Manoel S. D’Agrella-Filho
University of São Paulo
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Earth and Planetary Science Letters | 2003
Maria Irene Bartolomeu Raposo; Manoel S. D’Agrella-Filho; Roberto Siqueira
The Juiz de Fora Complex is mainly composed of granulites and granodioritic–migmatite gneisses and is a cratonic basement of the Ribeira fold belt, which was formed during the last stage of Brasiliano orogeny. Samples widely distributed over the studied region (SE Brazil, Rio de Janeiro State) were collected for paleomagnetic and magnetic anisotropy determinations. After measurement of anisotropy of low-field magnetic susceptibility (AMS) the same specimens were submitted to both alternating field (for anisotropy of remanent magnetization, ARM, determinations) and thermal demagnetization for paleomagnetic analyses. Demagnetization processes allowed isolating an eastern, steep, positive-inclination characteristic remanent magnetization (ChRM) direction from the rocks. The studied specimens are strongly heterogeneous at sample scale regarding the amount of magnetic minerals even for specimens from the same core. Rock magnetism indicates that there is a complex mixture of magnetic minerals with both low and high coercivity which are associated with coarse-grained (titano)magnetite and either fine-grained (titano)magnetite or ‘titanohematite’ grains, respectively. These magnetic minerals are responsible for both remanent direction and magnetic anisotropies, even though for some specimens paramagnetic minerals are the carriers of AMS fabric. The ChRM direction found for the rocks should represent the cooling phase of the Brasiliano event in the area, and it was acquired after the magnetic fabric was formed. The AMS and ARM fabrics are coaxial and tectonic in origin, and compare favorably with the mesoscopic-scale fabrics in the adjacent areas. However, ARM measurements have reached only low-coercivity grains and it could be determined in few specimens. Then only an estimate of ARM effect on ChRMs correction is shown. The rocks are strongly magnetically anisotropic (>50%) and foliated. The paleomagnetic directions from each specimen within a site tend to approach its AMS or ARM foliation (Kmax–Kint or ARMmax–ARMmin plane). These directions were corrected, specimen-by-specimen from the sites, taking into account the pole of AMS or ARM foliation (Kmin or ARMmin) and the value of the degree of anisotropy (P). Results show that even for specimens with high P, the angular difference between corrected and uncorrected magnetization direction may be insignificant if the geomagnetic field at the time of acquired magnetization was approximately parallel to the magnetic foliation of the specimens. Our data set suggests that AMS is a powerful tool to correct the ChRM deviation from paleofields in rocks with complex magnetic mineralogy such as those from the Juiz de Fora Complex.
Archive | 2017
Manoel S. D’Agrella-Filho; Umberto G. Cordani
This chapter, based on paleomagnetic and geologic-geochronological evidence, discusses the position of the Sao Francisco craton and other South American and African cratonic blocks within paleo-continents, since the formation of Columbia supercontinent in the Paleoproterozoic up to the fragmentation of Pangea in the Mesozoic. In Paleoproterozoic times, between ca. 2.0 and 1.8 Ga, two large independent landmasses were formed. The first one involved several cratonic blocks that were leading to the formation of Laurentia. Later, Laurentia, proto-Amazonia, West Africa and Baltica amalgamated to form the nucleus of the supercontinent Columbia at about 1.78 Ga. The second landmass encompassed the Sao Francisco-Congo, Kalahari, Rio de la Plata and Borborema-Trans-Sahara, forming the Central African block. For the Sao Francisco-Congo and Kalahari cratons, two robust Paleoproterozoic poles are available. One is from the Jequie charnockites of Bahia (Sao