Mitsuhiro NAGATA 1 , Yoshimi HAYASHI 2 , Tomokazu SAKASHITA 2 , Yuta KAWAGOE 1 , Yoshikazu KOUCHI 1 , Satoshi HIRASAWA 3 , Masato FUJITA 3 , Koshi YAMAMOTO 4 , and Shigeru OTOH 1 1 Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan ² Department of Earth Sciences, Faculty of Science, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan 3 Toyama Science Museum, 1-8-31 Nishinakano-machi, Toyama 939-8084, Japan 4 Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
ABSTRACT
We examined the zircon U-Pb age of the following samples to constrain (1) the minimum age of sedimentation of the Tetori Group ( sensu stricto ) and (2) the period of deposition of the Tetori Group ( s.s.) in the Jinzu Region: a tuff sample of the Hayashidani Andesite in the Hakusan Region, three samples of felsic dikes cutting the Tetori Group ( s.s. ) in the Hakusan and Jinzu Regions, and a tuff sample from the Tetori Group ( s.s. ) in the Jinzu Region. As a result, the ages of the Hayashidani Andesite and felsic dikes fell between 109 Ma and 104 Ma (middle Albian). The tuff from the Tetori Group ( s.s.) in the Jinzu Region yielded the concordia age of 120.9 ± 1.1 Ma (2σ; early Aptian), which we interpret as the age of sedimentation. Our geochronologic study, together with previous studies, suggests that the period of sedimentation of the Tetori Group ( s.s.) included the early Aptian and terminated by the middle Albian. Moreover, recently reported Albian zircon fission-track ages from the Tetori Group ( s.s. ) are likely rejuvenation ages caused by the heat from the igneous activity that formed the felsic dikes. Key words : Cretaceous, Jinzu Group, Tetori Group, U-Pb age, zircon長田充弘・林芳美・坂下智和・川越雄太・高地吉一・平澤 聡・藤田将人・山本狮志・大藤 茂(2018) 手取層群の堆積はいつ終了したか. 福井県立恐竜博物館紀要17：9–26.鉄義の手取層群の堆積年代下限値を求め、神通区の狭義の手取層群の堆積期間を拘束するため、以下の試料のジルコンU-Pb年代を測定した：白山区の林谷安山岩の凝灰岩，白山・神通両区で狭義の手取層群を切る珪長質岩脈３試料、および神通区の狭義の手取層群中の凝灰岩である。林谷安山岩の凝灰岩および珪長質岩脈３試料の年代は，104–109 Ma（白亜紀Albian期中葉）であった．また，神通区の手取層群中の凝灰岩から，堆嶺年代と解釈される120.9 ± 1.1 Ma (Aptian期前半)のコンコーディア年代が得られた.本研究と先行研究から,鉄義の手取層群の堆積期間はAptian期前半を含み、Albian期中葉には終了していたと考えられる．今回，神通区から得られた珪長質岩脈の年代は，近年先行研究で得られたAlbian期に相当するジルコンフィッション・トラック（FT）年代値に近い。このFT年代は，岩脈をもたらした火成活動による若返り年代と見られる.
INTRODUCTION
The Tetori Group ( sensu lato ; sensu Sano, 2015) is distributed in Fukui, Ishikawa, Gifu, Toyama, and Niigata Prefectures, central Japan and consists mostly of Middle Jurassic to Early Cretaceous marine to terrestrial clastic rocks (e.g., Maeda, 1961; Sakai et al., 2012; Sano, 2015; Yamada, 2017; Fig. 1). It overlies unconformably or tectonically Early Jurassic and older rocks of the Hida and Hida Gaien Belts, and middle Cretaceous andesitic to rhyolitic volcanic and pyroclastic rocks, in turn, cover the Tetori Group ( s.l. ). Maeda (1961) divided the distribution area of the Tetori Group ( s.l. ) into the Hakusan and Jinzu Regions; the former includes the Tedorigawa, Kuzuryugawa, Shokawa, and Furukawa–Kamitakara areas surrounding Mt. Hakusan, whereas the latter ranges from the Jinzu basin (the area drained by the Jinzugawa River) to the Northern Japan Alps. The Tetori Group ( s.l. ) in the Hakusan Region was traditionally subdivided into the Kuzuryu, Itoshiro, and Akaiwa Subgroups in ascending order (Maeda, 1961). On the other hand, several opinions of the lithostratigraphic subdivision have been proposed for the Tetori Group ( s.1. ) in the Jinzu Region. For example, Kawai and Nozawa (1958) subdivided the group into the Higashisakamori, Nagatogawa, and Atotsugawa Formations, in ascending order, and correlated them with the Kuzuryu, Itoshiro, and Akaiwa Subgroups, respectively. Maeda (1961) expressed a different opinion that there is no apparent difference between the Itoshiro and Akaiwa Subgroups in the Jinzu Region and lumped them as the undivided Itoshiro-Akaiwa Subgroup. The depositional age of marine beds in the Kuzuryu and Itoshiro Subgroups has been relatively well constrained using index fossils such as ammonoids. For example, the Kaizara and Yambarazaka Formations of the Kuzuryu Subgroup in the Itoshiro area, Fukui Prefecture, have been correlated with the upper Bathonian- Callovian and Oxfordian, respectively (Sato and Westermann, 1991). Further, the Itsuki Formation of the