An attempt to apply three geothermometers in the interpretation of low-temperature mylonites in the southern segment of the Tanlu fault zone

Abstract：Owing to lack of formation temperature data of mylonites, such problems as the formation depth, metamorphic evolution, deformation mechanism, exhumation history of mylonites, relation with host rocks and explanation of isotopic age data are difficult to interpret correctly and reasonably. The muscovite-chlorite geothermometer, muscovite-biotite geothermometer and chlorite compositional geothermometer that are suited to the low-temperature conditions have only been used in the interpretation of regional metamorphism of sedimentary rocks and have never been used for mylonites of ductile shear zones. In this paper an attempt is first made to apply the above-mentioned three geothermometers in mylonite by using syn-tectonic neogenic muscovite, biotite and chlorite of mylonites in a ductile shear zone in the southern segment (Anhui segment) of the Tanlu fault zone. According to the neogenic mineral assemblages and deformation behaviors of feldspars, the deformation temperatures of the Tanlu fault zone range between 350℃and 450℃. The temperatures obtained using the muscovite-biotite geothermometer and chlorite compositional geothermometer range between 672℃ and 1116 ℃ and between 470℃and 520 ℃ respectively, being higher than the deformation temperatures of mylonites in the ductile shear zone, which suggests that the two geothermometers are not applicable to calculation of the formation temperatures of low-temperature mylonite. The temperatures obtained using the muscovite-chlorite geothermometer coincide with the deformation temperatures of mylonites in the Tanlu ductile shear zone, which suggests that among the above-mentioned three geothermometeres, only the muscovite-chlorite geothermometer is applicable to the measurements of the formation temperatures of low-temperature mylonites.