2023 Volume 118 Issue 1 Article ID: 221219a
Oligocene to Miocene volcanic rocks from the Toyama basin of the SW Japan arc, that were formed during back-arc spreading in the Japan Sea, were examined to reveal their petrogenesis and temporal change of arc volcanism during the Japan Sea opening. The arc volcanism in the Toyama basin initiated with rhyolitic pyroclastic flows (Tori Formation) containing hecatolite (moonstone) in 23-22 Ma. Enriched Sr-Nd isotope (SrI = 0.70769-0.70944; NdI = 0.51203-0.51224) suggests that contemporaneous andesitic magma (Kamiwazumi and Matsunagi Formations) mixed or assimilated basement granitoids and gneisses of the Hida belt to generate rhyolitic magma. Subsequently, andesitic volcanism (Iwaine Formation) occurred in 18-17 Ma after magmatic hiatus. Andesitic lavas of the Iwaine Formation are composed of high magnesian andesite (HMA), high-Sr andesite and tholeiitic andesite. HMA has Mg# > 64, high Cr and Ni concentrations, not so high Th/Yb and (La/Sm)N ratios, and slightly enriched Sr-Nd isotope (SrI = 0.70482; NdI = 0.51279). High-Sr andesite has relatively low SiO2 content (<60 wt%), high Sr (>2000 ppm) and K2O contents (3.98 wt% in the maximum), indicating that it is low-SiO2 adakite. These geochemical characteristics suggest that HMA and high-Sr andesite were produced by partial melting of the mantle wedge saturated by H2O derived from slab fluid and metasomatized by slab melt, respectively. Although chemical variation diagrams suggest tholeiitic andesite seems to have been generated from basaltic magma, it has enriched Sr-Nd isotope (SrI = 0.70713-0.70756; NdI = 0.51237-0.51241). Thus, tholeiitic andesite is considered to have been produced by AFC (assimilation and fractional crystallization) after generation of basaltic parental magma. Andesitic magmatism of the Iwaine Formation was followed by rhyolitic magmatism of the Iozen Formation in 17-16 Ma. The petrogenesis of the rhyolite from the Iozen Formation can be explained by low-rate mixing between andesitic magma (Iwaine Formation) and the Hida belt. The petrogeneses of the andesites, especially HMA and high-Sr andesite, are related to slab melting. Because the old and cold Pacific plate was subducting beneath the Toyama basin during the Japan Sea opening, additional heat source such as upwelling of the asthenospheric mantle into the mantle wedge is required. Moreover, back-arc spreading in the Japan Sea was driven by upwelling of the asthenospheric mantle into the mantle wedge.
Extensive arc volcanism occurred especially in the Japan Sea side of the Japan arc during back-arc spreading in the Japan Sea (Japan Sea opening). As continents have grown through subduction processes in the earth’s history (Tamura et al., 2016; Sawada et al., 2018), back-arc spreading is considered to have provided complementary information on tectono-thermal evolution of active continental margins (Wolfram et al., 2019). Therefore, arc volcanism during back-arc spreading is considered to be a significant key to reveal process of evolution of volcanic arc.
Whole-rock geochemical variations of Oligocene to Miocene basaltic rocks from the NE Japan arc, especially in Sr-Nd isotopic compositions of basalts and andesites (e.g., Hanyu et al., 2006; Shuto et al., 2006, 2015), suggest that temporal change of basaltic volcanism was caused by upwelling of the hot asthenosphere into the mantle wedge (Yoshida, 2009; Shuto et al., 2015). Meanwhile, sub-alkalic (partly alkalic) basalts to dacites were erupted in the western Japan Sea side of the SW Japan arc from 25 to 17 Ma (Kimura et al., 2003, 2005). Just after termination of the Japan Sea opening, andesitic and rhyolitic volcanism including high-magnesian andesite (HMA) occurred in the Pacific side of the SW Japan arc due to initiation of subduction by the Philippine Sea plate (e.g., Nakada and Takahashi, 1979; Tatsumi, 2006). Nevertheless, temporal change of arc volcanism during the Japan Sea opening in the Toyama basin, situated in the easternmost part of the SW Japan arc, is not well understood yet compared to the NE and western SW Japan arcs (Yamada and Takahashi, 2021).
