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[SCG44-04] Zircon U–Pb ages of plutonic rocks from Iwafune area, Ibaraki Prefecture
Keywords:U-Pb age of zircon, Iwafune pluton, Granite, Diorite, Gabbroic Diorite
The Yamizo Mountains consist of Yamizo, Torinoko, Keisoku and Tsukuba Mountain Blocks. The Yamizo, Torinoko, and greater part of Keisoku Mountain Blocks consist of the Yamizo Group and several plutons. The diorites and gabbros of the plutons are of Early Cretaceous in age (109–102 Ma: Ejima et al., 2018; Ejima et al., 2019), and the granites of Late Cretaceous–Paleocene in age (69–63 Ma: Ejima et al., 2019). On the other hand, southern part of the Keisoku and Tsukuba Mountain Blocks mainly consist of plutonic rocks, such as granites and gabbros are of Late Cretaceous–Paleocene in age (66–61 Ma: Koike and Tsutsumi, 2017; Koike and Tsutsumi, 2018). The Iwafune pluton occurs in northern part of the Keisoku Mountain Block. The Iwafune pluton consists mainly of diorite with minor gabbroic diorite and granite which is distributed in southeast. Shibata et al. (1973) reported biotite K–Ar age of the Iwafune diorite is 64.5 ± 2.5 Ma. However, the petrographic characteristics of the Iwafune diorite are similar to diorite of the Fukuroda pluton with about 100 Ma ages in the Torinoko Mountain Block.
Therefore, in order to reexamine age of igneous activity of the Iwafune pluton, U–Pb dating of zircons extracted from the Iwafune plutonic rocks was carried out using laser ablation inductively coupled plasma mass spectrometry combined with cathodoluminescence (CL) imaging on the zircons.
The CL images of zircons separated from diorite, gabbroic diorite and granite show that these zircons have oscillatory zoning and broad-banded zoning which are characteristic internal texture for magmatic zircon. The zircon U–Pb ages of the diorite, gabbroic diorite and granite are 64.4 ± 0.6 Ma (n = 37), 64.5 ± 0.7 Ma (n = 24) and 64.6 ± 0.8 Ma (n = 41), respectively. The diorite, gabbroic diorite and granite of the Iwafune pluton are same intrusive age, and there is no difference between these ages of the Iwafune pluton and the K–Ar age of previous study (Shibata et al.,1973). Moreover, the zircon U–Pb ages of the Iwafune pluton are same as those of granites in the Yamizo Mountain Block (69–63 Ma: Ejima et al., 2019), and granites and gabbros in the Tsukuba Mountain Block (66–61 Ma: Koike and Tsutsumi, 2017; Koike and Tsutsumi, 2018).
Therefore, in order to reexamine age of igneous activity of the Iwafune pluton, U–Pb dating of zircons extracted from the Iwafune plutonic rocks was carried out using laser ablation inductively coupled plasma mass spectrometry combined with cathodoluminescence (CL) imaging on the zircons.
The CL images of zircons separated from diorite, gabbroic diorite and granite show that these zircons have oscillatory zoning and broad-banded zoning which are characteristic internal texture for magmatic zircon. The zircon U–Pb ages of the diorite, gabbroic diorite and granite are 64.4 ± 0.6 Ma (n = 37), 64.5 ± 0.7 Ma (n = 24) and 64.6 ± 0.8 Ma (n = 41), respectively. The diorite, gabbroic diorite and granite of the Iwafune pluton are same intrusive age, and there is no difference between these ages of the Iwafune pluton and the K–Ar age of previous study (Shibata et al.,1973). Moreover, the zircon U–Pb ages of the Iwafune pluton are same as those of granites in the Yamizo Mountain Block (69–63 Ma: Ejima et al., 2019), and granites and gabbros in the Tsukuba Mountain Block (66–61 Ma: Koike and Tsutsumi, 2017; Koike and Tsutsumi, 2018).