A major Late Jurassic fluid event at the basin/basement unconformity in western France: 40Ar/39Ar and K-Ar dating, fluid chemistry, and related geodynamic context

Abstract : The Malm-Lower Cretaceous period is often considered to represent a period of time markedly devoid of crustal deformations, and of heat and mass transfers in western France, although numerous evidences of such transfers are reported in varied basins from western Iberia to western Africa. Migrating evaporitic-derived brines and seawater interacted with both the sedimentary cover and the basement in the Poitou High located at the boundary between Aquitaine and Paris Basins. Occurring after sedimentation, this activity induced significant alterations of the sediments by dolomitization and silicification, and the basement by adularization and locally by concentration of F-Ba (Pb-Zn) assemblages that are typical of carbonate-hosted metal deposits. Thus, metals were leached from metamorphic/granitic basement already weathered and peneplaned during Triassic time, and were redeposited by brines during this main fluid event. The main objective of this study was to decipher the timing of this major fluid/heat/mass transfer on the basis of 40Ar/39Ar and K-Ar dating of epigenetic minerals having similar paragenetic and geochemical signatures over an extended area from the Atlantic Ocean coast to the French Massif Central. A Late Jurassic fluid stage (146-156 Ma) significantly younger than what is generally reported in Western Europe was obtained. This fluid event occurred within a segment of a continental margin that was located away from main rift zones and far (more than 500 km) from major deformation areas such as the main rift or orogenic zones all over Western Europe. These results yield a new concept based on wide wavelength relationships between fluid movements and geodynamic events affecting both the sedimentary basins and their basements.