In situ U/Pb dating of impact-produced zircons from the Vargeao Dome (Southern Brazil)

The VargeAo impact structure was formed in the Serra Geral basaltic and rhyodacitic to rhyolitic lava flows of southern Brazil, that belong to the Parana-Etendeka large igneous province. The Chapeco-type rhyodacites contain small baddeleyite crystals recently dated at 134.3 (+/- 0.8) Ma, which is regarded as the age of this acid volcanism coeval to the flood basalt eruption. Inside the impact structure, a brecciated rhyodacitic sample displays fine veinlets containing numerous lithic fragments in a former melt. This impact breccia contains newly formed zircons, either in the veins or at the contact between a vein and the volcanic matrix. The zircons are 1050m in length, clear and nearly unzoned. In situ laser-ablation dating of the zircons provides a concordant Early Aptian age of 123.0 +/- 1.4Ma that is regarded as the age of the impact event. As in situ age determination ensures the best possible selection of the analyzed mineral grains, the methodology employed in this study also represents a promising method for dating other impact structures.