GEOTHERMOBAROMETRY OF GRANITIC MIGMATITE LEUCOSOMES FROM THE SOTOMÓ-CHAIQUENES HIGH GRADE METAMORPHIC COMPLEX, CHILE

The undated Sotomó-Chaiquenes High Grade Metamorphic Complex (SChMC) resides south of Calbuco Volcano, Chile, located within the Alerce Andino National Park. The SChMC consists of gneisses and schists with migmatites. Plagioclase, muscovite, and biotite from three migmatite leucosomes, two two-mica granites and one garnet muscovite granite, were analyzed using the EPMA from the Florida Center for Analytical Electron Microscopy, FIU. Twelve biotites, 11 muscovites, and 22 plagioclases were analyzed from one two-mica granite. Eleven biotites, 8 muscovites, and 13 plagioclases were analyzed from a second two-mica granite. Thirteen muscovites and 12 plagioclases were analyzed from the garnet muscovite granite. The first two-mica granite has an Ab range of 65-76%, muscovite X_Ms of 0.92-0.93, and biotite X_Fe of 0.38-0.45. The second two-mica granite has an Ab range of 66-75%, muscovite X_Ms 0.92-0.94, and biotite X_Fe 0.28-0.47. The garnet muscovite granite has an Ab range of 76-94%, muscovite X_Ms of 0.90-0.93, and the garnet is spessartine-rich almandine. The muscovites in all samples have primary compositions and do not indicate a hydrothermal origin. The biotites in the two-mica granites have compositions that are primary with slight re-equilibriation. The biotites plot in the peraluminous field on biotite discrimination diagrams. The spessartine-rich almandines along with the muscovite suggested the garnet muscovite granite is also peraluminous. The biotite thermometer of Luhr et al. (1984) was used to estimate crystallization temperatures of the two-mica granite leucosomes. The plagioclase-muscovite thermometer of Green and Usdansky (1986) was used to estimate the crystallization temperate of the garnet muscovite granite. The phengite barometer of Massonne and Schreyer (1987) was used to estimate the pressure of crystallization for all granite leucosomes. The two-mica granites yielded estimated temperatures and pressures of 641°C ± 13 and 5.59 kbar ± 0.41, and 644°C ± 32 and 6.56 kbar ± 0.48. The garnet muscovite granite yielded estimate temperature and pressure of ~708°C and 6.69 kbar ± 1.63. The peraluminous compositions and calculated temperatures and pressures suggest that the SChMC migmatites leucosomes are S-type granites formed from partial melting of continental crust at depth.