Itoshiro Subgroup has been correlated with the Hauterivian (–Barremian), because it yields common brackish fauna with the Hauterivian (–Barremian) Ryoseki Formation of the Monobegawa Group in central Shikoku, the Outer Zone of Southwest Japan (Kozai et al., 2002). The depositional age of non-marine beds in the Itoshiro and Akaiwa Subgroups, on the other hand, was hard to constrain, because of the following reasons. (1) The non-marine beds yield no index fossils; (2) there are significant lateral facies changes; and (3) there are no key beds that are useful to correlate the Tetori Group ( s.l. ) in different areas or regions. Some recent studies proposed the revision of the stratigraphic subdivision of the Tetori Group ( s.l. ) of Maeda (1961). For example, Matsukawa et al. (2014a, b) intended to subdivide the Tetori Group ( s.l. ) in the Jinzu Region into the Tetori (former Kuzuryu Subgroup) and Jinzu (former Itoshiro and Akaiwa Subgroups) Groups. Matsukawa et al. (2014a, b) suggested that the Jinzu Group differs in lithostratigraphy from the Itoshiro and Akaiwa Subgroups in the Hakusan Region; the Itoshiro Subgroup in the Hakusan Region contains marine to brackish deposits whereas the Jinzu Group does not. Matsukawa et al. (2014a) also suggested that the Jinzu Group is younger than the Itoshiro and Akaiwa Subgroups in the Hakusan Region that yield no Aptian– Albian or younger fossils; they obtained Aptian–Albian zircon fission track (FT) ages from four tuffaceous sandstone samples from the lower part of the Jinzu Group. However, the FT age of the three samples among the four from the Yamanomura area, Gifu Prefecture, is inconsistent with the stratigraphy; i.e., Aptian beds overlies an Albian bed. Gifu-ken Dinosaur Fossil Excavation Party (1997) mapped many dikes and sills in the Yamanomura area. Sano (2015) and Yamada (2017), on the other hand, proposed to separate the marine Kuzuryu Subgroup from the Tetori Group ( s.l. ), because (1) the sedimentary environments of the Kuzuryu Subgroup (mostly marine) and the Itoshiro and Akaiwa Subgroups (primarily non-marine) are significantly different and (2) the Kuzuryu Subgroup is some five million years older than the upper subgroups. Sano (2015) and Yamada (2017) subdivided the Tetori Group ( s.l. ) into the Kuzuryu (former Kuzuryu Subgroup) and Tetori Groups ( sensu stricto ; former Itoshiro and Akaiwa Subgroups). Sano (2015) further recognized four depositional stages (DS) in the Middle Jurassic to early Late Cretaceous strata in the Hakusan Region. DS1 is the stage of deposition of mainly marine strata, represented by the Middle– Late Jurassic Kuzuryu Group. DS2–3 is the stage of deposition of mainly brackish to fluvial strata, represented by the Berriasian– Aptian Tetori Group ( s.s. ). DS4 is the stage of deposition of volcanic rocks covering the Tetori Group ( s.s. ), together with the intrusion of some plutonic bodies. We will follow the terminology of Sano (2015) and Yamada (2017) for the Tetori Group ( s.l. ) in the Hakusan Region and will follow the terminology of Kawai and Nozawa (1958) for the Tetori Group ( s.l. ) in the Jinzu Region. In this study, we present some new zircon U-Pb age data from (1) an andesitic tuff of DS4 that covers the Tetori Group ( s.s. ) in the Itoshiro-Oyama area of the Hakusan Region, (2) felsic intrusive rocks of DS4 that cut the Tetori Group in the Yamanomura area of the Jinzu Region, and (3) a tuff layer of the Tetori Group ( s.s. ) in the Oyama area of the Jinzu Region. The age of andesitic tuff and felsic dikes of DS4 will constrain the minimum age of sedimentation of the Tetori Group. The age of the tuff layer in the Jinzu Region will constrain the age of deposition of the Tetori Group ( s.s. ) there. Further, we will discuss the temporal relation between the Tetori Group and the Kanmon Group in the Renge, Akiyoshi, Maizuru and Ultra-Tamba Belts, to the south of the Hida and Hida Gaien Belts,  Inner Zone of Southwest Japan. The Kanmon Group and the uppermost part of the Tetori Group commonly contain red beds, indicative of the similar sedimentary environment (Maeda, 1961). The Kanmon Group also contains abundant andesitic to rhyolitic volcanic and pyroclastic rocks.
