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Paleomagnetism and 40Ar/39Ar geochronology of the high-grade metamorphic rocks of the Jequié block, São Francisco Craton: Atlantica, Ur and beyond

2011; Elsevier BV; Volume: 185; Issue: 3-4 Linguagem: Inglês

10.1016/j.precamres.2011.01.008

1872-7433

Manoel S. D’Agrella-Filho, Ricardo I.F. Trindade, E. Tohver, Liliane Janikian, Wilson Teixeira, Chris M. Hall,

Geochemistry and Geologic Mapping

Paleomagnetic, magnetic anisotropy data and 40Ar/39Ar ages are presented for high-grade metamorphic rocks of the Jequié block, São Francisco Craton. Jequié charnockites and enderbites gneisses from the eastern border of the block present northern and steep-downward directions, carried by Ti-poor titanomagnetite with high unblocking temperatures (550–600 °C). A mean direction for 12 sites of enderbites from the eastern sector yielded a magnetic component A (Dm = 47.0°, Im = 75.7°, α95 = 6.3°, K = 48.8) with a corresponding paleomagnetic pole at 339.6°E, 5.4°N (A95 = 11.2°). Sites sampled on other metamorphic rocks including granulite-facies, tonalites and dacites yield different magnetic components. Anisotropy of magnetic susceptibility measured for all sampling sites shows a high degree of anisotropy (P = 1.121–1.881), with NE- or NW-trending magnetic lineations and vertical magnetic foliations. These data were used to correct the mean site directions for all components. While the other components presented larger scatter after anisotropy correction, the component A shows a slightly tighter clustering of magnetic directions (Dm = 61.2°, Im = 76.5°, α95 = 5.4°, K = 66.2) giving a new, anisotropy corrected paleomagnetic pole at 342.1°E, −0.5°N (A95 = 9.6°). 40Ar–39Ar plateau ages of 2035 ± 4 Ma (hornblende) and 1876 ± 4 Ma (biotite) obtained for one of the samples with component A imply a very low cooling-rate of 1.4 °C/Ma for these rocks. Based on these ages and corrected unblocking temperatures of the magnetic component A, we argue that the characteristic magnetization of the Jequié metamorphic rocks was acquired by high temperature thermo-chemical processes during regional cooling of the adjacent Itabuna–Salvador–Curaçá belt between 2089 and 1985 Ma (pole age likely at 1.99 Ga). Comparison of this pole with available paleomagnetic poles from South America and Africa suggests that neither Atlantica nor Ur have ever existed, and dispersed continental fragments dominated the paleogeography at ca. 2.0 Ga ago.