Early Cretaceous partial melting recorded by pelitic gneiss from the Nagasaki Metamorphic Complex, western Kyushu, Japan: initiation of Cretaceous high-T metamorphism at eastern margin of Eurasia

ABSTRACT

We present Early Cretaceous partial melting recorded by pelitic gneiss from the Nagasaki Metamorphic Complex, western Kyushu, Japan. The existence of the partially melted rock in the complex may imply initiation of long-lived high-T metamorphism in proto-southwest Japan at Cretaceous eastern margin of Eurasia. The gneisses are composed of three garnet-bearing layers and one felsic lens, and they show mylonitic foliation. Application of Zr-in-rutile geothermometer and quartz-in-garnet Raman geobarometer for inclusion rutile and quartz in garnet provided prograde condition of 670–692°C and 1.01–1.26 GPa and peak condition of 755–769°C and 1.17–1.38 GPa. These conditions correspond to those of lower crustal depths between volcanic arc and forearc where thermal advection by wedge mantle corner flow occurs. Presence of the felsic lens whose mineral assemblage can undergo partial melting under the peak condition and presence of a crystallized melt inclusion in garnet suggest that distinct partial melting occurred during the metamorphism. The concordant U–Pb ages of metamorphic zircon rims are ranging from 131.5 ± 4.3 to 120.1 ± 3.9 Ma. The mean square weighted deviation (MSWD) of the metamorphic age was as large as 6.4, implying a significant duration of high-T condition for growth of metamorphic zircon. The obtained ages are within the period of the magmatic hiatus that has previously proposed around southwest Japan and Korean Peninsula (ca. 160–120 Ma) that is due to stop of wedge mantle corner flow by flatting of subducting slab. Our results suggest that high-T metamorphism associated with partial melting at lower crustal depths along Early Cretaceous proto-southwest Japan has already been initiated at the late stage of the hiatus (ca. 130 Ma). This may be due to slab rollback and trench retreat at eastern margin of Eurasia.

GRAPHICAL ABSTRACT

KEYWORDS:

• Slab rollback
• wedge mantle corner flow
• partial melting
• melt inclusion
• Nagasaki Metamorphic Complex
• Zr-in-rutile geothermometry
• quartz-in-garnet Raman geobarometry
• zircon U–Pb age

Acknowledgments

We thank Tomoaki Kubo (Kyushu University) for use of laser Raman microspectrometer and Takaaki Noguchi (Kyushu University) for advising on Raman spectroscopy and data analysis. Kazuhiko Shimada (Kyushu University) is thanked for technical support with the mineral chemical analyses. Hayato Yodoya (Kyushu University) is thanked for assistance for sample preparation for whole-rock chemical analysis. We are grateful to Tatsuki Tsujimori (Tohoku University), Takeshi Imayama (Okayama University of Science), and an anonymous reviewer for constructive reviews and Robert J. Stern (University of Texas at Dallas) for editorial efforts. This work was financially supported by JSPS KAKENHI Grant Numbers JP C25400518 to T. Ikeda and JP 20K04116 to K. Miyazaki and by the Late Professor Tatsuro Matsumoto Scholarship Grant Number MT18005 to Y. Mori.

Disclosure statement

No potential conflict of interest was reported by the author(s).

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Additional information

Funding

This work was supported by the Japan Society for the Promotion of Science under KAKENHI [JP C25400518], [JP 20K04116]; Kyushu University under the Late Professor Tatsuro Matsumoto Scholarship [MT18005].