GEOLOGIC SETTING
Itoshiro-Oyama area in the Hakusan Region (Fig. 2)
The Itoshiro-Oyama area is along the Itoshirogawa River in Gujo City, Gifu Prefecture. The Tetori Group in this area unconformably covers Paleozoic beds (Shimozaisho Group) composed mostly of limestone and mudstone (Kawai, 1959) and consists of the Nobu-dani (Conglomerate) and Oyama Formations in ascending order. The two comformable formations strike east and dip 20°–40°S. The Nobu-dani Formation consists mostly of conglomerate with minor amounts of yellow brown to white sandstone. The sandstone and the matrix of the conglomerate have partly become reddish. The conglomerate contains subangular to round pebbles of limestone and mudstone (Maeda, 1957). The Oyama Formation consists of conglomerate and partly reddish or greenish sandstone and mudstone. The conglomerate mainly contains pebbles of granite, gneiss, and sandstone. Limestone- and mudstone-pebbles are rare. Although Maeda (1957) subdivided the Oyama Formation into the sandstone-rich Lower Member and the mudstone-rich Upper Member, we cannot distinguish the lithostratigraphic difference. Maeda (1957, 1961) correlated the Nobu-dani and Oyama Formations with the Kitadani Formation in the Takinami area, Katsuyama City, Fukui Prefecture (Fig.1). Kawai (1959), on the other hand, correlated the two formations with the Upper Cretaceous Asuwa Group. Andesitic to dacitic dikes cut part of the Tetori Group and the Paleozoic basement, and the Hayashidani Andesite (Tanase et al., 1994; Wakita et al., 1992), striking east and dipping 30°–50°S, sporadically occurs on top of the Tetori Group. We interpret that the Hayashidani Andesite conformably covers the Tetori Group because the attitude of bedding of the two lithostratigraphic units is subparallel to each other, although we have not confirmed the contact between the two. The Omodani Rhyolite (Kawai, 1956, 1959), consisting of dacitic to rhyolitic lava and pyroclastic rocks, unconformably covers the Tetori Group and the Hayashidani Andesite. The Omodani Rhyolite strikes east and dips 10°–50°S.
Jinzu Region
Yamanomura area (Fig. 3)—The Yamanomura area is in the northern part of Hida City, Gifu Prefecture. Four members of the Tetori Group in the Jinzu Region make a conformable succession in this area; they are the Ioridanitoge Conglomerate and Inotani Alternation (Nakanomatanokkoshi Sandstone) Members of the Nagatogawa Formation, and Minamimatadani Conglomerate and Wasabu Alternation Members of the Atotsugawa Formation, in ascending order (Kawai and Nozawa, 1958; Harayama et al., 1991). The type localities for the Minamimatadani Conglomerate and Wasabu Alternation Members are in this area. The Ioridanitoge Conglomerate Member consists mainly of cobble to boulder conglomerate with some layers of fine to coarse feldspathic arenite. The cobbles and boulders of the conglomerate are mostly of granite, gneiss, and quartz porphyry (Kawai and Nozawa, 1958; Gifu-ken Dinosaur Fossil Excavation Party, 1997). The Inotani Alternation Member consists mostly of coarse arkosic sandstones interbedded with thinner shales and rare tuffaceous rocks (Kawai and Nozawa, 1958). The Minamimatadani Conglomerate Member consists of conglomerate and pebbly sandstone. The conglomerate contains pebbles and cobbles, of various rock types: felsic volcanic rocks, black mudstone, granite, gneiss, quartz porphyry, dark gray shale, arkosic granule sandstone, and chert. The Wasabu Alternation Member consists mostly of medium to coarse, bedded or massive feldspathic arenites interbedded with dark gray shales and rare conglomerates and tuffaceous rocks. Many andesitic to rhyolitic dikes cut the Tetori Group in this area (Gifu-ken Dinosaur Fossil Excavation Party, 1997; Kawai and Nozawa, 1958). The dikes are mostly altered, and the surrounding rocks, including the Tetori Group, have suffered contact metamorphic effects. Oyama area (Fig. 4)—The Oyama area of Toyama City is in the northwestern part of the Jinzu Region. In this area, there is a dinosaur-footprint site (Matsukawa et al., 1997; Hirasawa et al., 2010; Toyama Dinosaur Research Group, 2002) that also yields fossil dinosaur teeth. However, there is no unified view of the stratigraphic horizon of this remarkable site, because the Minamimatadani Conglomerate Member, separating the Inotani and Wasabu Alternation Members, thins out in this area. Yamada et al. (1988) and Hirasawa et al. (2010) placed the site in the Inotani and Wasabu Alternation Members, respectively, and Matsukawa et al. (2014a) lumped the two alternation members into the Inotani Formation of the Jinzu Group.
