The pseudotachylite from Champagnac in the Rochechouart Meteorite Crater, France
Authors: Reimold W. U. , Oskierski W. , Huth J. 
Published: February 2008

As part of a comprehensive study on pseudotachylites from established impact structures, cryptoexplosion structures, and diverse tectonic settings, a vein system containing melt rock and hydrothermal crystallizations from the Champagnac quarry in the basement of the Rochechouart impact structure, northwest Haut Limousin, France was characterised by petrographic and geochemical means. The lack of characteristic shock phenomena in melt clasts (deformation is restricted to kink bands in mica and local planar fracturing in quartz and feldspar), the angular shapes of clasts, and the structural field relations lead to the conclusion that the melt fraction of these vein fillings from the Champagnac quarry represents pseudotachylite produced by friction during lateral shear movements. These veins differ from the impact breccia dyke fillings (Al breccias) reported by Lambert (1981). Bulk chemical analysis of vein fillings and defocussed beam electron probe results obtained on glassy pseudotachylite favor in situ formation of melt and rule out the possibility that it represents injected impact melt. Compositional variations are observed in this vein system and are interpreted to result from varied contributions of mafic and felsic components (mica plus opaque minerals versus quartz and feldspar). Glass analyses indicate that total melting as well as partial melting of parent rock occurred. According to petrographic evidence, the temperature at melt formation is estimated at 700–750°C, and the pressure is estimated to have been less than 1 GPa. The hydrothermal component of vein fillings was generated simultaneously with the pseudotachylite melt and produced distinctive petrographic and geochemical effects within the vein system: (1) sericitization in clasts and adjacent host rock, which altered andesitic feldspar to pure albite; and (2) selective removal of Sr from vein fillings. A Rb‐Sr isotope study on vein fillings yielded consistently low Sr and 87Sr contents, but highly varied Rb/Sr ratios and an apparently well‐defined isotopic equilibration among seven vein‐filling samples. An apparent 217‐m.y.‐isochron results. It is shown that the Rb‐Sr data cannot be explained by mixing processes and are not in agreement with normal radiogenic evolution of the Sr isotope system. Isotopic equilibration by diffusion seems improbable. Therefore the following genetic process for the Champagnac pseudotachylite vein system is suggested: (1) Clastic breccias were formed by impact‐induced lateral slip along preexisting planes of weakness. Locally these breccias were transformed into pseudotachylite by friction melting. (2) Circulation of hydrothermal fluids (700°C, high PH2O) led to sericitization of plagioclase of the breccia clasts and immediate host rocks and modified the Sr isotopic system of pseudotachylite. (3) During the subsequent cooling period hydrothermal phases crystallized and the various textures of pseudotachylite matrix and clasts developed, before (4) long‐term tectonic activity (crosscutting fractures) and additional hydrothermal modifications took place.