Early Cretaceous Tono Plutonic Complex is the largest granitic body in the Kitakami Mountains. The Tono Plutonic Complex makes up a zoned pluton more felsic inward, which consists of the Marginal facies, Main facies, Central facies and gabbroic rocks. The Marginal facies is composed of quartz diorite and tonalite. The Main facies rocks are exposed widely and composed of granodiorite and tonalite. The Central facies rocks, which consist mainly of leucocratic granodiorite and trondhjemite, were intruded slightly later into the north-central part. The Main facies rocks make continuous trends in variation diagram of many elements. The Central facies rocks are characterized by low contents of K2O, Rb, Y, Zr, heavy rare earth elements(REE) and high contents of SiO2 and Na2O. Chemical compositions of the Marginal facies vary locally, but they change continuously to those of the Main facies as a whole. Light REE are enriched relative to heavy REE in the granitic rocks. The Central facies rocks have the chemical composition similar to the Archean high-Al TTD and TTG concerning major elements and many trace elements. Abundances of lithophile elements in the Main facies and Marginal facies rocks have characteristics similar to arc volcanic rocks. The Central facies rocks also have similar characteristics for incompatible elements, with low abundance of heavy REE and Y. The Main facies rocks were probably crystallized from a subduction-related magma which evolved under relatively shallow depth and genetically related to the Marginal facies. The Central facies rocks were solidified from a felsic magma distinct from the Main facies. The parental magma of the Central facies may have been produced by partial melting of basaltic source under high pressure and hydrous condition.
View full abstract