SAMPLE DESCRIPTIONS
Itoshiro-Oyama area in the Hakusan Region
Hayashidani Andesite (Fine tuff; Sample IO1)—Sample IO1 was collected from an outcrop of fine massive andesitic tuff along the Takigadani in Gujo City, Gifu Prefecture (35° 57′ 34.8″ N, 136* 45' 4.0" E; Fig. 2). The tuff consists of plagioclase, and opaque minerals, including alteration minerals such as chlorite (Fig. 5a). The tuff also contains devitrified glass replaced by chlorite or sericite and few lapilli. The lapilli have phenocrysts of plagioclase and clinopyroxene. The tuff grades upwards into cross-laminated, reddish purple tuffaceous sandstone. Felsic dike (Sample IO2)—Sample IO2 was collected from an outcrop of a felsic dike along the Itoshirogawa River (35* 57* 34.8" N, 136° 45' 28.6" E; Fig. 2). The dike cuts the Nobu-dani Formation of the Tetori Group. Although the dike forms no evident metamorphic contact aureole, the Tetori Group in the vicinity of the dike is substantially hardened. Phenocrysts, mostly plagioclase, occupied 30 vol% of the sample (Fig. 5b). The other constituent minerals, including alteration minerals, were quartz, chlorite, and opaque minerals. Maeda (1957) described these dikes as quartz porphyries.
Jinzu Region
Felsic dike (Garnet dacite: Sample YM1)—Sample YM1 was collected from an outcrop of a felsic dike along the Utsubodanigawa River, Hida City, Gifu Prefecture (36* 23* 42.59" N, 137* 25' 16.73" E; Fig. 3). The dike cuts the Wasabu Alternation Member of the Tetori Group ( s.s. ). Although the dike forms no evident metamorphic contact aureole, the Tetori Group around the dike is partly bleached because of alteration. Gifu- ken Dinosaur Fossil Excavation Party (1996, 1997) has already reported the distribution of garnet-bearing felsic rocks near this location. Phenocrysts occupied 30 vol% of the sample, among which 90 vol% were quartz and plagioclase, and 10 vol% or less were garnet (Fig. 5c, d). The other constituent minerals, including alteration minerals, were chlorite and opaque minerals. Felsic dike (Sample YM2)—Sample YM2 was collected from an outcrop of a felsic dike along the Mizuhoradani, Hida City, Gifu Prefecture (36° 24′ 18.35″ N, 137° 25′ 16.73″ E; Fig. 3). The felsic dike cut the Tetori Group and the basement Hida Granite (Gifu-ken Dinosaur Fossil Excavation Party, 1997). Although the dike forms no evident metamorphic contact aureole, the Tetori Group around the dike is partly bleached because of alteration. The constituent minerals, including alteration minerals, were quartz, plagioclase, biotite, chlorite, and opaque minerals (Fig. 5e). Matsukawa et al. (2014a) reported Albian zircon fission-track ages from tuffaceous sandstone samples taken near the location. Tuff (Sample OY1)—Sample OY1 was collected from the tuff bed at Oyama dinosaur footprint site, southwestern Oshimizu, Toyama City (Toyama Dinosaur Research Group, 2002: 36° 31° 43.70" N, 137° 16' 6.33" E) just above the best-preserved footprint horizon (upper surface of Bed E; Fig. 4). The tuff consisted of quartz, plagioclase, biotite, and devitrified glass replaced by chlorite with the diameter of about 0.03 mm (Fig. 5f). The quartz crystals sometimes showed wavy extinction, and the plagioclase and biotite crystals displayed columnar or acicular euhedral form. Board of Education of Toyama Prefecture (2003) reported detailed description of the sample. Hirasawa et al. (2010) interpreted that the tuff is an ash-fall deposit.
