Fission track analysis of apatites from Sao Francisco craton and Mesozoic alcaline-carbonatite complexes from central and southeastern Brazil

Thermal histories on seven Brazilian apatites were obtained by fission track analysis using Monte Carlo simulations. The apatites were collected from two distinct geotectonic provinces. One group, originated from Sao Francisco craton, represents a typical cratonic domain with Proterozoic and Eopaleozoic rocks and yielded Permo-Triassic ages (counted since the instant when temperature was low enough so that the damage produced by fission tracks in apatite started to be preserved). The common thermal history accepted by all samples of this group is a linear cooling from similar to 90 to similar to 25 degrees C for the last 240 Ma, in agreement with present day thermal gradient and denudation rates. The other group, from Mesozoic alkaline-carbonatite complexes, in central and southeastern Brazil, yielded Cretaceous ages, close to those of the intrusions. For the samples of central Brazil, fission track analysis suggests a slow cooling from similar to 95 degrees to similar to 85 degrees C between 90 and 60 Ma ago, followed by a faster cooling from similar to 85 degrees to similar to 27 degrees C for the last 60 Ma. Otherwise, two trends exist for the samples of southeastern Brazil. The primary one is an increase in tempera ture from similar to 75 degrees to similar to 95 degrees C, which occurred between 140 and 60 Ma ago. In this period, there is also another trend: a cooling from similar to 100 degrees to similar to 80 degrees C. However, both trends are followed by a common thermal history during the last 60 Ma: a cooling from approximately similar to 80 degrees to similar to 25 degrees C. (C) 1997 Elsevier Science Ltd.