Francisco Craton), and the other from the Limpopo high-grade metamorphics in South Africa (Kalahari Craton). They support the possible link between these two cratonic blocks at ca. 2.0 Ga. Columbia may have remained united until 1.25 Ga, when Baltica and Amazonia/West Africa broke apart. Their paleomagnetic record seems to indicate that both executed clockwise rotations, until they collided with Laurentia along the Grenville belt at ca. 1.0 Ga., culminating with the formation of Rodinia. For the Central African block, however, there are no reliable paleomagnetic poles available between 1.78 and 1.27 MA. Nevertheless, during this time interval, the geological-geochronological evidence indicates that no continental collisional episodes affected the Sao Francisco-Congo craton, where important intra-plate tectonic episodes occurred. Most probably, this large continental block drifted alone since the end of the Paleoproterozoic and did not take part of Columbia or Rodinia. At the end of the Mesoproterozoic, ca. 1100 MA, the robust Umkondo pole of the Kalahari craton, as part of the Central African block, and the equally robust Keweenawan pole of Laurentia at the center of Rodinia, indicated that these landmasses were very far apart. At that time a large oceanic realm, the Goias-Pharusian Ocean, was indeed separating Amazonia-West Africa from the Central African block. This ocean closed by a continued subduction process that started at ca. 900 MA and ended in a collisional belt with Himalayan-type mountains at ca. 615 MA, as part of the few continental collisions which formed Gondwana. However, the age of the final convergence is still a matter of debate, because paleomagnetic measurements for the Araras Group, which occurs within the Paraguay belt at the eastern margin of the Amazonian craton, would indicate that a large ocean was still in existence between it and Sao Francisco craton close to the Ediacaran/Cambrian boundary. Consensus about this matter awaits for further paleomagnetic data. Gondwana collided with Laurasia during the late Paleozoic, at about 300 Ma, originating Pangea, which not much later started splitting apart, near the Permian/Triassic boundary. As part of this present-time plate tectonic regime, the Sao Francisco Craton (in South America) started separation from the Congo craton (in Africa) in Jurassic times, giving rise of the present-day oceanic lithosphere of the Atlantic Ocean.
Precambrian Research | 2006
Eric Tohver; Manoel S. D’Agrella-Filho; Ricardo I. F. Trindade
Precambrian Research | 2008
Franklin Bispo-Santos; Manoel S. D’Agrella-Filho; I. G. Pacca; Liliane Janikian; Ricardo I. F. Trindade; Sten-Åke Elming; Jesué A. da Silva; Márcia Aparecida de Sant’Ana Barros; Francisco Egidio Cavalcante Pinho
Precambrian Research | 2004
Ricardo I. F. Trindade; Manoel S. D’Agrella-Filho; Marly Babinski; Eric Font; Benjamim Bley de Brito Neves
Terra Nova | 2008
Liliane Janikian; Renato Paes de Almeida; Ricardo I. F. Trindade; Antonio Romalino Santos Fragoso-Cesar; Manoel S. D’Agrella-Filho; Elton Luis Dantas; Eric Tohver
Precambrian Research | 2012
Franklin Bispo-Santos; Manoel S. D’Agrella-Filho; Ricardo I. F. Trindade; Sten-Åke Elming; Liliane Janikian; Paulo M. Vasconcelos; Bruno M. Perillo; I. G. Pacca; Jesué A. da Silva; Márcia Aparecida de Sant’Ana Barros
Precambrian Research | 2004
Manoel S. D’Agrella-Filho; I. G. Pacca; Ricardo I. F. Trindade; Wilson Teixeira; M. Irene B. Raposo; T. C. Onstott
Precambrian Research | 2014
Franklin Bispo-Santos; Manoel S. D’Agrella-Filho; Liliane Janikian; Nelson Joaquim Reis; Ricardo I. F. Trindade; Maria Anna Reis
Precambrian Research | 2014
Franklin Bispo-Santos; Manoel S. D’Agrella-Filho; Ricardo I. F. Trindade; Liliane Janikian; Nelson Joaquim Reis
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Márcia Aparecida de Sant’Ana Barros
Universidade Federal de Mato Grosso
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