ANALYTICAL METHOD
Zircon separates were prepared and analyzed following the procedures described in Kawagoe et al. (2012). A brief description of the analytical method is given below. Zircon grains were separated from each sample and were mounted in acrylic resin and polished. The internal textures, solid phase inclusions, and microcracks of the zircons were observed using transmitted and reflected optical microscopy and monochromatic cathodoluminescence (CL) scanning electron microscope (SEM) imaging. CL images were captured on a JEOL JSM-5910LV SEM equipped with a JEOL MP-Z01118T CL detector in the Fukui Prefectural Dinosaur Museum. All zircon grains exhibit clear oscillatory zoning in CL images (Fig. 6). A portion of each grain, having no inclusions, microcracks, and detritus core, was chosen for analyses. U-Pb dating was carried out on the laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) equipped in the Graduate School of Environmental Studies, Nagoya University. The ICPMS instrument was an Agilent 7700x quadrupole- based ICPMS connected with LA system (New Wave Research NWR-213), which used the frequency quintupled Nd-YAG 213-nm wavelength. NIST (National Institute of Standards and Technology, U.S.A.) SRM 610 glass with the recommended 206Pb/238U value of 0.2236 and the Nancy 91500 zircon with the
238U- 206Pb age of 1062.4 ± 0.4 Ma (Wiedenbeck et al., 1995) were used as standard materials. The measurement conditions, given in Table 1, were based on a protocol by Kouchi et al. (2015): energy density of 11.7 J/cm 2 , pulse repetition rate of 10 Hz, pre-ablation time of 8 s, ablation time of 10 s, and the ablation pit size of 25 µm. The peaks of 202 Hg, 204 (Hg+Pb), 206 Pb, 207 Pb, 208 Pb, 232 Th, and 238U were monitored. Data were acquired in sequences of 28 analyses, consisting of 5 analyses of gas blank, 4 NIST SRM 610 glass standard, one Nancy 91500 zircon, nine unknown, 4 SRM 610 standard, and five gas blank. Note that Kawagoe et al. (2012) and Takeuchi et al. (2015) also carried out zircon U-Pb dating with the same instruments and measurement conditions. After the analyses, we rejected the following analytical data: (1) the data from a spot containing an inclusion or a crack, (2) the data from a zircon grain that leaped during laser ablation, and (3) the data with the amount of common lead ( 206 PbC; Stacey and Kramers, 1975) of 5% or more. We plotted all the remaining data on a concordia diagram using Isoplot 4.15 (Ludwig, 2012). Then we chose concordant spots (spots with concordant 238 U-206Pb and 235U-207Pb data sets) with the error ellipse (20) overlapping the concordia line (Matsui et al., 2018). The slope of the ellipse is represented by the rho (ρ) value, the correlation coefficient of error in the 206 Pb/ 238 U- 207 Pb/ 238 U ratios. We defined the igneous age of each rock sample as follows. Since pyroclastic and volcanic rocks tend to contain older inherited and/or exotic zircons, we calculated the concordia age of several spots that form the youngest age cluster; i.e., the youngest spot and other spots with the error bar (2σ) of the 238U-206Pb age overlapping with that of the youngest spot. The data from inherited and exotic zircons, exhibiting significantly older ages, were eliminated from the calculation. We will mainly use the concordia age of concordant spots in the following description and discussion. The age assignments to the geologic time scale in this study are based on International Commission for Stratigraphy (2018).
RESULTS
We conducted three cycles of measurements of zircons for each rock sample and calculated the following values for the zircon and glass standards. One is the weighted mean 238 U- 206 Pb and 207 Pb- 206 Pb age of the three spots on the Nancy 91500 standard zircon for each rock sample (Table 1). The others are percent relative standard deviations (%SD) of the 206 Pb/238 U , 207 Pb/206 Pb, and 208 Pb/232 Th ratios of eight spots on the SRM 610 standard for each cycle (%2SD in Table 1), a measure of repeatability of the measurements. The weighted mean 238 U- 206 Pb ages of the Nancy 91500 zircon all gathered around its recommended age of 1062.4 ± 0.4 Ma within the range of error. The %2SD values of the 206Pb/238U， 207Pb/206Pb and 208Pb/232 Th ratios of the SRM 610 glass were all below 5.0.
