We discovered and mapped a 1200 km-long zone of enhanced electrical conductivity through a large array of magnetometers, over a narrow corridor parallel to the Paraguay and Araguaia belts which surround the Amazon craton. The anomaly has been further studied using magnetotelluric measurements made along a 300 km profile crossing the Paraguay belt; modeling shows the anomaly at this location as a very strong 100 km-wide upper- to mid-crust conductor, located under a region where surficial rocks show an increase in deformation and metamorphism within the belt. From the high conductivity of the anomalous structure, its observed geometry and surface rock exposures, the most likely interpretation of its source is graphitized biogenic material in metasediments, now deeply underthrust in a Neoproterozoic or Early Cambrian suture zone. This result strongly supports the hypothesis that the collision between the Amazon plate and the western Paraná block has probably closed an ocean in the Paraguay belt region, contrary to former propositions of an ensialic evolution for this belt. Although the precise age of this ocean closure have been a matter of debate, our finding significantly constrains the tectonic setting of West Gondwana amalgamation involving the Amazon plate and its surrounding blocks.