Fluid–rock reactions in an evaporitic mélange, Permian Haselgebirge, Austrian Alps

Abstract

Tectonically isolated blocks of carbonate rocks present within the anhydritic Haselgebirge mélange of the Northern Calcareous Alps record a complex history of deformation and associated deep‐burial diagenetic to very low‐grade metamorphic reactions. Fluids were hot (up to ≈ 250 °C) and reducing brines charged with carbon dioxide. Individual carbonate outcrops within the mélange record different regimes of brine–rock reactions, ranging from pervasive dolomite recrystallization to dedolomitization. Early diagenetic features in these carbonates were almost entirely obliterated. Matrix dolomite alteration was related to thermochemical sulphate reduction (TSR) recognized by the replacement of anhydrite by calcite + pyrite ± native sulphur. Pyrite associated with TSR is coarsely crystalline and characterized by a small sulphur isotope fractionation relative to the precursor 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)‐activated 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, including quartz, K‐feldspar, albite and K‐mica. 40Ar/39Ar analysis of authigenic K‐feldspar yielded mostly disturbed step‐heating spectra which suggest variable cooling through the argon retention interval for microcline during the Late Jurassic. This timing coincides with the recently recognized subduction and closure of the Meliata‐Hallstatt ocean to the south of the Northern Calcareous Alps and strongly suggests that the observed deep‐burial fluid–rock reactions were related to Jurassic deformation and mélange formation of these Permian evaporites.