Progressive sequence of reactions of the Ryoke metamorphism in the Yanai district, southwest Japan: the formation of cordierite

摘要: The compositions of biotite and muscovite were examined in terms the paragenesis metamorphic grade low- to medium-grade pelitic rocks Ryoke metamorphism Yanai district, southwest Japan. that coexist with K-feldspar have a higher K component an A′KF diagram than those lacking K-feldspar. This fact reflects increase K2O content muscovite, but it not only also octahedral vacancy. At beyond cordierite isograd, where coexists neither chlorite nor K-feldspar, shows illite, Aliv □xii−1 Si−1, Tschermak components, Alvi R+−1 □xii R+ denote interlayer vacancy (Fe+Mg+Mn), respectively. A reaction define isograd is proposed by treating this chemical change as being responsible for first appearance cordierite, i.e. K,Al-poor biotite+phengitic muscovite=K,Al-rich biotite+cordierite+quartz+water .By key metamorphism, set progressive established typical low-pressure type metamorphism. Some textures one high-grade areas, K-feldspar-cordierite zone, suggest further two prograde reactions taken place, andalusite+biotite+quartz=cordierite+K-feldspar+water and  andalusite=sillimanite.quartz=cordierite+K-feldspar+water This implies zone probably has P–T  path involving isobaric heating.