Hakusan Region
Andesitic tuff (Sample IO1)—We obtained analytical data from 27 spots in 27 zircon grains and adopted all the data sets (Table 2). Then we chose 27 concordant data sets out of 27 (Fig. 7). The 238 U-206 Pb ages of the concordant spots clustered in 106– 112 Ma and 238–283 Ma, and the 238 U- 206 Pb age of the youngest concordant spot was 106.4 ± 4.3 Ma (2σ). The concordia age of the youngest cluster (7 spots) was 109.3 ± 1.6 Ma (MSWD = 2.7, probability = 0.10). The weighted mean of the 238U-206Pb ages of the youngest cluster was 109.3 ± 1.7 Ma (MSWD = 1.5, probability = 0.17) . Felsic dike (Sample IO2)—We obtained analytical data from 27 spots in 27 zircon grains and adopted 25 data sets (Table 2). Then we chose 17 concordant data sets out of 25 (Fig. 6). The 238U- 306 Pb ages of the concordant spots clustered in 103–121 Ma, and the 238 U-206 Pb age of the youngest concordant spot was 102.6 ± 3.3 Ma. The concordia age of the youngest cluster (10 spots) was 104.3 ± 1.5 Ma (MSWD = 15, probability = 0.000). The weighted mean of the 238U-206 Pb ages of the youngest cluster was 104.3 ± 1.4 Ma (MSWD = 1.5, probability = 0.17).
Jinzu Region
Felsic dike (Garnet dacite: Sample YM1)—We obtained analytical data from 27 spots in 27 zircon grains and adopted all the data sets (Table 2). Then we chose 20 concordant data sets out of 27 (Fig. 7). The 238 U- 206 Pb ages of the concordant spots clustered in 107–121 Ma, and the 238 U-206 Pb age of the youngest concordant spot was 107.0 ± 3.1 Ma. The concordia age of the youngest cluster (14 spots) was 108.9 ± 0.7 Ma (MSWD = 7.6, probability = 0.006). The weighted mean of the 238 U-206 Pb ages of the youngest cluster was 108.9 ± 0.9 Ma (MSWD = 1.2, probability = 0.24). Felsic dike (Sample YM2)—We obtained analytical data from 27 spots in 27 zircon grains and adopted all the data sets (Table 2). Then we chose 25 concordant data sets out of 27 (Fig. 7). The 238U-206Pb ages of the concordant spots clustered in 105–146 Ma, and the 238 U-206 Pb age of the youngest concordant spot was 105.4 ± 2.1 Ma. The concordia age of the youngest cluster (10 spots) was 106.9 ± 1.0 Ma (MSWD = 9.6, probability = 0.002). The weighted mean of the 238U-206 Pb ages of the youngest cluster was 106.9 ± 0.8 Ma (MSWD = 0.89, probability = 0.54). Fine tuff (Sample OY1)—We obtained analytical data from 27 spots in 27 zircon grains and adopted 25 data sets (Table 2). Then we chose 23 concordant data sets out of 25 (Fig. 7). The 238U-206Pb ages of the concordant spots clustered in 115–129 Ma and scattered at 209 and 251 Ma. The 238 U-206 Pb age of the youngest concordant spot was 115.5 ± 6.8 Ma. The concordia age of the youngest cluster (19 spots) was 120.9 ± 1.1 Ma (MSWD = 3.0, probability = 0.081). The weighted mean of the 238U-206Pb ages of the youngest cluster was 120.9 ± 1.1 Ma (MSWD = 1.01, probability = 0.44).
DISCUSSION
Igneous activity that terminated the deposition of the Tetori Group We studied the zircon U-Pb ages of the andesitic tuff of the Hayashidani Andesite overlying the Tetori Group (s.s.) in the Itoshiro-Oyama area (IO1) and of felsic dikes cutting the Tetori Group ( s.s. ) in the Itoshiro-Oyama (IO2) and Yamanomura (YM1, YM2) areas. We interpret that the concordia ages of the youngest age-cluster of each sample is a measure of the consolidation age of the magma that formed each sample from the following reasons. (1) The closure temperature of the zircon U-Pb system, ca. 900 ℃ (Cherniak and Watson, 2000), lies between the temperatures of normal rhyolitic and basaltic magmas; and (2) normal life-span of an arc volcano is one million years or less, comparable to the error of the U–Pb dating of 100-million-year-old zircon grains. Thus, we suggest that the volcanic or pyroclastic rocks of samples IO1, IO2, YM1, and YM2 were formed at 109.3 ± 1.6 Ma, 104.3 ± 1.5 Ma, 108.9 ± 0.7 Ma, and 106.9 ± 1.0 Ma, respectively, in a middle Albian continuous igneous activity. Accordingly, we propose that the deposition of the Tetori Group ( s.s. ) in the Hakusan and Jinzu Regions terminated by ca. 104–109 Ma (middle Albian), which corresponds to the inferred numerical age of DS4 of Sano (2015).
