Polyhalite microfabrics in an Alpine evaporite m¨¦lange: Hallstatt, Eastern Alps

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• 作者：Anja Schorn ; ^{anja.schorn2@stud.sbg.ac.at} ; Franz Neubauer¹ ; ^{franz.neubauer@sbg.ac.at} ; Manfred Bernroider² ; ^{manfred.bernroider@sbg.ac.at}
• 关键词：Polyhalite ; Microfabric ; Haselgebirge ; Metamorphic reaction ; Evaporite ; Vein
• 刊名：Journal of Structural Geology
• 出版年：2013
• 出版时间：January, 2013
• 年：2013
• 卷：46
• 期：Complete
• 页码：57-75
• 全文大小：5465 K

文摘

In the Hallstatt salt mine (Austria), polyhalite rocks occur in 0.5-1?m thick and several metre long tectonic lenses within the protocataclasite to protomylonite matrix of the Alpine Haselgebirge Fm..?Thin section analysis of Hallstatt polyhalites reveals various fabric types similar to metamorphic rocks of crust-forming minerals, e.g. quartz and feldspar. Polyhalite microfabrics from Hallstatt include: (1) polyhalite mylonites, (2) metamorphic reaction fabrics, (3) vein-filling, fibrous polyhalite and (4) cavity-filling polyhalite. The polyhalite mylonites contain a wide range of shear fabrics commonly known in mylonitic quartzo-feldspathic shear zones within the ductile crust and developed from a more coarse-grained precursor rock. The mylonites are partly overprinted by recrystallised, statically grown polyhalite grains. Metamorphic reaction fabrics of polyhalite fibres between bl?dite (or astrakhanite) [Na₂Mg(SO₄)₂.4H₂O] and anhydrite have also been found. According to previous reports, bl?dite may occur primarily as nodules or intergrown with l?weite. Reaction fabrics may have formed by exsolution, (re-)crystallisation, parallel growth or replacement. This fabric type was only found in one sample in relation with the decomposition of bl?dite at ca. 61?¡ãC in the presence of halite or slightly above, testifying, therefore, a late stage prograde fabric significantly younger than the main polyhalite formation.