Fluid-rock reactions in an evaporitic melange, Permian Haselgebirge, Austrian Alps

Tectonically isolated blocks of carbonate rocks present within the anhydrit ic Haselgebirge melange of the Northern Calcareous Alps record a complex hi story of deformation and associated deep-burial diagenetic to very low-grad e metamorphic reactions. Fluids were hot (up to approximate to 250 degrees C) and reducing brines charged with carbon dioxide. Individual carbonate ou tcrops within the melange record different regimes of brine-rock reactions, ranging from pervasive dolomite recrystallization to dedolomitization. Ear ly diagenetic features in these carbonates were almost entirely obliterated . Matrix dolomite alteration was related to thermochemical sulphate reducti on (TSR) recognized by the replacement of anhydrite by calcite + pyrite +/- native sulphur. Pyrite associated with TSR is coarsely crystalline and cha racterized by a small sulphur isotope fractionation relative to the precurs or Permian anhydrite. Carbonates associated with TSR show low Fe/Mn ratios reflecting rapid reaction of ferrous iron during sulphide precipitation. As a result, TSR-related dolomite and calcite typically show bright Mn(II)-ac tivated cathodoluminescence in contrast to the dull cathodoluminescence of many (ferroan) carbonate cements in other deep-burial settings. In addition to carbonates and sulphides, silicates formed closely related to TSR, incl uding quartz, K-feldspar, albite and K-mica. Ar-40/Ar-39 analysis of authig enic K-feldspar yielded mostly disturbed step-heating spectra which suggest variable cooling through the argon retention interval for microcline durin g the Late Jurassic. This timing coincides with the recently recognized sub duction and closure of the Meliata-Hallstatt ocean to the south of the Nort hern Calcareous Alps and strongly suggests that the observed deep-burial fl uid-rock reactions were related to Jurassic deformation and melange formati on of these Permian evaporites.