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The Mesozoic tholeiitic Ceará-Mirim dyke swarm has a general east-west trend cutting the Precambrian basement of northeastern Brazil. The dykes occur mainly in the State of Rio Grande do Norte (RN) but enter the neighbouring State of Ceará to the west where they trend SW-NE (Fig.1 ). This swarm was subdivided into six parallel sub-swarms by SIAL (1975, 1976) w~g attributed an Ear1y Cretaceous age to them. Available K-Ar radiometric dates (EBERT & BROCHINI, 1968; CORDANI, 1974; RODRIGUES, 1976; SIAL, 1976; HORN et ai., 1988; MACEDO, unpublished data) vary between 214 and 216 Ma. HORN et aI. (1988) used a procedure which allowed the removal of argon-Ioss effects to conclude that the ages might be situated between Middle Jurassic and Ear1y Cretaceous. Paleomagnetic data presented by GUERREIRO & SCHULT (1983) and BUCKER et aI. (1986), corresponding to samples from the eastern part of the swarm (between the cities of Lages and Angicos, RN), suggest that the emplacement of the sub-swarms was not simultaneous since they show distlnct rnagnetlzation directions. New paleomagnetic results that confirm the above conclusion are presented here for the western part of the swarm, where the dykes show a SW-NE structural orientation. Five parallel sub-swarms were identifled' each with its characteristic rnagnetization direction (Fig. 2), although sub-

Paleomagnetism of the Ceará-Mirim dyke swarm, northeastern Brazil