2008-02-27

第３０５回 †

日時　平成１９年２月２７日　水曜日　午後５時より
場所　東京工業大学　石川台６号館　４０４号室　

＜講師＞　Simon R. Wallis　先生　（名古屋大学　地質・地球生物学講座）
＜題名＞　「Partial melting as a fundamental process in the formation and exhumation of the Sulu UHP belt」

The Sulu-Dabie ultrahigh-pressure (UHP) belt is one of the great coesite-bearing terrains of the world. The constituent continental rocks are mainly acidic gneiss with blocks of dense eclogite. These rocks have been subducted to depths in excess of 90 km within the mantle and similar regions are an important part of many continental collision zones. Petrologically estimated peak metamorphic conditions suggest it is likely that the UHP gneisses melted, but very little evidence has been presented to support this idea, and UHP rocks are generally considered to have remained in a solid state throughout their evolution. In the Sulu region, the presence of potassium feldspar-rich intrusions synchronous with UHP metamorphism at around 220 Ma combined with Rb-Sr isotopic data provides concrete evidence in favour of synmetamorphic partial melting. Partial melting of acidic gneisses can account for their lack of well-preserved UHP minerals. U-Pb zircon dating suggests UHP conditions were maintained between 240-220 Ma. Lu-Hf garnet dating suggests eclogite facies metamorphism occurred at 241 Ma. Thermal modeling shows the gap of 20 my between eclogite facies metamorphism and the onset of melting under UHP conditions is a suitable time scale for significant heating after continental collision, which could lead to melting. Partially molten gneiss would be both weak and buoyant in the mantle．This combination can help explain the problem of how to exhume UHP terranes and predicts a thick UHP root zone to continental collision zones. The size of dense eclogite blocks within the molten gneisses can be used to constrain exhumation rates and the viscosity of the molten continental crust.