チェコ共和国・ボヘミア地塊のZrcadlová Hut’採石場に産する珪長質グラニュライトが経験した最高温度圧力条件

<p>Several conflicting pressure-temperature (P-T) conditions of granulite in the Bohemian Massif have been proposed, and we estimated a P-T path of felsic granulite from the Zrcadlová Hut’ of the Blanský les body in the Bohemian Massif. The felsic granulite is mainly composed of garnet, biotite, quartz, plagioclase, and potassium feldspar, with or without pyroxenes. Garnet in two granulite samples occurs as porphyroblasts and preserves prograde zoning. Some of the porphyroblasts contain kyanite as inclusions. These felsic granulites also contain coronitic garnet surrounding spinel + plagioclase symplectites. These symplectites can be considered as pseudomorphs after kyanite. The pyroxene-bearing felsic granulite sample also contains garnet with prograde zoning, although the garnet grains are relatively small (<2 mm in diameter). The peak P-T conditions of the felsic granulite containing kyanite inclusions in prograde zoned garnet were estimated to be >1100 °C, 2.3 GPa based on the combination of garnet-biotite geothermometer and garnet-aluminosilicate-quartz-plagioclase barometer. The pyroxene-bearing felsic granulite contains clinopyroxene + plagioclase symplectite that has formed during a low-pressure stage, and its P-T conditions were estimated to be 750 °C, 0.5-0.8 GPa, by orthopyroxene-clinopyroxene geothermometer and jadeite-quartz-albite barometer. It is likely indicating that the pressure was rapidly reduced from the maximum-pressure stage, and then the relatively rapid cooling occurred. Such rapid decompression and cooling are needed to retain the prograde zoning in garnet. Index minerals of ultrahigh pressure such as coesite are not found. Although ultrahigh pressure conditions were not reached, the estimated pressure is relatively high (>2.0 GPa) compared with previous studies.</p>