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IntroductionDURING the past 15 years, studies of the major iron ore districts and copper districts in the Lake Superior region were completed and abundant new .data on the geology and geochronology of the Precambrian rocks became available. During this same period, there was renewed interest in the base metal potential of the .region, stimulated largely by the discovery of the famous Kidd Creek massive sulfide orebody in Ontario. Accordingly, there has been a gradual shift away from traditional studies of the classic mining areas to studies of broader geologic environments and of the sequential events of the geologic history of large areas. I am pleased to have had a part in these later studies.An outcome of the recent work (Morey and Sims, 1976) has been the recognition that the Lake Superior region is underlain by t•vo fundamentally different basement rock terranes--an older gneiss terrane (3,000 m.y. or more old) and a younger greenstone terrane (2,700-2,750 m.y. old). In this paper, I will discuss the contrasting effects these two types of basement rocks had on later geologic events, with emphasis on the tectonic evolution of the middle Precambrian basin in which the great iron-formations were formed. Finally, I will compare the middle Precambrian basin of this region with the Labrador trough, a Proterozoic basin of approximately equivalent age and tectonic setting that also contains valuable ore deposits.The subdivision of the Precambrian used in this paper is given in the explanation for the geologic map (Fig. 2). The three-fold subdivision--lower, middle, and upper Precambrian--which is based on orogenies, is widely accepted in .the Lake Superior region (James, 1958; Goldich, 1968; Lumbers and

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The Early Proterozoic Penokean Orogen developed along the southern margin of the Archean Superior craton. The orogen consists of a northern deformed continental margin prism overlying an Archean basement and a southern assemblage of oceanic arcs, the Wisconsin magmatic terranes. The south-dipping Niagara fault (suture) zone separates the south-facing continental margin from the accreted arc terranes. The suture zone contains a dismembered ophiolite.The Wisconsin magmatic terranes consist of two terranes that are distinguished on the basis of lithology and structure. The northern Pembine-Wausau terrane contains a major succession of tholeiitic and calc-alkaline volcanic rocks deposited in the interval 1860-1889 Ma and a more restricted succession of calc-alkaline volcanic rocks deposited about 1835-1845 Ma. Granitoid rocks ranging in age from about 1870 to 1760 Ma intrude the volcanic rocks. The older succession was generated as island arcs and (or) closed back-arc basins above the south-dipping subduction zone (Niagara fault zone), whereas the younger one developed as island arcs above a north-dipping subduction zone, the Eau Pleine shear zone. The northward subduction followed deformation related to arc-continent collision at the Niagara suture at about 1860 Ma. The southern Marshfield terrane contains remnants of mafic to felsic volcanic rocks about 1860 Ma that were deposited on Archean gneiss basement, foliated tonalite to granite bodies ranging in age from about 1890 to 1870 Ma, and younger undated granite plutons. Following amalgamation of the two arc terranes along the Eau Pleine suture at about 1840 Ma, intraplate magmatism (1835 Ma) produced rhyolite and anorogenic alkali-feldspar granite that straddled the internal suture.L'orogtne du PCnokten d'lge prottrozoique prtcoce s'est CdifiC le long de la marge sud du craton Suptrieur de I'Archten. L'orogtne inclut, dans sa partie nord, un prisme de marge continentale dtformt qui repose sur un socle archten, et dans le sud un assemblage d'arcs ocCaniques, les terranes magmatiques de Wisconsin. La zone de failles Niagara (suture) inclinCes vers le sud stpare la face sud de la marge continentale d'avec les terranes d'arcs accrttts. La zone de suture renferme une ophiolite dCmembrte.Les terranes magmatiques de Wisconsin sont form& de deux terranes qui diffirent par leurs lithologies et leurs structures. Le terrane de Pembine-Wausau du nord est composC principalement d'une succession de roches volcaniques tholttitiques et calco-alcalines dkposk durant l'intervalle 1860 -1889 Ma et d'une succession d'ttendue plus restreinte des roches volcaniques calco-alcalines dCposte aux environs de 1835-1845 Ma. Des roches granitoides d'lges qui varient de 1870 B 1760 Ma recoupent les roches volcaniques. La succession la plus vieille a crCC les arcs insulaires et (ou) les bassins d'arrikrearc fermts sus-jacents B la zone de subduction plongeant vers le sud (zone de failles Niagara), tandis que la plus jeune succession a formt des arcs insulaires B l'aplomb de la zone de subducti...

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