PROGRESSIVE SEQUENCE OF REACTIONS OF THE RYOKE METAMORPHISM IN THE YANAI DISTRICT, SOUTHWEST JAPAN - THE FORMATION OF CORDIERITE

The compositions of biotite and muscovite were examined in terms of th e paragenesis and the metamorphic grade in low-to medium-grade pelitic rocks of the Ryoke metamorphism in the Yanai district, southwest Japa n. The biotite and muscovite that coexist with K-feldspar have a highe r K component in an A'KF diagram than those in rocks lacking K-feldspa r. This fact reflects an increase in the K2O content in muscovite, but in biotite it reflects an increase of not only the K2O content bur al so of the octahedral vacancy. At higher metamorphic grade beyond the c ordierit-isograd, where cordierite coexists with neither chlorite nor K-feldspar, the biotite shows an increase in illite, K Al-iv square(-1 )(xii) Si-1, and Tschermak components, Al-vi Al-iv R-+2(-1) Si-1, wher e square(xii) and R2+ denote the interlayer vacancy and (Fe + Mg + Mn) , respectively. A reaction to define the cordierite isograd is propose d by treating this chemical change as being responsible for the rst ap pearance of cordierite, i.e. K,Al-poor biotite + phengitic muscovite = K,Al-rich biotite + cordierite + quartz + water. By treating this as a key reaction in medium-grade metamorphism, a set of reaction in a pr ogressive metamorphism is established for the Ryoke metamorphism, a ty pical low-pressure type metamorphism. Some textures in one of the high -grade areas, the K-feldspar-cordierite zone, suggest that a further t wo prograde reactions have taken place, i.e. andalusite + biotite + qu artz = cordierite + K-feldspar + water and and alusite = sillimanite. This implies that this zone probably has a P-T path involving isobaric heating.