The Toyama basin, one of the large sedimentary basins in the Japan arc, is characterized by voluminous andesites and rhyolites (Fig. 1). The Late Oligocene to Middle Miocene strata related to the Japan Sea opening in the Toyama basin can be subdivided into Nirehara (Tori), Iwaine, Iozen, Kurosedani, and Higashibessho Formations (Nakajima et al., 2019; Yamada and Takahashi, 2021). Of these formations, the Tori (rhyolite), Iwaine (andesite), and Iozen (rhyolite) are composed mainly of volcanic rocks. The Tori Formation is a rift-type rhyolite known for its occurrence of hecatolite (moonstone) (Ishida et al., 1998; Ayalew and Ishiwatari, 2011). The mineral chemical composition of hecatolite in so-called ‘moonstone rhyolite’ suggests that the rhyolitic magma was relatively dry, produced by melting of anhydrous rocks such as granulite (Ishida et al., 1998). Contemporaneously, basaltic to andesitic volcanism occurred in the Noto peninsula to form the Kamiwazumi and Matsunagi Formations (Yamada and Takahashi, 2021). Although Okamura et al. (2016) reported whole-rock geochemical compositions of these andesites, the petrogenesis has not been revealed. The petrogenesis of andesites including calc-alkaline and tholeiitic andesites, HMA, and adakite from the Iwaine Formation has been studied in detail by Ishiwatari and Ohama (1997), Takahashi and Shuto (1999), Tsuchihashi and Ishiwatari (2006), Ishiwatari et al. (2007), and Sato et al. (2013). Ishiwatari and Ohama (1997) discussed that the andesitic magma of the Iwaine Formation assimilated the continental crust, based on xenocryst and whole-rock geochemistry. Sato et al. (2013) concluded that hot state of the mantle wedge due to upwelling of the asthenosphere caused melting of subducting slab and the mantle wedge. Nevertheless, petrogenesis of the rhyolite from the Iozen Formation has not been revealed.
Although the andesites from the Iwaine Formation have been studied in detail, most of the research focused on specific rock types such as HMA and adakite. Additionally, the magma generation process of the rhyolites has not been revealed. Accordingly, information on petrogenesis of andesites and rhyolites in the Toyama basin is not enough to discuss the temporal change of arc volcanism related to the Japan Sea opening in the Toyama basin. To understand this, we examined whole-rock geochemistry (major and trace elements and Sr-Nd isotopes) of andesites and rhyolites from the Toyama basin. In this paper, we discuss the petrogenesis of volcanic rocks from the Tori, Iwaine and Iozen Formations and temporal change of arc volcanism in the Toyama basin during the Japan Sea opening. Our results further give implications for the mantle-crust dynamics during the Japan Sea opening.
The Japan Sea, separating the Eurasian continent and the Japan arc, opened rapidly during 21-15 Ma (Kano, 2018; Nakajima, 2018) because of (1) asthenospheric mantle injection into the mantle wedge (Tatsumi et al., 1989; Okamura et al., 1998; Shuto et al., 2006; Zadeh et al., 2013; Shuto et al., 2015), (2) rollback of the Pacific and Philippine Sea plates (Martin, 2011), (3) collision between the Indian subcontinent and the Eurasian continent (Kimura and Tamaki, 1986; Jolivet et al., 1994; Ren et al., 2002; Horne et al., 2017), and/or (4) oblique subduction of the Pacific plate (Yin, 2010).
