40Ar/39Ar and paleomagnetic results from Liberia and the Precambrian APW data base for the West African Shield

Some key issues in reconstructions of Proterozoic supercontinents

“…Geological and paleomagnetic data suggest that the Amazonia and São Francisco Cratons in South America collided with the West African and Congo Cratons in Africa, respectively, along the 2.1-2.0 Ga Transamazonian and Eburnean Orogens (McElhinny and McWilliams, 1977;Onstott and Hargraves, 1981;Onstott et al, 1984;Onstott and Dorbor, 1987;Bertrand and Jardim de Sá, 1990;Ledru et al, 1994;D'Agrella et al, 1996;Rogers, 1996). However, whether South America and West Africa were divorced and then remarried to form part of Gondwana, or remained largely coherent from their amalgamation at 2.1-2.0 Ga until their incorporation into Gondwana is still controversial.…”

“…On the other hand, however, both geological and paleomagnetic data suggest that the São Francisco and Amazonia Cratons were once joined, respectively, with the Congo and West African Cratons along the 2.1-2.0 Ga Transamazonian/Eburnean orogens (McElhinny and McWilliams, 1977;Onstott and Hargraves, 1981;Onstott et al, 1984;Onstott and Dorbor, 1987;Bertrand and Jardim de Sá, 1990;Ledru et al, 1994;D'Agrella et al, 1996;Rogers, 1996;Nomade et al, 2003). On the map of the classical Bullard et al (1965) fit of Africa and South America, the structural trend of the Transamazonian orogen along the eastern margin of the São Fig.…”

“…Paleomagnetic and isotopic data also suggest that South America (São Francisco and Amazonia Cratons) and West Africa (Congo and West African Cratons) may have existed as a single landmass after the Transamazonian/Eburnean orogenic events (McElhinny and McWilliams, 1977;Onstott and Hargraves, 1981;Onstott et al, 1984;Onstott and Dorbor, 1987;D'Agrella et al, 1996;Nomade et al, 2003). In the late 1970's and early 1980's, the consensus of opinion by most paleomagnetists was that the Congo and West African Cratons collided, respectively, with the Amazonia and São Francisco Cratons about 2.0 Ga ago (McElhinny and McWilliams, 1977;Fig.…”

Tectonic Interpretation of40Ar/39Ar Ages on Country Rocks from the Central Sector of the Río Negro-Juruena Province, Southwest Amazonian Craton

“…The specimen orientation in the tumbler was reversed with each consecutive step to correct for any rota tion-induced remanence magnetization (Hillhouse, 1977). Thermal demagnetization was performed in a Schonstedt TSD-1 with a mea sured ambient field of 20 nT and a measured field accuracy of i 5° C in temperature (Onstott and Dorbor, 1987).…”

“…All specimens were measured on a Schonstedt spinner magne tometer interfaced to an Apple IIe computer at the Princeton University Laboratory, Prince ton, New Jersey. Alternating field demagnetiza tion was performed with a two-axis tumbler in ambient DC fields of less than 30 nT and with peak alteration fields up to 1100 Gauss (130 mT) (Onstott and Dorbor, 1987). The specimen orientation in the tumbler was reversed with each consecutive step to correct for any rota tion-induced remanence magnetization (Hillhouse, 1977).…”