SHRIMP U-Pb dating of zircons related to the partial melting in a deep subduction zone: Case study from the Sanbagawa quartz-bearing eclogite

Dehydrated fluid from a subducting cold slab is considered to cause deep focused earthquakes and island‐arc volcanism. 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 the Sanbagawa metamorphic belt, central Shikoku, Japan. The discovery is significant because the melt may play an important role in deep‐focused earthquakes and the melt itself may directly contribute to the origin of island‐arc magmatism. 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 (SHT16, 75) 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 (SHT15, 76) 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.