Abstract

&Ar/ 39Ar incremental heating analyses were carried out on tholeiitic dyke swarms of Santa Catarina (Plorian6polis) and Rio de Janeiro and alkaline dykes from the proximity of Rio de Janeiro City, southeastern Brazil. Most of the plagioclases from the tholeiitic dykes display disturbed age spectra, with younger apparent ages most often clearly correlated with alteration phases. However, a calculated weighted mean age from the less disturbed released fractions of the NE-oriented Santa Catarina dyke swarm gives an age of 129.4 f 0.3 Ma which is contemporaneous with the Ponta Grossa Dyke Swarm (PGDS) and at least the northern Paran6 Continental Flood Basalts (CPB). The Rio de Janeiro tholeiitic dykes display age spectra affected by alteration and probably excess argon, but with low-temperature ‘mini-plateau ages’ between 130.6 and 129.8 Ma, which are concordant with previous data obtained on the PGDS and Rio de Janeiro dyke swarms by Renne et al. (1996a) and Turner et al. (1994). These new geochronological data when added to the previous ones show that the initiation of Gondwana’s breakup in the region was characterized by a volcano-tectonic system with a geometry more complicated than the rift-rift-rift system suggested by many authors. The different rift arms of this structural feature were operating during a brief time period, mainly from 131.5 to 129 Ma, and were contemporaneous at least with the northern part of the Pam&Etendeka Province (PEP). Single grain and bulk sample biotite from the alkaline, mainly coast-parallel dyke swarm near Rio de Janeiro City display plateau ages around 80 Ma (81.8 f 1.8 and 78.8 f 2.8 Ma) and 70 Ma (72.5 f 0.5 and 69.7 f 0.2 Ma). These magmatic intrusions refer to Late Cretaceous flexuring and vertical movements and rift basin formations (Almeida, 1976) due the northward progressing rifting motions separating South America and Africa during South Atlantic Ocean formation.