Petrogenesis of the Paleoproterozoic rapakivi A-type granites of the Archean Carajas metallogenic province, Brazil

Three Paleoproterozoic A-type rapakivi granite suites (Jamon, Serra dos Carajas, and Velho Guilherme) are found in the Carajas metallogenic province, eastern Amazonian craton. Liquidus temperatures in the 900-870 degreesC range characterize the Jamon suite, those for Serra dos Carajas and Velho Guilherme are somewhat lower. Pressures of emplacement decrease from Jamon (3.2 +/- 0.7 kbar) through Serra dos Carajas (2.0 +/- 1.0 kbar) to Velho Guilherme (1.0 +/- 0.5 kbar). Oxidizing conditions (NNO+0.5) characterized the crystallization of the Jamon magma, the Velho Guilherme magmas were reducing (marginally below FMQ), and the Serra dos Carajas magmas were intermediate between the two in this respect. The three granite suites have Archean T-DM model ages and strongly negative epsilon(Nd) values (-12 to -8 at 1880 Ma), and they were derived from Archean crust. The Jamon granite suite may have been derived from a quartz dioritic source, and the Velho Guilherme granites from K-feldspar-bearing granitoid rocks with some sedimentary input. The Serra dos Carajas granites either had a somewhat more mafic source than Velho Guilherme or were derived by a larger degree of melting. Underplating of mafic magma was probably the heat source for the melting. The petrological and geochemical characteristics of the Carajas granite suites imply considerable compositional variation in the Archean of the eastern Amazonian craton. The oxidized Jamon suite granites are similar to the Mesoproterozoic magnetite-series granites of Laurentia, and they were derived from Archean igneous sources that were more oxidized than the sources of the Fennoscandian rapakivi granites. The Serra dos Carajas and Velho Guilherme granites approach the classic reduced rapakivi series of Fermoscandia and Laurentia. No counterparts of the Mesoproterozoic two-mica granites of Laurentia have been found, however. Following the model of Hoffman [Hoffman, R, 1989. Speculations on Laurentia's first gigayear (2.0 to 1.0 Ga). Geology 17, 135-138], the origin of the similar to1.88 Ga Carajas granites is related to a mantle superswell beneath the Trans-Amazonian supercontinent. This caused breakup of the continent and was associated with magmatic underplating and resultant crustal melting and generation of A-type granite magmas. The Paleoproterozoic continent that included the Archean and Trans-Amazonian domains of the Amazonian craton was assembled at similar to2.0 Ga; its disruption was initiated at similar tosimilar to1.88 Ga, at least 200 Ma earlier than in Laurentia and Fennoscandia. The Carajas granites were related to the breakup of the supercontinent, not to subduction processes. (C) 2004 Elsevier B.V. All rights reserved.