Presentation Information
[T16-O-9]Two pulses of Paleocene magmatism in the Noto Peninsula, central Japan: New evidence from U–Pb zircon geochronology
*Mitsuhiro NAGATA1, Raiki YAMADA2, Sota NIKI3, Takafumi HIRATA4 (1. Department of Earth and Environmental Sciences, College of Humanities and Sciences, Nihon University, 2. Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology , 3. Institute for Space and Earth Environmental Research, Nagoya University, 4. Geochemical Research Center, the University of Tokyo)
Keywords:
Cenozoic,Ishikawa Prefecture,Paleocene,U–Pb dating,zircon
Magmatism was widespread in Southwest Japan during the Early Cretaceous to early Paleogene (ca. 110–60 Ma), prior to the opening of the Sea of Japan. However, previous studies in central Japan have focused mainly on volcanic rocks (e.g., Nohi Rhyolite) in the Hida Mountainland and plutonic rocks of the Ryoke–San’yo Belt, whereas igneous rocks in the northern part of central Japan have received comparatively little attention. Although several volcanic rocks in the Noto Peninsula have yielded Paleocene–Eocene whole-rock K–Ar dates (Tsukamoto, 1988), the reliability of these dates has not been re-evaluated. In this study, U–Pb zircon dating was conducted on these volcanic rocks to reassess their erupution ages and examine their geological significance.
The geology of the Noto Peninsula is generally characterized by plutono-metamorphic rocks of the Hida Belt unconformably overlain by Oligocene and younger volcanic and sedimentary rocks (e.g., Ozaki, 2010). This study focuses on the Okakuma (Ohkakuma) and Odawara formations (proposed herein), both of which have previously yielded whole-rock K–Ar dates of ca. 57–46 Ma(Tsukamoto, 1988). The Okakuma Formation is composed of rhyolitic lava and rhyolitic pyroclastic rocks (Tomioka and Sato, 2007), for which a K–Ar whole-rock date of 45.7 Ma has been reported (Tsukamoto, 1988).
Although the Odawara Formation is currently commonly included within the Konosuyama Formation (e.g., Ozaki, 2010), we distinguish it from the Konosuyama Formation in this study because (1) it yielded significantly older K–Ar whole-rockdates (56.7 and 52.1 Ma) than those reported for the Konosuyama Formation (ca. 33–20 Ma; Shibata et al., 1981; Tsukamoto, 1988; Chen et al., 2020), and (2) it differs petrographically from the Konosuyama Formation, particularly in its groundmass texture (Umeka and Ishiwatari, 2004). We therefore provisionally define the Odawara Formation, with its type locality at Odawara and the adjacent Sakeo area in Noto Town, Hōsu District, Ishikawa Prefecture. This formation is composed mainly of andesitic lava and small amounts of andesitic pyroclastic rocks.
Zircon grains separated from rhyolite and andesite samples of the Okakuma and Odawara formations, respectively, were analyzed using LA–MC–ICP–MS at the Geochemical Research Center, Graduate School of Science, The University of Tokyo. Weighted mean 206Pb/238U dates of 64.8 ± 0.5 Ma and 61.2 ± 0.3 Ma (2σ) were obtained, respectively.
These dates are ca. 10–20 Myr older than the previously reported K–Ar dates, suggesting that the K–Ar isotopic system was rejuvenated by thermal effects associated with post-Oligocene magmatism. Accordingly, the Okakuma and Odawara formations are interpreted to have formed during the Danian–Selandian stages of the Paleocene. Similar rejuvenation of K–Ar dates has also been reported from the Shogawa (Volcano–Plutonic) Complex (e.g., Futomiyama Group) in central Japan (e.g., Tanase et al., 2005; Kaneko et al., 2019). Therefore, the Paleocene volcanic rocks of the Noto Peninsula may represent the northern extension of the magmatic activity associated with the Shogawa Complex. This study provides U–Pb zircon geochronological evidence for two pulses of Paleocene magmatism in the Noto Peninsula and places important chronological constraints on the distribution and magmatic history of Paleocene magmatism in the Southwest Japan arc prior to the opening of the Sea of Japan.
