Mafic dyke swarms at 1882, 535 and 200 Ma in the Carajas region, Amazonian Craton: Sr-Nd isotopy, trace element geochemistry and inferences on their origin and geological settings

摘要

The Carajas-Rio Maria region, together with the Rio Maria domain of the Central Amazonian province, comprises the eastern margin of the Amazonian Craton with the Neoproterozoic Araguaia belt. This region hosts several basaltic dyke swarms whose U-Pb baddeleyite ages highlighted three intrusive events at 1882, 535 and 200 Ma. New geochemical and Sr-Nd isotopic data were obtained for the different groups of the Carajas dykes allowing new insights on i) the mantle source composition beneath the Carajas region through time and ii) the geodynamic setting of the intrusive events. The 1882 Ma swarm is coeval to the Uatuma SLIP event which is one of the oldest intraplate events of the proto-Amazonian craton. Trace elements and isotopic values suggest that the dyke parent melt for those dykes have a crustal component derived from a sedimentary source similar to GLOSS (GLObal Subducting Sediment compositions). This is consistent with the emplacement of the dykes in a supra-subduction setting or in a post-collisional setting. Trace and isotopic values of the 535 Ma dyke swarm are consistent with an enriched mantle source from EMII component. These geochemical features suggest an enrichment of the mantle from an oceanic lithosphere poor in sediments, different to that of the 1882 Ma source. The age of this swarm matches magmatic activity during a post-collisional extensive-transtensive event recorded in the marginal Araguaia belt after the amalgamation of the Amazonian Craton to the Western Gondwana during Neoproterozoic. The 200 Ma dyke swarm which is related to the CAMP (Central Atlantic Magmatic Province) and opening of the Atlantic Ocean shows trace element composition similar to Atlantic E-MORB. The coupled isotopic values are consistent with an enriched mantle source with EMII component. These particular geochemical features suggest that the plume activity responsible for the CAMP near the rifting zone has not affected the mantle beneath the Carajas region.