Period of deposition of the Tetori Group in the Jinzu Region
The period of deposition of the Tetori Group ( s.s. ) in the Jinzu Region included ca.121 Ma (early Aptian) and terminated by 104–109 Ma (middle Albian). The concordia ages of the youngest zircon cluster from the OY1 tuff in the Jinzu Region was 120.9 ± 1.1 Ma. We interpret that this is the best measure of the age of deposition of the OY1 tuff as discussed in the previous section. Recently, Sakai et al. (2015) reported the zircon U-Pb age of 121.2 ± 1.1 Ma from a tuff sample of the Akaiwa Formation, a lithostratigraphic unit of DS3 of Sano (2015), of the Tetori Group in the Hakusan Region. The age of the tuff samples from the Jinzu and Hakusan Regions are concordant within error, indicating that terrestrial clastic rocks of DS3 with ca. 121 Ma tuff commonly occur in both the Jinzu and Hakusan Regions (Fig. 9). Matsukawa et al. (2014a) interpreted that the Inotani Formation of the Jinzu Group ( sensu Matsukawa et al., 2014a) was deposited in the Aptian–Albian or later because they obtained Aptian– Albian zircon fission-track ages from four tuffaceous sandstone samples of the formation. Three samples among the four were collected from the Yamanomura area, and their ages were 112.5, 102.6, and 121.0 Ma, in ascending order. The age of the uppermost sample was identical with the Aptian zircon U-Pb age of the OY1 tuff, whereas the ages of the lower two samples corresponded to the Albian. Maeda and Takenami (1957) and Gifu-ken Dinosaur Fossil Excavation Party (1997) have reported many dikes and sills, including the felsic dikes YM1 and YM2 we sampled, around the locations of the lower two tuffaceous sandstone samples. The zircon U-Pb ages of YM1 and YM2 are 108.9 ± 0.7 Ma and 106.9 ± 1.0 Ma, respectively, and lie between the two zircon fission-track ages of the lower tuffaceous sandstone samples. Considering (1) the irregular zircon fission-track ages concerning the stratigraphic horizons and (2) the relatively low closure temperature (ca. 250°C; Hurford, 1986), we suggest that these Albian fission-track ages are rejuvenation ages caused by
the heating from ca. 104–109 Ma intrusive rocks. Matsukawa et al. (2014a) also reported an Albian zircon fission-track age from a tuffaceous sandstone sample of the Nagatogawa area, where intrusive rock bodies also occur (Maeda and Takenami, 1957; Matsukawa et al., 2014a, b; Shigeno, 2003). Although we have not obtained a zircon U-Pb age of the intrusive rocks in the Nagatogawa area, we suppose that the Albian zircon fission- track age would also be an annealed age. In conclusion, the Tetori Group ( s.s. ) in the Hakusan and Jinzu Regions commonly includes terrestrial sedimentary rocks of DS3 ( sensu Sano, 2015) with 121 Ma (early Aptian) tuff. The fact does not support the idea of Matsukawa et al. (2014a) that the age of the Tetori Group ( s.s. ) in the Hakusan Region and Jinzu Region (Jinzu Group sensu Matsukawa et al., 2014a) differs from each other.
