40AR/39AR DATING OF THE ARAGUAINHA IMPACT STRUCTURE, MATO-GROSSO, BRAZIL

Pursuing the exploration of the Araguainha impact structure (Engelhardt et al., 1992), we present 40Ar/39Ar ages (1) of biotite samples from the granite, which forms the central uplift of the structure, and (2) of a melt rock, formed by the impact. Total degassing ages of biotites from granite samples range from 326 to 481 Ma. The variation is explained by Ar losses due to the oxidation of divalent Fe and by removal of K. The K loss depends on the time that the granite was exposed to weathering at particular outcrops. The oldest age of the least oxidized biotite from a granite sample, collected at a site most recently exposed, signifies that the ascending granite passed the 300 degrees isotherm earlier than 481 Ma ago. Early Devonian Furnas sandstones, the oldest sediments exposed by the impact, were deposited on this granite basement 410-396 Ma ago. The 40Ar/39Ar analyses of two size fractions of an impact melt rock, resulting in plateau ages of 245.5 +/- 3.5 Ma and 243.3 +/- 3.0 Ma, respectively, indicate that the Araguainha impact occurred close to the Permian-Triassic boundary. Pursuing the exploration of the Araguainha impact structure (Engelhardt et al., 1992), we present 40Ar/39Ar ages (1) of biotite samples from the granite, which forms the central uplift of the structure, and (2) of a melt rock, formed by the impact. Total degassing ages of biotites from granite samples range from 326 to 481 Ma. The variation is explained by Ar losses due to the oxidation of divalent Fe and by removal of K. The K loss depends on the time that the granite was exposed to weathering at particular outcrops. The oldest age of the least oxidized biotite from a granite sample, collected at a site most recently exposed, signifies that the ascending granite passed the 300 degrees isotherm earlier than 481 Ma ago. Early Devonian Furnas sandstones, the oldest sediments exposed by the impact, were deposited on this granite basement 410-396 Ma ago. The 40Ar/39Ar analyses of two size fractions of an impact melt rock, resulting in plateau ages of 245.5 +/- 3.5 Ma and 243.3 +/- 3.0 Ma, respectively, indicate that the Araguainha impact occurred close to the Permian-Triassic boundary. Pursuing the exploration of the Araguainha impact structure (Engelhardt et al., 1992), we present 40Ar/39Ar ages (1) of biotite samples from the granite, which forms the central uplift of the structure, and (2) of a melt rock, formed by the impact. Total degassing ages of biotites from granite samples range from 326 to 481 Ma. The variation is explained by Ar losses due to the oxidation of divalent Fe and by removal of K. The K loss depends on the time that the granite was exposed to weathering at particular outcrops. The oldest age of the least oxidized biotite from a granite sample, collected at a site most recently exposed, signifies that the ascending granite passed the 300 degrees isotherm earlier than 481 Ma ago. Early Devonian Furnas sandstones, the oldest sediments exposed by the impact, were deposited on this granite basement 410-396 Ma ago. The 40Ar/39Ar analyses of two size fractions of an impact melt rock, resulting in plateau ages of 245.5 +/- 3.5 Ma and 243.3 +/- 3.0 Ma, respectively, indicate that the Araguainha impact occurred close to the Permian-Triassic boundary.