Journal of Economic Geology (Jul 2019)
Comparison of the Thermometery-Barometery methods by using Thermocalc and Theriak-domino to study Calc- silicate Hornfelses in Cheshin, Hamedan
Abstract
Introduction The pseudo-section method, first proposed by Hensen (1971), is used today by scientists to determine the thermodynamic conditions of mineral crystallization and modeling in metamorphic lithology (Hoschek, 2004; Omrani et al., 2013). The principle of this method is based on the fact that in order to have a complete chemistry of a rock sample with certain minerals in equilibrium, only one equilibrium pressure and temperature can be considered. However, the whole rock chemistry is fixed, mineral changes show some changes in temperature and pressure and new thermodynamic conditions. The superiority of using the pseudo-section method of temperature and pressure calculation is knowing the composition of minerals (microprobe data). By knowing the whole chemistry of the rock and the type of minerals found in the rock, the range of temperature and the pressure of rock formation can be determined. However, possession of mineral chemistry data and microprobe data can help us in more advanced computing and modeling. Accessing the whole rock's chemistry data is easier and calculating the temperature and pressure by pseudo-section methods has high capabilities (Moazzen et al., 2015). This paper investigates the results of this method for calc-silicate hornfelses of the Cheshne area of Hamedan. In this study, the theriak-domino program and the thermodynamic data database of minerals (Powell and Holland, 1988) have been used. In this research study, we tried to determine the temperature and the pressure for the studied rocks by using the two software: Thermocalc and Theriak-domino. Furthermore, the zoning of garnet and clinopyroxene have been studied by the pseudo-section method. Materials and methods After a careful examination of the area, a number of samples were taken. Sampling was done based on the collection of the best samples that represent the whole of the studied rocks. After the preparation of thin sections of suitable samples, according to the objective of the study, three samples of calc-silicate hornfels of Hamedan area were selected for chemical decomposition of the main elements. During the sampling, samples were taken from fresh and non-rough sections of the rock and were selected such as to represent the actual changes in their chemical composition and mineralogy. These samples were sent to Kanpajoh for analysis. The main elements were analyzed by the X-ray fluorescence (XRF) method. In addition, it should be noted that the data from chemical minerals, which was presented in the article by Ghorbani et al. (2016)+, was used in this study (Ghorbani et al., 2016). Results The major oxidative disintegrations in three samples of calc-silicate hornfelses show that the most abundant oxide in these samples is SiO2, which averages about 45.04. Decreasing calcium in the Grossular garnets can indicate a decrease in pressure. In fact, by describing the garnet in calc-silicate rocks in the Hamedan area, a prograde metamorphism has been created that has reduced the amount of this element after decreasing the temperature and pressure due to the retrograde metamorphism and the uplift and removal of pressure of the upper floors and the influence of the fluids. The combined variations of the garnets are in the Grossular range. The Garnets are the ultimate members of the Pyrope, the Almandine and the Spessartine poor (Ghorbani et al., 2016). The combination of clinopyroxenes in the En-Wo-Fs graph shows that most of the analyzed points are located within the diopside (Ghorbani et al., 2016). Discussion We supposed that all solid solution minerals were pure final members, and unit activity for solid solution phases (such as diopside and garnet) and pure phases (such as quartz). Then, the equilibrium reactions at 3.3 kb pressure were calculated by the Thermocalc software and plotted on P-T charts. The calculated temperature for the garnet and clinopyroxene minerals was calculated to be about 550 ° C and the calculated pressure was 2.5 to 3.5 kb. In this paper, using the whole rock chemistry and minerals chemistry, the method of calculating the temperature and pressure was applied to the pseudo-section method. For this purpose, the percentage of the main oxides was calculated as the molar percentage of the elements, then molar percentage of the elements was used as input for the Theriak-domino program. The pressure and temperature calculated by the Theriak-domino software package are from 2.5 to 3.5 kb and 500 to 550 degrees C, respectively. References Ghorbani, H., Moazzen, M. and Saki, A., 2016. Investigate the Mineral Chemistry and P-T Estimation of Formation of Diopside, Garnet and Coexisting Minerals in the Calc- silicate Hornfelses from the Alvand Metamorphic Aureole, Hamadan, West of Iran. Journal of Earth Science, 26(101): 139–146. (in Persian with English abstract) Hensen, B.J., 1971. Theoretical phase relations involving cordierite and garnet in the system MgO-FeO- Al2O3-SiO2. Contributions to Mineralogy and Petrology, 33(3): 191–214. Hoschek, G., 2004. Comparison of calculated P-T pseudosections for a kyanite eclogite from the Tauern Window, Eastern Alps, Austria. European Journal of Mineralogy, 16(1): 59–72. Moazzen, M., Torabi Asl, S. and Omrani, H., 2015. Temperature and pressure of metamorphism, phase stability and chemical zoning in garnet and biotite of metamorphic rocks of Jandag region using quasi-sectional method. Iranian Journal of Crystallography and Mineralogy, 23(4): 651–660. (in Persian with English abstract) Omrani, H., Moazzen, M., Oberhänsli, R., Tsujimori, T., Bousquet, R. and Moayyed, M., 2013. Metamorphic history of glaucophane-paragonite-zoisite eclogites from the Shanderman area, northern Iran. Journal of Metamorphic Geology, 31(8): 791–812. Powell, R. and Holland, T.J.B., 1988. An internally consistent thermodynamic dataset with uncertainties and correlations. III. Application methods, worked examples and a computer program. Journal of Metamorphic Geology, 6(2): 173–204.
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