FIGURE 9. Stratigaphy and correlation of the Early Cretaceous in the Inner Zone of Southwest Japan. Abbreviations—Alt.: Alternation Member, Cgl.: Conglomerate Member, Fm.: F Commencement of Early Cretaceous igneous activity in central and Southwest Japan Takeuchi et al. (2015, 2017) already reported concordant data with us. Takeuchi et al. (2015) studied the U-Pb age of detrital zircons from the Mizukamidani Formation of the Tetori Group and that of the garnet dacite dike cutting the Tetori Group in the Asahi area, eastern Toyama Prefecture (Fig. 1). They obtained a cluster age of 123 Ma from the Mizukamidani Formation and that of 109 Ma from the garnet dacite dike. Thus they concluded that the Tetori Group in the Asahi area continued its deposition until 123 Ma or later, but cut by the dike of 109 Ma. Moreover, Takeuchi et al. (2017) reported the zircon U-Pb age of 109.2 ± 0.8 Ma from a welded-tuff sample of the Oyashirazu Formation that overlies the Tetori Group in the Asahi area and consists of andesitic to dacitic volcanic rocks. The ages of the Oyashirazu Formation and the Hayashidani Andesite (109.3 ± 1.6 Ma; this study) are concordant within error. Andesitic to rhyolitic volcanic or pyroclastic rocks have also been reported from the Inner Zone of Southwest Japan. Kusuhashi et al. (2013) reported the zircon U-Pb age of 106.4 ± 0.4 Ma from the andesite of the Sawada Formation of the Sasayama Group (Hayashi et al., 2017) in Hyogo Prefecture. The Kisa Andesite in Hiroshima Prefecture and associated rhyolitic tuff yield the zircon U-Pb ages of 111.5 ± 1.5 Ma and 111.8 ± 0.9 Ma, respectively (Hayasaka et al., 2016). The rhyolitic tuff of the Shimonoseki Subgroup of the Kanmon Group in Yamaguchi Prefecture yields the zircon U-Pb age of 106.4 ± 1.0 Ma (Miyazaki et al., 2018). It is noteworthy that the Kanmon and Sasayama Groups both consist of fluvial clastic rocks covered with andesitic to rhyolitic volcanic and pyroclastic rocks and indicate a similar environmental change with that of DS3 (fluvial)–DS4 (volcanic) of Sano (2015). In the present-day Inner Zone of Southwest Japan, andesitic to rhyolitic igneous activity seems to have regionally commenced at ca. 104–112 Ma. Stratigraphy and correlation of the Early Cretaceous in the Inner Zone of Southwest Japan are summarized in Fig. 9.
CONCLUSIONS
We obtained LA-ICPMS U-Pb ages of igneous zircons from (1) a tuff sample of the Hayashidani Andesite of DS4 (Sano, 2015) that covers the Tetori Group ( s.s. ) in the Hakusan Region, (2) felsic dikes that cut the Tetori Group ( s.s. ) in the Hakusan and Jinzu Regions, and (3) a tuff sample (OY1) in the Tetori Group ( s.s. ) in the Jinzu Region. The results of the study and their implications can be summarized as follows. 1. The depositional age of the tuff of the Hayashidani Andesite was 109.3 ± 1.6 Ma, whereas the ages of the felsic dikes in the Hakusan and Jinzu Regions were 104.3 ± 1.5 Ma, 108.9 ± 0.7 Ma, and 106.9 ± 1.0 Ma. These DS4 ages strongly suggest that the deposition of the Tetori Group ( s.s. ) terminated by 104–109 Ma. Albian zircon fission-track ages from the Tetori Group ( s.s. ) of Matsukawa et al. (2014a), supporting their idea that the Tetori Group ( s.s.) in the Jinzu Region (Jinzu Group sensu Matsukawa et al., 2014a) is of Aptian–Albian or younger age, are likely rejuvenation ages caused by the heating from the 104–109 Ma felsic dikes. 2. The depositional age of the OY1 tuff in the Tetori Group ( s.s. ) of the Jinzu Region was 120.9 ± 1.1 Ma and is identical with that of a tuff sample in the Akaiwa Formation (DS3) of Hakusan Region (121.2 ± 1.1 Ma; Sakai et al., 2015). These ages suggest that terrestrial clastic rocks of DS3 with ca.121 Ma tuff commonly occur both in the Hakusan and Jinzu Regions.
ACKNOWLEDGMENTS
We would like to express our sincere gratitude to Professor Shin-ichi Sano of the University of Toyama for various discussions and instructions during the course of this study. We thank Dr. Kazuhiro Miyazaki and Dr. Daisuke Sato of the Geological Survey of Japan, AIST for giving us advice on Cretaceous igneous activity in Southwest Japan. Critical reviews of the submitted manuscript by Emeritus Professor Kenji Konishi of Kanazawa University, Professor Toshihiro Yamada of Osaka City University, Tatsuki Tsujimori of Tohoku University, and Dr. Soichiro Kawabe of the Fukui Prefectural Dinosaur Museum are greatly appreciated. This study was supported by JSPS KAKENHI Grant Number 17K05746. Board of Education of Toyama Prefecture. 2003. Report of the Geological Survey of the Tetori Group in Toyama Prefecture. Board of Education of Toyama Prefecture, Toyama, 109 p.* Cherniak, D. J., and E. B. Watson. 2000. Pb diffusion in zircon. Chemical Geology 172: 5–24. 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