ZIRCON CHRONOLOGY AND GARNET CHEMISTRY RELATED TO DEHYDRATION MELTING IN A DEEP SUBDUCTION ZONE: CASE STUDY FROM THE SANBAGAWA QUARTZ-BEARING ECLOGITE

The Sanbagawa high P–T metamorphic rocks have been considered as typical cold oceanic material subducted during the Cretaceous. However, we have discovered an eclogite outcrop exhibiting partial melting texture. In order to confirm the age of partial melting of Sanbagawa metamorphic rocks, we dated zircons from both the melted portion and the host eclogite using the U–Pb SHRIMP age-dating at the Korean Basic Science Institute. Zircons from the melted portion are rounded and have sector zoning. The core and mantle yield U–Pb age in the 130–113 Ma range (120 Ma average), and the rim ages are in the 115–104 Ma range. Zircons from the eclogite have a homogenous core with thin mantle and rims. The U–Pb ages are concentrated at 123–112 Ma. These pieces of evidence suggest that the eclogite metamorphism occurred at about 120 Ma and subsequent partial melting happened at about 110 Ma. REE concentration of the zircon core and mantle indicates the eclogite facies recrystallization. Rims of the zircons decrease HREE and increase MREE, suggesting decomposition of phengite and omphacite and formation of melt and garnet. Garnet REE signature supports the dehydration reaction.