. Other plutonic rocks in east-central Minnesota may correlate with CAC tonalites, but further study is required. |
