Late Cretaceous CHIME monazite ages of Sanbagawa metamorphic rocks from Nushima, Southwest Japan

Abstract

The Sanbagawa metamorphic rocks of epidote–amphibolite facies are extensively exposed in Nushima at the southern end of the Hyogo Prefecture, Japan. Chemical U–Th–total Pb isochron method (CHIME) ages were determined for monazite grains from the Sanbagawa metamorphic rocks. CHIME monazite ages are 92.3 ± 3.2 and 90.3 ± 4.7 Ma for two chlorite–rich rocks collected from the reaction zone between pelitic schists and serpentinites. The monazite grains occur as isolated phases or aggregates with rutile/ilmenite in chlorite–rich matrix, and thus the CHIME monazite ages are interpreted to represent recrystallization close to the peak stage of prograde epidote–amphibolite facies metamorphism. Considering the U–Pb zircon ages reported in literature, the main stage of Sanbagawa prograde metamorphism up to the epidote–amphibolite facies is estimated to have been at 80–92 Ma. Comparison of the age data between the Sanbagawa eclogite and epidote–amphibolite facies stages suggests the possibility that the formation of eclogite facies rocks at different depths and periods of the Sanbagawa subduction continued for more than 20–30 million years.