PALEOMAGNETISM AND AR-40 AR-39 DATING OF THE EARLY CRETACEOUS FLORIANOPOLIS DIKE SWARM (SANTA-CATARINA-ISLAND), SOUTHERN BRAZIL/

The Florianopolis (FL) basaltic dikes are well exposed on Santa Catari na Island, close to the present southeastern border of the Parana Magm atic Province in Southern Brazil, and their intrusion was related to t he tectonic and magmatic evolution of rifting in the southern Atlantic . The dikes are vertical or subvertical, and cut the crystalline basem ent rocks, mainly Late Proterozoic granites. Most of the dikes exhibit NE trends, corresponding to structural trends of the crystalline base ment. A few NW-trending dikes can also be observed, occasionally cross cutting the NE dikes. Samples from 73 dikes were collected for paleoma gnetic work, and subjected to both alternating field and thermal demag netization. Characteristic magnetizations of normal and reversed polar ities interpreted as original thermoremanences are carried by Ti-poor magnetites showing remanent coercive forces in the range 18-45 mT. The corresponding paleomagnetic pole (FL pole) is located at 3.3 degrees E, 89.1 degrees S (N = 65; alpha(95) = 2.6 degrees; k = 47), Samples f or Ar-40/Ar-39 dating were collected at the same paleomagnetic sites. Plagioclase samples from nine dikes were analyzed by stepwise heating; the apparent age spectra are strongly discordant due to excess Ar-40 contamination, but five samples yielded plateau ages ranging from 119. 0 +/- 0.9 Ma to 128.3 +/- 0.5 Ma. One additional sample yielded an iso chron age of 121.5 +/- 0.5 Ma. The inferred ages apparently define two modes at similar to 119-122 Ma and similar to 126-128 Ma, although th e data set is probably insufficient to determine whether dike intrusio n was continuous or episodic. The ages of 128-119 Ma are coeval with t he final stages of the rifting at this latitude, suggesting that the e mplacement of the FL dikes is related to extension of the continental crust just prior to the formation of oceanic crust. By comparing the F L pole with the existing paleomagnetic poles for the NW Ponta Grossa d ike swarm(mainly 130.5 Ma) to the north, the Parana volcanics (mainly 133-132 Ma), and the Central Alkaline Province (127-130 Ma) on the wes tern side of the Parana. Province it is concluded that the differences in pole positions reflect plate displacements, and therefore the FL p ole is probably dominated by the dikes with ages younger than 127 Ma. (C) 1998 Elsevier Science B.V. All rights reserved.