6:15 PM - 7:30 PM
[SMP43-P08] Petrological studies of spinel and quartz-bearing paragneiss from Zayetkwin-Onzon area, central Myanmar
Keywords:spinel, paragneiss, P-T conditions, Mogok Metamorphic Belt, Myanmar
Coexisting spinel and quartz are newly found in a garnet-biotite gneiss collected from the Zayetkwin-Onzon area. This sample contains porphyroblastic garnet and cordierite, and biotite, plagioclase, quartz, and graphite in the matrix. Spinel and sillimanite coexisting with quartz, plagioclase, biotite, and ilmenite occur only as inclusions in cordierite. Spinel is a Zn-poor spinel-hercynite solid solution with XMg [= Mg/(Mg + Fe2+)] = 0.34 - 0.35, YAl [=Al/(Al + Fe3+)] = 0.97 - 0.99, TiO2 = 0.0 - 0.2 wt%, and ZnO = 1.8 - 2.3 wt%. The matrix assemblage gives pressure/temperature estimates of 0.7 - 0.8 GPa/780 - 840℃ using a garnet-biotite geothermometer and garnet-biotite-plagioclase-quartz geobarometer. Biotite grains in the spinel-bearing sample and associated paragneisses contain a distinctly high TiO2 content of up to 6.9 wt% (0.39 per formula unit for O = 11), which probably progressed mainly because of the TiR-2 substitution (R is the sum of divalent cations and represents vacancy in the octahedral sites). The fluorine content is up to 2.0 wt%, and the chlorine content is less than 0.1 wt%. The Ti-rich biotite suggests temperatures of 800℃ or higher if Ti is employed in the biotite geothermometer, as proposed by Henry et al. (2005).
The occurrence of a spinel-quartz-cordierite-sillimanite assemblage in the Zayetkwin-Onzon sample and the high-temperature estimates of around 800℃ suggest granulite facies equilibrium of the Mogok metamorphic rocks. Orthopyroxene-bearing garnet-gneisses were reported from the Mogok area, about 80 km NE of the Zayetkwin-Onzon area (Yonemura et al., 2013). These data suggest wide distributions of granulite facies metamorphic rocks in the northern part of the MMB.