GEOLOGY AND GEOCHRONOLOGY OF CARBONATITES AND ASSOCIATED ALKALINE ROCKS PERIPHERAL TO THE PARANA BASIN, BRAZIL-PARAGUAY

Apatite and titanite fission-track ages have been determined for carbo natite complexes and associated alkaline rocks of the Paranaiba provin ce, Brazil, and the Amambay and Concepcion provinces, Paraguay. Close agreement between previously determined K-Ar ages and the apatite fiss ion-track ages indicates that the complexes were emplaced at shallow l evels and quickly cooled to the low temperatures (<70-degrees-C) requi red for the retention of fission tracks in apatite. The complexes in P aranaiba province, with the exception of the somewhat older Catalao I (114 Ma), were emplaced between 79 Ma and 88 Ma, in two apparently sep arate pulses of activity ca. 80 Ma and ca. 88 Ma. The alkaline complex es of the Amambay and Concepcion provinces exhibit a greater range of emplacement ages, from 86 Ma to 147 Ma, with most of the complexes emp laced between 111 Ma and 126 Ma. At both Chiriguelo and Cerro Sarambi, much younger ages obtained for transgressive carbonatite units indica te a renewal of the igneous activity that led to mineralization.