FISSION-TRACK ANALYSIS OF APATITES FROM SAO-FRANCISCO CRATON AND MESOZOIC ALKALINE-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 colle cted from two distinct geotectonic provinces. One group, originated fr om Sao Francisco craton, represents a typical cratonic domain with Pro terozoic and Eopaleozoic rocks and yielded Permo-Triassic ages (counte d since the instant when temperature was low enough so that the damage produced by fission tracks in apatite started to be preserved). The c ommon thermal history accepted by all samples of this group is a linea r cooling from similar to 90 to similar to 25 degrees C for the last 2 40 Ma, in agreement with present day thermal gradient and denudation r ates. The other group, from Mesozoic alkaline-carbonatite complexes, i n central and southeastern Brazil, yielded Cretaceous ages, close to t hose of the intrusions. For the samples of central Brazil, fission tra ck analysis suggests a slow cooling from similar to 95 degrees to simi lar to 85 degrees C between 90 and 60 Ma ago, followed by a faster coo ling from similar to 85 degrees to similar to 27 degrees C for the las t 60 Ma. Otherwise, two trends exist for the samples of southeastern B razil. 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 tren ds are followed by a common thermal history during the last 60 Ma: a c ooling from approximately similar to 80 degrees to similar to 25 degre es C. (C) 1997 Elsevier Science Ltd.