The formation history of the Japan Sea can be divided into four stages (Kano, 2018; Nakajima, 2018; Yamada and Takahashi, 2021): Stage I (44-28 Ma), stage II (28-23 Ma), stage III (23-18 Ma), and stage IV (18-15.3 or 13.5 Ma). In the early stage of the Japan Sea opening (stage I; 44-28 Ma), rifting initiated and progressed slowly (Kano et al., 2007; Kano, 2018; Nakajima, 2018). During this period, in the SW Japan arc, large-scale felsic magmatism resulted in formation of calderas (Imaoka et al., 2011), in contrast to the emplacement of Sr-Nd isotopically depleted basalts in the NE Japan arc (Shuto et al., 2015). During stage II (28-23 Ma), regional uplift, caused by upwelling of the asthenospheric mantle into the mantle wedge, formed unconformities in the entire Japan arc (Kano et al., 2007; Kano, 2018). During stage III (23-18 Ma), subsidence by rifting parallel to the continental margins (Hayashida et al., 1991) repeated, with minor volcanic activity in the eastern Japan Sea at 23 Ma (Kano, 2018), and unconformities are found between 19 and 17 Ma (Kano et al., 2007). In stage IV (18-15.3 Ma), the Japan arc was separated rapidly from the continent, with clockwise rotation of the SW Japan arc (e.g., Hoshi et al., 2015; Hoshi, 2018) and strike-slip faulting accompanied by rifting and rotations in strike-slip basins of the NE Japan arc (e.g., Hosoi et al., 2023). In the NE Japan arc, rifting continued until 13.5 Ma (Stage IV; Kano, 2018; Nakajima, 2018). Most of the volcanic rocks including the studied volcanic rocks in this study related to the Japan Sea opening were formed during this period.
Geological setting and geochronology of the Toyama basinThe Toyama basin, one of the large Cenozoic sedimentary basins in the Japan arc, is thought to form a large rift basin that extended from the Toyama trough in the Toyama Bay to the Hokuriku region of the SW Japan (Ishiyama et al., 2017; Nakajima et al., 2021; Takeuchi, 2021). The thickness of Cenozoic sediments and volcanic rocks in the Toyama basin is considered to be up to ∼ 5 km in the maximum according to the results of seismic survey by Ishiyama et al. (2017). The basement of the Toyama basin forming the Hida mountains consists of the Hida belt, the Tetori Group and the Futomiyama Group. The Hida belt is composed mainly of Paleozoic to Mesozoic paragneiss including marble, orthogneiss, amphibolite, sedimentary-origin schist and granitoids (Takahashi et al., 2010, 2018; Cho et al., 2021; Yamada et al., 2021). The Tetori Group is composed of Middle Jurassic to Lower Cretaceous fossiliferous (e.g., plants, mollusks, and dinosaurs) conglomerate, sandstone and mudstone (Sano, 2015). Late Cretaceous (∼ 70 Ma) rhyolites with granitic plutons are distributed as the Futomiyama Group covering the Hida belt and Tetori Group (Sudo, 1979; Ganzawa, 1983; Kaneko et al., 2019). Cenozoic strata composed of sedimentary and volcanic rocks related to the Japan Sea opening unconformably cover those basement rocks.
The Late Oligocene to Middle Miocene strata related to the Japan Sea opening in the Toyama basin can be subdivided into Nirehara (Tori), Iwaine, Iozen, Kurosedani and Higashibessho Formations (Nakajima et al., 2019; Yamada and Takahashi, 2021). In the Nanto area (study area), Late Oligocene to Middle Miocene strata are distributed as the Tori, Iwaine and Iozen Formations (Ganzawa, 1983; Fig. 2).
The Tori Formation, which is the equivalent to the Nirehara Formation (Yamada and Takahashi, 2021), is composed mainly of conglomerate and rhyolitic welded tuff (Yamasaki and Miyajima, 1970; Sudo, 1979; Ganzawa, 1983). The Tori Formation is distributed in the western part of Toyama Prefecture (including the study area), whereas the Nirehara Formation (without rhyolites) is distributed in the central and eastern parts of Toyama Prefecture (out of the study area). Rhyolitic welded tuff is not observed in the Nirehara Formation. According to reported zircon fission track (FT) and K-Ar ages, Yamada and Takahashi (2021) considered that the Tori Formation was formed from 23 to 22 Ma (stage III). Contemporaneously active andesites are distributed in the Noto peninsula (Fig. 1) as the Kamiwazumi and Matsunagi Formations. These formations are composed mainly of aphyric andesite and pyroxene andesite (Yoshikawa et al., 2002). Yoshikawa et al. (2002) considered that the Kamiwazumi and Matsunagi Formations were formed in 23-20 Ma and 23-19 Ma, respectively, based on stratigraphy and radiometric ages.
