Natural and experimental fluorine substitution in biotite: Implications for fluid-rock thermochronometry and application to the Seridó Belt, northeastern Brazil

We found intra-grain F-in-biotite zonation within an orthogneiss sample from the Paleoproterozoic Caicó complex basement nearby the metasedimentary belt and intruded plutons. The fit of the natural profile by modeled profiles for cooling rates below 10 °C Myr−1, obtained from available Seridó Belt U-Pb and 40Ar/39Ar ages, indicates that the orthogneiss cooled from 475 °C. It suggests the existence of a thermal gradient as the inner part of the belt cooled from 623 °C.