Applicability of Geothermometer to Granitic Mylonites in Nabu Ductile Shear Zone,Southeast Guangxi
The measurement of metamorphic and deformation temperature of mylonites is of great significance for the interpretation of deformation evolution process and thermochronological isotope dating results.In this paper it only discusses the applicability of each geothermometers in the Nabu ductile shear zone.Using the plastic deformation characteristics,such as the mineral assemblage,dynamic recrystallization characteristics of minerals and EBSD(electron back-scattered diffraction)quartz c-axis fabrics in mylonites,the metamorphic deformation temperature range of the Nabu ductile shear zone is estimated to be 400 to 550℃.Based on the electron probe microanalysis(EMPA),the chlorite compositional geothermometer,muscovite-chlorite geothermometer,Ti-in-muscovite and Ti-in-biotite geothermometers were applied to calculate the metamorphic deformation temperature of the granitic mylonite samples in the Nabu ductile shear zone.The corresponding temperatures are 305 to 325℃,390 to 500℃,395 to 492℃and 473 to 565℃,431 to 574℃.Through comparative analysis,it is found that the results obtained by using chlorite compositional geothermometer are obviously lower than the temperature range estimated by using plastic deformation characteristics and the results calculated by other geothermometers.Combined with the results of previous studies,the chlorite compositional geothermometer is only suitable for the temperature calculation of low temperature metamorphic deformation.The results obtained by the other three geothermometers are all within the error range of the temperature estimated by using the plastic deformation characteristics,indicating that they are suitable for the calculation of metamorphic deformation temperature of the Nabu ductile shear zone.Among them,the muscovite-chlorite geothermometer is suitable for low temperature mylonite,and Ti-in-muscovite and Ti-in-biotite geothermometers are suitable for relatively high temperature mylonite.
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