Lower Proterozoic volcanic rocks and their setting in the southern Lake Superior district

Studies of lower Proterozoic volcanic rocks in Wisconsin and northern Michigan reveal the existence of two different Penokean-age (about 1,900 to 1,800 m.y. old) geologic terranes. The terranes are in contact along the east-west trending Niagara fault and are confined between Archean craton to the north and progressively younger Proterozoic magmatic provinces to the south. The northern terrane shows some similarity to Andean-type continental margins. The geologic history of the area includes evidence of rifting, continental arc volcanism, and later orogenesis (collision?). Rocks in this environment were sutured to a southern terrane, the Penokean volcanic belt, which developed from an island arc and basin-type environment, probably flanking the continental margin. The northern Penokean terrane contains thick units of sedimentary rocks, both platformal sequences and turbidites. Volcanic units are less abundant than sedimentary rocks and are typified by basalt flows and by lesser amounts of basaltic and rhyolitic volcaniclastic rocks. Penokean andesites are almost entirely absent from the northern terrane. Penokean calc-alkalic plutonic rocks are rare, whereas gabbroic sills and mafic layered complexes are common. The volcanic rocks display bimodal, tholeiitic trends with high iron enrichment. During what is considered to be the peak of the Penokean orogeny, the northern terrane rocks were affected by multiple deformations and metamorphic episodes. Tectonic styles in this region result from Archean basement uplifts and gneiss domes that exerted vertical stresses. Gneiss domes are outlined in the surrounding rocks by nodal patterns of metamorphic mineral zones. Recent work and the integration of previous studies in the Penokean volcanic belt have shown the contrast between this region and the northern terrane. South of the Niagara fault, metavolcanic rocks are much more abundant than metasedimentary rocks. Sedimentary units appear to have been derived from varied, discontinuous sources and included conglomerates, graywackes, argillites, graphitic shales, tuffaceous sandstones, and dolomites. Penokean calc-alkalic volcanic suites and calc-alkalic plutons are uniquely abundant in the Penokean volcanic belt. However, minor quantities of tholeiitic volcanic and intrusive rocks occur within calc-alkalic suites in northeastern Wisconsin, just south of the Niagara fault. This area may have been the site of interarc magma genesis. Tectonism in the Penokean volcanic belt can be distinguished by the lack of Archean basement, mantled gneiss domes, and metamorphic nodes. Greenschist-facies metamorphism was widespread throughout the belt. Higher-grade metamorphism was restricted to the vicinity of plutons and areas of locally intense deformation. In spite of similarities, some features of modern plate-tectonic orogenies are not displayed by the Penokean. This leads to the conclusion that the Penokean may represent a transitional style of tectonism, not like Archean, nor exactly like modern plate-tectonic activity.