DATING OF K-SULPHATES AND OF THEIR DEFORMATION STRUCTURES: PRELIMINARY RESULTS FROM GERMAN ZECHSTEIN AND ALPINE HASELGEBIRGE

Evaporite mélanges often form decollément surfaces of major extensional and contractional allochthons because of the very low shear resistance of halite. The most common cases of evaporite mélanges are such (1) at passive continental margins, where they are deformed during gravity-driven extension (raft tectonics), (2) in external foreland fold-thrust belts within a convergent geodynamic setting, and (3) in salt diapirs. In all these cases, halite is strongly deformed by late-stage deformation and only anhydrite-rich sulphate lenses preserve early deformational stages. Dating of associated K-sulphates may allow the recognition of early stages of deformation although this method is poorly applied. Knowledge of the limitations of K-sulphate chronometers (alunite, langbeinite and polyhalite occur in nature) may allow, therefore, dating of full history of evaporite mélanges.

Polyhalite [K2Ca2Mg(SO4)4·2H2O] commonly occurs in sedimentary evaporite successions. Under laboratory conditions, polyhalite can be synthesized by a reaction of gypsum with appropriate solutions in the ternary system K2SO4–MgSO4–H2O at room temperatures (Wollmann 2010, PhD Thesis University of Freiberg) and start dehydration at 255°C to a maximum of 355°C.

We successfully applied ⁴⁰Ar/³⁹Ar dating of polyhalite and langbeinite from German Zechstein and from Alpine Haselgebirge in Northern Calcareous Alps (NCA). Polyhalite from Morsleben gave an age of 28.68 ± 0.11 Ma, which may represent the age of crystal growth. Dating of deformed langbeinite from Neuhof gave a slightly scattered age pattern at ca. 150 Ma, implying recrystallization during ductile flow. In NCA, four different microstructural types of polyhalite rocks from Permian to Lower Triassic evaporites of Altaussee and Berchtesgaden salt mines have been dated. Polyhalite substituting euhedral halite in mudrock record an age of 234–235 Ma. Polyhalite in veins yield ages of ca. 232–233 Ma. Large polyhalite grains show two different growth stages at ca. 225 Ma (core) and ca. 208 Ma (rim), which we interpret to date two stages of fluid migration through the host anhydrite. Polyhalite from a mylonitic polyhalite rock gave a staircase pattern with an age at ca. 113 Ma (Cretaceous ductile shear) and an age of Ar loss at ca. 44 Ma consistent with regional tectonic events.