Late Cretaceous age of eclogite facies metamorphism of the Sanbagawa belt in the Asemi River area, Shikoku (SW Japan): Evidence from detrital zircon

摘要

Within the Sanbagawa metamorphic belt, the timing of high-pressure and retrograde metamorphism is controversial. Recently, an area in the Asemi River region (central Shikoku, Japan) was delineated that had been subjected to ecologite fades metamorphism prior to retrograde metamorphism in the epidote-amphibolite facies which provides a new opportunity to constrain the age of high-pressure metamorphism. Detrital zircon from this area yields ages as young as 84 +/- 4 Ma. Hence, the protoliths were deposited not earlier than ca. 88 Ma and thus, eclogite facies and retrograde metamorphism occurred after 88 Ma, probably at 86-82 Ma as indicated by the age of metamorphic zircon and K-Ar and Ar-Ar ages for white mica.