Thursday, November 12, 2015

The PaleoPosition of the Amazonian Craton in the Calymmian MesoProterozoic

Reassessment of Aguapeí (Salto do Céu) Paleomagnetic pole of the Amazonian Craton and implications for Proterozoic supercontinents


D’Agrella-Filho et al


The Aguapei paleomagnetic pole obtained for mafic sills and dykes from Salto do Céu region at the western margin of the Amazonian Craton constrains its links with Baltica and Laurentia in Rodinian reconstructions. A new U-Pb age on baddeleyites at 1439 ± 4 Ma for the intrusives, constrasts strongly with a previous Ar-Ar age at 981 ± 2 Ma, with important consequences for paleogeographic reconstructions. We report new paleomagnetic and magnetic anisotropy results for sills from the Salto do Céu region and reassess the paleomagnetic data in view of the new geochronological age. A total of 155 samples were collected from thirteen new paleomagnetic sampling sites, five of them corresponding to sedimentary rocks located at the borders of the sills in an attempt to perform baked contact tests. After thermal and alternating field demagnetization, the sills provided a characteristic magnetic component at Dm = 208.2°, Im = 68.5° (N = 8, α95 = 6.4°), with a corresponding paleomagnetic pole at 46.4°S; 277.0°E (A95 = 10.2°). Directions obtained in this study are similar to those reported previously for other mafic sills and dykes in the same region. A pole integrating the new results for the sills with those of the previous Aguapeí pole is situated at 56.0°S; 278.5°E (A95 = 7.9°). This new combined Salto do Céu pole supersedes the previous Aguapeí pole. Magnetic mineralogy studies, optical and electronic microscopy indicates PSD magnetite as the main magnetic carrier in these rocks. The baked contact tests were inconclusive, but the similarity between Salto do Céu pole and other high-quality poles with ages around 1420-1430 Ma suggests they carry a primary thermoremanence of that age. The Salto do Céu and other coeval poles are compatible with a connection between Amazonia and Baltica at the Mesoproterozoic in a paleogeographic configuration slightly different from SAMBA (South America-Baltica). At the same time, the new geochronological and paleomagnetic data imply that all paleogeographic interpretations for the position of Amazonia in Rodinia based on the previously published Aguapeí pole (and now renamed as Salto do Céu pole) must be revised.

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