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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...

The Precambrian Rio Paraiba do Sul Shear Belt comprises a 200-km-wide anastomosing network of NE-SW trending ductile shear zones extending over 1000 km of the southeastern coast of Brazil. Granulitic, gneissic-migmatitic, and granitoid terrains as well as low-to medium-grade metavolcanosedimentary sequences are included within it. These rocks were affected by strong contractional, tangential tectonics, due to west-northwestward oblique convergence of continental blocks. Subsequent transpressional tectonics accomodated large dextral, orogen-parallel movements and shortening. The plutonic Socorro Complex is one of many deformed granites with a foliation subparallel to that of the shear belt and exposes crosscutting relationships between its tectonic, magmatic, and metamorphic structures. These relationships point to a continuous magmatic evolution related to regional thrusts and strike slip, ductile shear zones. The tectonic and magmatic structural features of the Serra do Lopo Granite provide a model of emplacement by sheeting along shear zones during coeval strike-slip and cross shortening of country rocks. Geochronological data indicate that the main igneous activity of Socorro Complex spanned at least 55 million years, from the late stage of the northwestward ductile thrusting (650 Ma), through right-lateral strike slip (595 Ma) deformation. The country rocks yield discordant age data, which reflect a strong imprint of the Transamazonian tectono-metamorphic event (1.9 to 2.0 Ma). We propose a model for the origin of calcalkaline granites of the Ribeira Belt by partial melting of the lower crust with small contributions of the lithospheric mantle during transpressional thickening of plate margins, which were bounded by deep shear zones. The transpressional regime also seems to have tbcused granite migration from deeper into higher Crustal levels along these shear zones. M A.S. Basel, Nb and Pb Isotope studies bearing on the crustal evolution of southeastern Brazil, Rev. , Approximation of terrestrial lead isotope evolution by two-stage model, Earth Planet. Sci. Lett., 26, 207-221, 1975. Steiger, R.H., and E. Jager, Subcommission on geochronology: Convention on the use of decay constant in geo-and costoo-chronology, Earth Planet. Sci. Lett., 36, 359-362, 1977. Tommasi, A, A Vauchez, L.A.D. Fernandes, and C C. Potschef, Magma-assisted strain localization in an orogen-parallel transcurrent shear zone in southern Brazil, Tectonics, 13, 421-437, 1994. UIbrich, H H G.J, S.R F. Vlach, V.A. Janasi, and A R. Alineida, Ascens•1o e coloca$,no de granit6ides proteroz6icos em ,'ireas do embasamento do Estado de S•1o Paulo, Brasil Meridional: paradoxos da amtlise geom•trica, Bol. lnstituto de Geoci•ncias-Universidade de Sc•o Paulo, Publ. Esp., 12, 143-144, 1992. Vlach, S.R.F., V.A. Janasi, and C.A.C.B. Vasconc•los, The Itu-belt: associated calcalkaline a•'d aluminous A-type late Brasiliano granitoids 2n the states of S•Io Paulo and Paramt SE-B•'azil, paper presented at the 36. Congresso Bra,zileiro de Geologia, Sociedade Br...

A 530–490 Ma tectono-metamorphic event, the Búzios orogeny, is recognized within the Ribeira Belt, along the coast of SE Brazil. Tectonic evolution started with a Late Neoproterozoic marine basin and volcanic activity at c. 610 Ma. The rocks in this basin were affected by high-grade metamorphism at c. 530 Ma, coeval with deformational phases D1–D2, which generated compressive low-angle tectonic structures with top-to-NW tectonic transport. Large recumbent folds with NW–SE axes parallel to the main stretching lineation formed during D3 as the Cabo Frio tectonic domain, the focus of this study, collided with the Oriental terrane to the NW. D4 sub-vertical shear zones are limited in extent. A new U–Pb age of 501±6 Ma is reported for zircon from an amphibolite-facies shear zone related to either D3 or D4. Post-tectonic 440 Ma pegmatites mark the final stage of tectono-magmatic activity. The Cabo Frio tectonic domain has African affinities and is exotic to the Ribeira Belt. Middle Cambrian deformational and metamorphic ages are also reported from the ‘Angolan’ Pan-African belt, the southern Kaoko and Damara belts in Namibia, and the Cuchilla Dionisio–Punta Del Este terrane in Uruguay. The occurrence of Cambrian metamorphic rocks along the present African and South American coastlines shows that Mesozoic rifting closely follows Palaeozoic sutures of West Gondwana.