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During Neoproterozoic times the Earth may have experimented several glacial events indicated by the presence of glacial deposits in several continents which are systematically capped by carbonates (cap carbonates ). Paleomagnetic studies indicate that some of these sequences have been deposited in low latitudes, leading to the idea that ice had covered the entire planet at these times. This hypothesis, the so-called snowball Earth hypothesis, evocates the most extreme climatic changes of the history of the planet which paved the way for the diversification of life species at the beginning of Cambrian times. Geochronologic and isotopic data of these rocks identify at least three glacial events during Neoproterozoic times. However, the available paleomagnetic data base is still insufficient to test the global extension of these deposits. According to the snowball Earth hypothesis, the transition from ice-house to green-house conditions must have been abrupt and rapid but this assumption remains speculative. The geographic and geochemical conditions of the depositional environment of these rocks, especially of the basal dolomitic sequence which could be used to test such scenarios, are also poorly documented. More paleomagnetic and geochemical data on well-preserved post-glacial Neoproterozoic successions are needed to properly address these issues. Paleomagnetic data obtained on the neoproterozoic carbonates of the Araras Group, Mato Grosso, revealed two different magnetic components. The bituminous limestones of the Guia Formation record a secondary magnetization acquired during the end of the Brasiliana orogenesis (∼520 Ma) and give a paleomagnetic pole at Lat = –82, 6◦ et Long = 292, 6◦ (N=13, A95=7,8, K=29,1). The basal dolostones of the Mirassol d’Oeste Formation, capping the glacial deposits of the Puga Formation (∼635 Ma), present primary magnetic signatures characterized by the occurrence of several geomagnetic reversals. The magnetic mineralogy study of these rocks identifies hematite and magnetite, both of detrital origin, as carriers of the magnetization. These results gives a paleomagnetic pole at Lat = 33, 6◦ and Long = 326, 9◦ (N=16 sites, A95=7,0, K=28,9) and indicate a low latitude deposition (±22◦) for the glacial deposits and constrain the paleogeographic position of the Amazon craton at these times. Chemoand magnetostratigraphic data confirm the primary nature of the dolostones and indicate that they have been deposited in a shallow and anoxic environment, possibly influenced by sulfate-reducing bacterial activity. Presence of several geomagnetic reversals in the Mirassol d’Oeste dolostones suggests a minimum interval of 105 years for the deposition of the cap dolostones. We suggest a microbially-mediated model for the precipitation of the primary dolomite, in an anoxic environment. In addition we discuss the paleogeographic context of neoproterozoic continental margins in the aftermath of a Neoproterozoic glacial event.

Paleomagnetismo dos carbonatos de capa do cráton amazônico (Brasil): implicações para as glaciações do neoproterozóico