40Ar/39Ar GEOCHRONOLOGY APPLIED TO DATING CRETACEOUS MAGMATIC EVENTS IN SOUTH AMERICA

Abstract

This is a collaboration project between the Ar laboratory of the São Paulo University and the Berkeley Geochronological Center (BGC) situated in the California (USA). It comprises two complementary parts, one related to the support, training and improvement of the analytical methods adopted by the São Paulo Ar Laboratory, and other complementary part investigating the timing of the Atlantic Ocean opening by using 40Ar/39Ar data on continental tholeiitic basalts from the southern and equatorial margins of the South American continent. The geological materials used in the research are the mafic lavas of the Paraná magmatic province and, in the northeast Brazil, the Rio Ceará-Mirim dyke swarm and Sardinha magmatism (Parnaíba basin). 40Ar/39Ar ages from flood basalts situated at the margin of the Paraná magmatic province indicated that the opening of the Southern Atlantic began at c. 133 Ma. In contrast, the timing of rifting in the equatorial Atlantic margin is poorly constrained in view of the paucity of 40Ar/39Ar ages for the Rio Ceará-Mirim and Sardinha basalts. Most of the available geochronological data refers to K-Ar ages spanning over 30 million years, which conflicst with the geologically rapid eruptions that formed the Paraná province. In order to fill this temporal hiatus, we intend to define precisely the ages of the magmatic activity related to the rifting stages of the equatorial segment, and to refine the duration of the rift-related Paraná magmatism through a broader sampling of the exposed basalts. These new results will be combined with other geological evidences to further provide more comprehensive information about the Atlantic tectonic evolution.The BGC was chosen as research partner based on its commitment with the development of the 40Ar/39Ar method. The relevant scientific contributions of the BGC staff include: 1) calibration of international Ar-dating standards, 2) re-evaluation of the 40K decay constant and 37Ar half-life, 3) optimization of irradiation procedures and 4) high-precise measurements of geological time boundaries. Therefore, besides the geological aspects examined in the project the scientific cooperation with the BCG must provide the necessary expertise and improvements of the Ar/Ar laboratory at the São Paulo University.