西彼杵半島長崎変成岩類の岩石学的研究 : とくに緑色岩複合岩体と反応促進型延性変形について  [in Japanese] Petrological study of the Nagasaki metamorphic rocks in the Nishisonogi Peninsula : with special reference to the greenrock complex and the reaction-enhanced ductility  [in Japanese]

The Nagasaki metamorphic rocks in the Nishisonogi Peninsula, western Kyushu, mainly consists of crystalline schists with small amounts of serpentinite and the greenrock complexes, which have a character of serpentine melange. The crystalline schists belong to the epidote-glaucophane subfacies, characterized by the common presence of glaucophane + hematite + rutile in graphite-free psammitic schists and by minor occurrence of epidote + glaucophane in metabasites. The greenrock complexs are thin (350 m in maximum) and elongated (8 km in maximum) masses in tectonic contacts with surrounding crystalline schists, although the general trends (N-S) of the masses are nearly the same as those of the crystalline schists. A small greenrock complex located in Nabezaki, occurs as a core of the recumbent fold of the crystalline schists, clearly indicating the tectonic origin of the complex. The greenrock complexes consist of variety of rocks such as epidote-glaucophane-barroisite schist, metagabbro, albitite, hornblendite, clinopyroxenite, omphacite-bearing rocks, and pelitic schists. These rocks occur as discrete platy or wedge-shaped bodies embedded in ultramafic schists, and usually show distinct flow folding. Some rocks such as jadeitite, rodingite, and epidote-garnet-crossite-barroisite rock occur as tectonic blocks included in ultramafic schists and serpentinite. The ultramafic schists, usually chlorite-actinolite schist, play a major role for the emplacement of the complex and also for the formation of the internal structure of the complex. They originate from the reaction zone developing between serpentinite- and metasediments/metabasites, and act as lubricants for the tectonic transposition of the constituent rocks of the complex. Coupling of deformation and reaction may explain the high ductility of the ultramafic schists. Two different mechanisms, hydrofracturing and reaction-enhanced ductility, can operate at the same time on the deformation of the reaction zone during its formation. The excess pore fluid pressure due to the devolatilization reactions will cause the hydrofracturing in the wall rock adjacent to the reaction zone, if the porosity of the wall rock is small. On the other hand, the porosity of the reaction zone is high enough to let the fluid escape through the reaction zone according to Darcy's law. The reaction zone will shrink due to the rearrangement of minerals according to the withdrawal of the fluid. Thus, the excess pore fluid pressure caused by the devolatilization reaction will enhance the ductility of the reaction zone.