9:15 AM - 9:30 AM
[SMP26-02] Preliminary report of Hf isotope systematics of the Ryoke granitoids in the Mikawa area, SW Japan
Keywords:magmatic flare-up, Ryoke belt, zircon, Hf isotope, granitoids
The Ryoke belt is known to record part of the circum Pacific, Cretaceous magmatic flare-up [1,2]. The Cretaceous magmatic pulse in the Ryoke belt is considered to be composed of at least three short-term pulses. Recent U-Pb zircon dating of granitoids and high-temperature/low-pressure type metamorphic rocks in the Ryoke belt and petrochronological approach correlating granitoid intrusions with metamorphism have revealed that the magmatic pulses younger from west (Yanai area) to east (Mikawa area), and that the intrusion depth of granitoids changes from shallow for the first pulse through deeper (middle crust) for the second, and shallow again for the third [1-4]. In this study, we used LA-MC-ICPMS to determine Hf isotope compositions preserved by zircons from granitoids that belong to all three magmatic pulses in the Mikawa area (for analytical methods and conditions, see [5]). Analyzed samples are those previously used for U-Pb zircon dating [1,6].
As a result, εHf(t) values of zircons from the granitoids in the Mikawa area showed slightly positive to negative values, corresponding to the two-stage DM model age (TDMC) between 1.1 and 1.75 Ga. Granitoids belonging to the first pulse (~99-95 Ma; Kamihara tonalite and Kiyosaki granodiorite) showed εHf(t) of -1.5 to -1.3 (arithmetic means of each sample, the same hereafter). Granitoids from the second pulse (~81-75 Ma) that intruded to the middle crust showed distinct εHf(t) from -6.3 to -5.2 (three samples) and -0.4. The granitoids that belong to the third pulse (~75-69 Ma) also showed variation. The Inagawa granite and Mitsuhashi granodiorite showed εHf(t) from -6.1 to -6.0, and the mylonitized Busetsu granite gave εHf(t) of -6.8. In contrast, the Shinshiro tonalite and non-mylonitized Busetsu granite gave εHf(t) of -3.5 to -2.0. For the Busetsu granite and Shinshiro tonalite, εHf(t) values and U-Pb ages similar to those of the 81-75 Ma granitoids suggests that some zircons are derived from the 81-75 Ma granitoids, and that the 81-75Ma granitoids were located in the intrusion pathways of the Busetsu granite and Shinshiro tonalite. In spite of the presence of inherited zircons, minor variation of εHf(t) in each pluton may indicate limited crustal assimilation close to the depth of emplacement. Neither the areal distribution nor the emplacement depth of each pluton is correlated with εHf(t) values, whereas εHf(t) values show correlation with the age of granitoids; εHf(t) is close to 0 for the first pulse (-1.5 to -1.3) and part of the second pulse (-0.4), followed by markedly lower values (-6.3 to -5.2) during the second pulse and wider variation (-6.8 to -2.0) for the third pulse.
From the TDMC point of view, a bimodal distribution of TDMC at ~1.2 Ga and ~1.5 Ga is observed in the Mikawa area. This characteristic is similar to that observed in the Yanai area [7] where TDMC of ~1.2 Ga and ~1.4 Ga are observed from zircon in granitoids. This observation may reflect a difference in magma source, mantle contribution or a different degree of assimilation of old crustal materials such as detrital zircons in the Mino-Tanba belt [8] and the Ryoke belt [4,9-12]. Combining Hf isotope data with additional information on petrology and geochemistry of the granitoids, together with additional data from contemporaneous mafic igneous rocks [e.g., 13] will help understanding the magma petrogenesis in the Ryoke belt.
[1] Takatsuka et al. 2018a, Lithos [2] Okudaira et al. 2024, Elements [3] Skrzypek et al. 2016 Lithos [4] Kawakami et al. 2022, Island Arc [5] Nakano et al. 2021, Gondwana Res. [6] Takatsuka et al. 2018b, Island Arc [7] Mateen et al. 2019, Geosciences J. [8] Fujisaki et al. 2014, JAES [9] Herzig et al. 1998, Geochem. J. [10] Kawakami et al. 2019, Lithos [11] Skrzypek et al. 2018 J. Pet. [12] Kawakami et al. 2013. CMP [13] Wang et al. 2021, IGR
As a result, εHf(t) values of zircons from the granitoids in the Mikawa area showed slightly positive to negative values, corresponding to the two-stage DM model age (TDMC) between 1.1 and 1.75 Ga. Granitoids belonging to the first pulse (~99-95 Ma; Kamihara tonalite and Kiyosaki granodiorite) showed εHf(t) of -1.5 to -1.3 (arithmetic means of each sample, the same hereafter). Granitoids from the second pulse (~81-75 Ma) that intruded to the middle crust showed distinct εHf(t) from -6.3 to -5.2 (three samples) and -0.4. The granitoids that belong to the third pulse (~75-69 Ma) also showed variation. The Inagawa granite and Mitsuhashi granodiorite showed εHf(t) from -6.1 to -6.0, and the mylonitized Busetsu granite gave εHf(t) of -6.8. In contrast, the Shinshiro tonalite and non-mylonitized Busetsu granite gave εHf(t) of -3.5 to -2.0. For the Busetsu granite and Shinshiro tonalite, εHf(t) values and U-Pb ages similar to those of the 81-75 Ma granitoids suggests that some zircons are derived from the 81-75 Ma granitoids, and that the 81-75Ma granitoids were located in the intrusion pathways of the Busetsu granite and Shinshiro tonalite. In spite of the presence of inherited zircons, minor variation of εHf(t) in each pluton may indicate limited crustal assimilation close to the depth of emplacement. Neither the areal distribution nor the emplacement depth of each pluton is correlated with εHf(t) values, whereas εHf(t) values show correlation with the age of granitoids; εHf(t) is close to 0 for the first pulse (-1.5 to -1.3) and part of the second pulse (-0.4), followed by markedly lower values (-6.3 to -5.2) during the second pulse and wider variation (-6.8 to -2.0) for the third pulse.
From the TDMC point of view, a bimodal distribution of TDMC at ~1.2 Ga and ~1.5 Ga is observed in the Mikawa area. This characteristic is similar to that observed in the Yanai area [7] where TDMC of ~1.2 Ga and ~1.4 Ga are observed from zircon in granitoids. This observation may reflect a difference in magma source, mantle contribution or a different degree of assimilation of old crustal materials such as detrital zircons in the Mino-Tanba belt [8] and the Ryoke belt [4,9-12]. Combining Hf isotope data with additional information on petrology and geochemistry of the granitoids, together with additional data from contemporaneous mafic igneous rocks [e.g., 13] will help understanding the magma petrogenesis in the Ryoke belt.
[1] Takatsuka et al. 2018a, Lithos [2] Okudaira et al. 2024, Elements [3] Skrzypek et al. 2016 Lithos [4] Kawakami et al. 2022, Island Arc [5] Nakano et al. 2021, Gondwana Res. [6] Takatsuka et al. 2018b, Island Arc [7] Mateen et al. 2019, Geosciences J. [8] Fujisaki et al. 2014, JAES [9] Herzig et al. 1998, Geochem. J. [10] Kawakami et al. 2019, Lithos [11] Skrzypek et al. 2018 J. Pet. [12] Kawakami et al. 2013. CMP [13] Wang et al. 2021, IGR