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.

Coexisting different types of zoned garnet in kyanite‐quartz eclogites from the Sanbagawa metamorphic belt: Evidence of deformation‐induced lithological mixing during prograde metamorphism

“…andItaya and Tsujimori (2015) reported 40 Ar/ 39 Ar amphibole and phengite ages of 87-95 Ma and K-Ar phengite ages of 84-89 Ma from the Seba eclogites and related metapelites, respectively. Similar K-Ar phengite and paragonite ages of 78-80 Ma were also described from the Western Iratsu eclogites(Itaya & Tsujimori, 2015) Suzuki et al (2018). discussed the relationships between the U-Pb, Sm-Nd, and Lu-Hf ages and equilibrium conditions of the Sanbagawa eclogites, and suggested a possibility that the quartz eclogites and other lithologies in the eclogite-unit formed at different depths and periods of the Sanbagawa subduction, which could have continued for more than 20-30 Ma(Figure 2).…”

“…Schematic diagram showing the pressure–temperature–time (P–T–t) paths of the eclogite and non‐eclogite units of the Sanbagawa belt (modified from Figure b of Suzuki, Enami, Maekawa, Kato, and Ueno ()). A09, Aoki et al (); I11, Itaya, Tsujimori, and Liou (); O04, Okamoto et al (); W09, Wallis et al (); S18, Suzuki et al ()…”

“…Suzuki et al () discussed the relationships between the U–Pb, Sm–Nd, and Lu–Hf ages and equilibrium conditions of the Sanbagawa eclogites, and suggested a possibility that the quartz eclogites and other lithologies in the eclogite‐unit formed at different depths and periods of the Sanbagawa subduction, which could have continued for more than 20–30 Ma (Figure ).…”

Metamorphic record of the Asemi‐gawa eclogite unit in the Sanbagawa belt, southwest Japan: Constraints from inclusions study in garnet porphyroblasts

“…In addition, Suzuki et al. () suggested that different peak eclogite facies metamorphic ages reported by Okamoto et al. () and Wallis et al.…”

“…In comparison with the U-Pb zircon ages reported from previous geochronological studies, they interpreted the main stage of the Sanbagawa prograde metamorphism, up to the epidote-amphibolite facies, at 92-81 Ma. In addition, Suzuki et al (2018) suggested that different peak eclogite facies metamorphic ages reported by Okamoto et al (2004) and Wallis et al (2009) may reflect their formation at different depths and periods during a single Sanbagawa subduction episode (e.g., Endo et al, 2012).…”