The amalgamation of the southern Río de la Plata craton involves two possibly coeval Rhyacian sutures associated with the Transamazonian orogeny,rather than a single one as previously envisaged,i.e.the El Cortijo suture zone and the Salado suture.We circumscribe the Tandilia terrane to the region between these two sutures.The El Cortijo suture zone runs along a roughly WNW oriented magnetic low aligned along the southern boundary of the Tandilia terrane,i.e.boundary between the Tandilia and Balcarce terranes.This extensive magnetic low,ca.300 km long,and ca.90 km wide,would be caused by demagnetization associated with shearing.At a more local scale,the trend of the El Cortijo suture zone often turns toward the EeW.At this scale,WNW trending tholeiitic dykes of Statherian age are seen to cut the Rhyacian El Cortijo suture zone.Spatially associated with the El Cortijo suture zone,there are small magnetic highs interpreted to be related to unexposed basic bodies of ophiolitic nature related to those forming part of the El Cortijo Formation.We envisage the pre-Neoproterozoic evolution of the Tandilia belt to have been initiated by the extension of Neoarchean(w2650 Ma)crust occurred during Siderian times(2500e2300 Ma),causing the separation between the Balcarce,Tandilia and Buenos Aires terranes,and the development of narrow oceans at both north and south sides of the Tandilia terrane,accompanied by w2300e2200 Ma sedimentation over transitional econtinental to oceanice crust,and arc magmatism developed in the Tandilia terrane.The island arc represented by the El Cortijo Formation was also developed at this time.At late Rhyacian times,it occurred in both the closure of the narrow oceans developed previously,the entrapment of the El Cortijo island arc,as well as anatectic magmatism in the Balcarce terrane. The amalgamation of the southern Río de la Plata craton involves two possibly coeval Rhyacian sutures associated with the Transamazonian orogeny, rather than a single one as previously envisaged, iethe El Cortijo suture zone and the Salado suture. We circumscribe the Tandilia terrane to the region between these two sutures. The El Cortijo suture zone runs along a roughly WNW oriented magnetic low aligned along the southern boundary of the Tandilia terrane, ieboundary between the Tandilia and Balcarce terranes. This extensive magnetic low, ca.300 km long, and ca .90 km wide, would be caused by demagnetization associated with shearing. At a more local scale, the trend of the El Cortijo suture zone often turns toward the EeW. At this scale, WNW trending tholeiitic dykes of Statherian age are seen to cut the Rhyacian El Cortijo suture zone. Spatially associated with the El Cortijo suture zone, there are small magnetic highs interpreted to be related to unexposed basic bodies of ophiolitic nature rela ted to those forming part of the El Cortijo Formation. We envisage the pre-Neoproterozoic evolution of the Tandilia belt to have been initiated by the extension of Neoarchean (w2650 Ma) crust occurred during Siderian times (2500e2300 Ma), causing the separation between the Balcarce, Tandilia and Buenos Aires terranes, and the development of narrow oceans at both north and south sides of the Tandilia terrane, accompanied by w2300e2200 Ma sedimentation over transitional econtinental to oceanice crust, and arc magmatism developed in the Tandilia terrane. Island by the El Cortijo Formation was also developed at this time. Late late Rhyacian times, it occurred in both the closure of the narrow oceans previously previously, the entrapment of the El Cortijo island arc, as well as anatectic magmatism in the Balcarce terrane.