Conditions of meteoric calcite formation along a Variscan fault and their possible relation to climatic evolution during the Jurassic-Cretaceous

Two calcite cements, filling karst cavities and replacing Lower Carbonifero us limestones at the Variscan Front Thrust, were precipitated after mid-Jur assic Cimmerian uplift and subsequent erosion but before late Cretaceous st rike-slip movement. The first calcite (stage A) is nonferroan and crystals are coated by hematite and/or goethite. These minerals also occur as inclus ions along growth zones. The calcite lattice contains < 0.07 mol.% Fe, but Mn concentrations can be as high as 0.72 mol.% in bright yellow luminescent zones. Primary, originally one-phase, all-liquid, aqueous inclusions have a final melting temperature between -0.2 degrees and +0.2 degrees C, indica ting a meteoric origin of the ambient water. The delta(13)C and delta(18)O values of the calcites are between -7.3 parts per thousand, and -6.3 parts per thousand, -7.8 parts per thousand, and -5.5 parts per thousand on the V ienna PeeDee Belemnite (VPDB) scale, respectively. The second calcite (stag e B) consists of ferroan (0.13-0.84 mol.% Fe) blocky crystals with Mn conce ntrations between 0.34 and 0.87 mol.%. Primary, single-phase aqueous fluid inclusions indicate precipitation from a meteoric fluid below 50 degrees C. The delta(13)C values of stage B calcites vary between -7.3 parts per thou sand and -2.1 parts per thousand VPDB and the delta(18)O values between -7. 9 parts per thousand and -7.2 parts per thousand VPDB. A precipitation temp erature below 50 degrees C for the stage A calcites and the presence of iro n oxide/hydroxide inclusions in the crystals indicate near-surface precipit ation conditions. Within this setting, the geochemistry of the nonferroan s tage A calcites reflects precipitation under oxic to suboxic conditions. Th e ferroan stage B calcites precipitated in a reducing environment. The evol ution from the stage A to stage B calcites and the associated geochemical c hanges are interpreted to be related to the change from semiarid to humid c onditions in western Europe during late Jurassic-Cretaceous times. A change in humidity can explain the evolution of groundwater from oxic/suboxic to reducing conditions during calcite precipitation. The typically higher delt a(13)C values of the stage B compared to the stage A calcites can be explai ned by a smaller contribution of carbon derived from soil-zone processes th an from carbonate dissolution in the groundwater under humid conditions. Th e small shift to lower delta(18)O between stage A and B calcites may be cau sed by a higher precipitation temperature or a decrease in the delta(18)O v alue of the meteoric water. This decrease could have been caused by a chang e in the source of the air masses or by an increase in the amount of rainfa ll during the early mid-Cretaceous. Although the latter interpretation is p referred, it cannot be proven.