Geochemical Characteristics and Variations of the Ryoke Granitoids, Southwest Japan: Petrogenetic Implications for the Plutonic Rocks of a Magmatic Arc

Geochemical characteristics and variations of the Ryoke granitoids, southwest Japan, have been elucidated, based on more than 70 analyses from the Mikawa and Kinki districts for major and trace elements including REE. The Ryoke granitoids are the products of a continental-margin arc magmatism of Pacific Asia during late-Cretaceous period, and represent its magmatic front. They are classified into the Older and Younger groups, according to their intrusive ages and textural and petrographic characteristics. The Older Ryoke granitoids were emplaced at ca. 20 km depth during 100-85 Ma, and deformed and recrystallized together with the host Ryoke metamorphic rocks. The Younger Ryoke granitoids are post-tectonic plutons, emplaced at ca. 15 km depth (85-75 Ma), and accompanied by the formation of contact aureoles. The majority belongs to I-type and ilmenite-series granitoids. The granitoids of both groups are geochemically quite similar: calcic (alkali-lime index ∼65), very low in Fe+3/Fe+2 ratio and both metaluminous and peraluminous. They have low Rb (<100 ppm) and actinide contents, and moderate abundances of other LIL and HFS elements. They fall exclusively within the volcanic arc granite field in all the trace element discrimination schemes. Trace element geochemistry and chondrite-normalized REE patterns suggest that the I-type Ryoke granitoids were generated by the dehydration melting of amphibolite, of basaltic and basaltic-andesitic composition, or hydrous melting of tholeiitic basalt at 1 GPa and higher pressures. The heat-source for the melting of protoliths should be the underplating basalts, of which remnants are seen as the mafic rocks enclosed and/or intruded by the Ryoke granitoids.