R6: Plutonic rocks, volcanic rocks and subduction factory

[R6-05] Zircon U-Pb age combined with trace element geochemistry constraining the source of Miocene granitoids of the Kagoshima Prefecture, Japan

*Hafiz Ur REHMAN¹, Marimo NAKABAYASHI¹, Yuki OTA¹, Kaushik DAS², Chung Sun LIN³, Hao YANG LEE³, Daisuke YAMASHITA⁴, Hiroshi YAMAMOTO¹ (1. Kagoshima Uni., 2. Hiroshima Uni., 3. IES, Academia Sinica, Taiwan, 4. Satsumasendai Kosh. Mus.)

Keywords:Zircon, U-Pb age, Miocene, Kagoshima Prefecture, Koshikishima

This study reports zircon U-Pb age and trace element geochemistry of the Miocene granitoids exposed in Kagoshima to constrain their magmatic source. More than 150 zircon grains were U-Pb age-dated along with trace element contents from four plutons (#1. Osumi, #2. Takakuma Yama, #3. Shibi san, and #4. Koshikishima). Zircon grains from Osumi yielded U-Pb age values in the range from 13.4 ~ 15.8 Ma in sample OsG-3, 13.9 ~ 16.8 Ma in OsG-9, 13.3 ~ 14.7 in OsG-12, 14.2 ~ 16.4 in OsG-36. Zircons from Takakuma Yama yielded ages of 12.7 ~ 15.2 Ma from TkG-1 and 12.8 ~ 15.9 Ma in TkG-2. Shibi Pluton yielded ages of 12.7 ~ 14.3 Ma and a hornfels sample (Shb-19) yielded U-Pb age scattered between 13.9 and 206 Ma, with three spots giving values of 1803 Ma, 1873 Ma, and 2194 Ma from the detrital core domains. Zircons separated from Koshikishima Island yielded relatively younger age of 9 ~ 12 Ma in Kos-2, 8.8 ~ 12.1 Ma in Kos-11, 7.8 ~ 10.4 Ma in Kos-19, 8.7 ~ 10.6 Ma in Kos-30, 9.3 ~ 11.4 Ma in Kos-35, and 9.4 ~ 12.3 Ma in Kos-42. Trace element data of zircons, particularly, rare-earth elements, show typical magmatic type patterns with pronounced positive Ce and negative Eu anomalies. Ti-in-zircon thermometry data revealed temperature of crystallization ranging from 609 ~ 895 ℃, with majority of grains showing fall within 650 to 780 ℃ range. Age-data from plutons #1~3 are more or less identical and suggest Miocene magmatism possibly due to the trench-parallel ridge subduction that triggered partial melting in the overlying continental crust. Proterozoic cores suggest incorporation of the recycled crust during the magma formation. Pluton #4, yielding relatively younger age, may have resulted from partial melting of crust enriched in mafic component. Zircon trace element data, along with whole-rock major and trace element contents, also indicate the involvement of mafic component to generate the Koshikishima granitoids in a different tectonic scenario than the others.