Polyhalite microfabrics in an Alpine evaporite mélange: Hallstatt, Eastern Alps

Area of Science:

• Geology
• Mineralogy
• Metamorphic Petrology

Background:

• Polyhalite rocks are found in tectonic lenses within the Alpine Haselgebirge Formation in the Hallstatt salt mine, Austria.
• These polyhalites exhibit diverse microfabrics analogous to metamorphic crustal rocks.

Purpose of the Study:

• To analyze the microfabrics of polyhalite rocks from the Hallstatt salt mine.
• To understand the metamorphic processes and formation conditions of these polyhalites.

Main Methods:

• Thin section analysis of polyhalite samples.
• Microstructural and fabric analysis.

Main Results:

• Identified four main polyhalite microfabrics: mylonites, metamorphic reaction fabrics, vein-filling, and cavity-filling.
• Polyhalite mylonites display shear fabrics indicative of ductile deformation.
• Metamorphic reaction fabrics involve polyhalite formation from blödite and anhydrite, suggesting a late-stage prograde event.

Conclusions:

• Polyhalite microfabrics in Hallstatt are comparable to those in metamorphic rocks.
• The study reveals complex formation histories, including ductile shear and later metamorphic reactions.