Chen et al., 2020, Abst., JpGU-AGU Joint Meet. 2020, SGL34-P07./Kaneko et al., 2019, J. Geol. Soc. Jpn., 125, 781–792./Ozaki, 2010, Seamless Geoinf. Coastal Zone, "North Coastal Zone of Noto Peninsula, Digital Geosci., Map S-1., 15p./Shibata et al., 1981, J. Jpn. Assoc. Mineral., Petrol. Econ. Geol., 76, 248–252./Tanase et al., 2005, Monogr., Assoc. Geol. Collab. Jpn., 53, 143–157./Tomioka and Sato, 2007, Bull. Earthq. Res. Inst., Univ. Tokyo, 82, 255-264./Tsukamoto, 1988, In Kaseno, ed., Geology of Ishikawa-ken, 225–226./Umeka and Ishiwatari, 2004, Abst., 111th Ann. Meet. Geol. Soc. Jpn., 180.
The geology of the Noto Peninsula is generally characterized by plutono-metamorphic rocks of the Hida Belt unconformably overlain by Oligocene and younger volcanic and sedimentary rocks (e.g., Ozaki, 2010). This study focuses on the Okakuma (Ohkakuma) and Odawara formations (proposed herein), both of which have previously yielded whole-rock K–Ar dates of ca. 57–46 Ma(Tsukamoto, 1988). The Okakuma Formation is composed of rhyolitic lava and rhyolitic pyroclastic rocks (Tomioka and Sato, 2007), for which a K–Ar whole-rock date of 45.7 Ma has been reported (Tsukamoto, 1988).
Although the Odawara Formation is currently commonly included within the Konosuyama Formation (e.g., Ozaki, 2010), we distinguish it from the Konosuyama Formation in this study because (1) it yielded significantly older K–Ar whole-rockdates (56.7 and 52.1 Ma) than those reported for the Konosuyama Formation (ca. 33–20 Ma; Shibata et al., 1981; Tsukamoto, 1988; Chen et al., 2020), and (2) it differs petrographically from the Konosuyama Formation, particularly in its groundmass texture (Umeka and Ishiwatari, 2004). We therefore provisionally define the Odawara Formation, with its type locality at Odawara and the adjacent Sakeo area in Noto Town, Hōsu District, Ishikawa Prefecture. This formation is composed mainly of andesitic lava and small amounts of andesitic pyroclastic rocks.
Zircon grains separated from rhyolite and andesite samples of the Okakuma and Odawara formations, respectively, were analyzed using LA–MC–ICP–MS at the Geochemical Research Center, Graduate School of Science, The University of Tokyo. Weighted mean 206Pb/238U dates of 64.8 ± 0.5 Ma and 61.2 ± 0.3 Ma (2σ) were obtained, respectively.
These dates are ca. 10–20 Myr older than the previously reported K–Ar dates, suggesting that the K–Ar isotopic system was rejuvenated by thermal effects associated with post-Oligocene magmatism. Accordingly, the Okakuma and Odawara formations are interpreted to have formed during the Danian–Selandian stages of the Paleocene. Similar rejuvenation of K–Ar dates has also been reported from the Shogawa (Volcano–Plutonic) Complex (e.g., Futomiyama Group) in central Japan (e.g., Tanase et al., 2005; Kaneko et al., 2019). Therefore, the Paleocene volcanic rocks of the Noto Peninsula may represent the northern extension of the magmatic activity associated with the Shogawa Complex. This study provides U–Pb zircon geochronological evidence for two pulses of Paleocene magmatism in the Noto Peninsula and places important chronological constraints on the distribution and magmatic history of Paleocene magmatism in the Southwest Japan arc prior to the opening of the Sea of Japan.
Chen et al., 2020, Abst., JpGU-AGU Joint Meet. 2020, SGL34-P07./Kaneko et al., 2019, J. Geol. Soc. Jpn., 125, 781–792./Ozaki, 2010, Seamless Geoinf. Coastal Zone, "North Coastal Zone of Noto Peninsula, Digital Geosci., Map S-1., 15p./Shibata et al., 1981, J. Jpn. Assoc. Mineral., Petrol. Econ. Geol., 76, 248–252./Tanase et al., 2005, Monogr., Assoc. Geol. Collab. Jpn., 53, 143–157./Tomioka and Sato, 2007, Bull. Earthq. Res. Inst., Univ. Tokyo, 82, 255-264./Tsukamoto, 1988, In Kaseno, ed., Geology of Ishikawa-ken, 225–226./Umeka and Ishiwatari, 2004, Abst., 111th Ann. Meet. Geol. Soc. Jpn., 180.
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