Measurements of the K-Ar ages of the eight trachybasaltic dikes from the R¿o de Los Molinos zone, Argentina, suggest that the dikes were intruded during at least two magmatic episodes at about 140 and 65 m.y. B.P. Normal and reversed magnetic remanences are observed in the older dikes; these suggest that during the older magmatic episode at least two events of intrusion occurred; a minimum period of 5000 years must have elapsed between these intrusions. The pole computed for the Upper Jurassic-Lower Cretaceous dikes (79¿S, 8¿E, α95=18¿) is reasonably close to the paleomagnetic poles of the Vulcanitas Cerro Colorado formation (118 m.y.) and the Serra Geral Formation (115--125 m.y.). This supports the interpretation that the South Atlantic Ocean was opened in Lower Cretaceous times.