隠岐島後の泥質ミグマタイトに含まれる流体包有物の研究

その他

This study examined composition of fluid inclusions in leucocratic and melanocratic parts of pelitic migmatites from Oki-Dogo to understand fluid activity throughout P-T history of high-T/low-P metamorphic terrane. Calculated fluid density and isochores range from 0.73 to 0.84g/<cm>^^^3 and from 450 to 540℃ at 3kbar, respectively, for H_2O, and from 0.7l to 0.87g/<cm>^^^3 and from 540 to 860℃ for CO_2 inclusions. The temperature range for the
CO_2 inclusions is almost consistent with previously estimated peak and retrograde conditions ; 750～820℃ at l.5～4kbar for peak and 580～630℃ at 2.9～3.9kbar for retrograde conditions. The range for the H_2O inclusions is, however, significantly lower than the retrograde condition. Our results therefore suggest that CO_2-bearing fluid was dominant during peak to retrograde metamorphism, while H_2O-rich fluid infiltrated at the last stage of metamorphism.
Graphite is the most probabie source of CO_2 because the CO_2 inclusion becomes dominant with increasing modal abmdance of graphite in migmatites. Origin of H_2O is not clearly understood, but probably associated with an uplifting of metamorphic terrane providing H_2O-bearing hydrothermal fluid from middle crust. The later H_2O infiltration brought about hydration of garnet and biotite to chlorite and muscovite, which marks the end of thermal events of Oki metamorphic rocks.