The Iwaine Formation, unconformably overlies the Tori Formation, is composed mainly of andesitic lava, pyroclastic rocks and clastic rocks (Sudo, 1979; Ganzawa, 1983). Based on zircon U-Pb, zircon FT, and K-Ar ages and paleomagnetism by previous studies, Nakajima et al. (2019) considered that the Iwaine Formation was formed from 17.5 to 17.2 Ma (stage IV).
The Iozen Formation, conformably covering the Iwaine Formation, is composed mainly of rhyolitic lava, pyroclastic rocks and clastic rocks (Inoue et al., 1964; Ganzawa, 1983). Yamada and Takahashi (2021) considered that the rhyolitic magmatism of the Iozen Formation occurred just after andesitic magmatism of the Iwaine Formation and lasted until ∼ 16 Ma (stage IV), based on reported zircon U-Pb, zircon FT, K-Ar ages.
The Kurosedani Formation is composed mainly of fossiliferous conglomerate and sandstone, meanwhile the Higashibessho Formation is composed mainly of massive mudstone (Hayakawa and Takemura, 1987; Nakajima et al., 2019). The Kurosedani and Higashibessho Formations are considered to have been deposited during 17-15 Ma (stage IV), based on magnetic polarity (Tamaki et al., 2006), diatom biostratigraphy (Yanagisawa, 1999) and radiometric ages (Nakajima et al., 2019).
Major element compositions of feldspar in rhyolites from the Tori and Iozen Formations were determined by electron probe micro analyzer (EPMA) using the JEOL JXA-8530F at Tono Geoscience Center of Japan Atomic Energy Agency (JAEA). Acceleration voltage, beam current and probe diameter were set as 15 kV, 20 nA, and 5 µm, respectively, during the analysis. Synthetic and natural minerals by Astimex Standards Ltd. were prepared and used as standard material.
Powders of rock samples for whole-rock geochemistry were prepared at Niigata University. Samples were cut into 2 × 2 × 3 cm sized chips by rock cutters, and the surfaces of the chips were cleaned by polishing. The chips were washed by deionized water and dried in a thermostatic chamber. Subsequently, the chips were crushed using a tungsten carbide mortar and powdered by an agate ball mill. The powdered samples were used for all whole-rock geochemical analyses.
Whole-rock major and trace element compositions were determined by X-ray fluorescence (XRF) spectrometry using the Rigaku RIX 3000 at Niigata University. The analysis was performed using the 1:2 glass bead method. We measured the compositions of 10 (SiO2, TiO2, Al2O3, FeO*, MnO, MgO, CaO, Na2O, K2O, and P2O5; FeO* is total iron) major and 11 trace elements (Ba, Ni, Pb, Th, Rb, Sr, Y, Zr, Nb, Cr, and V) following the analytical procedures of Takahashi and Shuto (1997).
Whole-rock concentrations of trace elements were determined using an Agilent 7500a quadrupole inductively coupled plasma mass spectrometer (ICP-MS) at Niigata University. Sample solution was prepared using two methods: acid digestion (AD) based on Neo et al. (2006) and alkali fusion after acid digestion (AFAD) following Senda et al. (2014). We measured the concentrations of 26 trace elements (Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Pb, Th, and U), following the analytical procedures of Neo et al. (2006). The accuracy of the trace element concentrations was assessed using the concentrations of the United States Geological Survey (USGS) reference material BHVO-2 (Eggins et al., 1997; Jochum et al., 2016). The averages of the trace element concentrations of BHVO-2 measured in this study are consistent with the reference values (Table 2; Eggins et al., 1997; Jochum et al., 2016). The nondissolution of zircon contained in the rhyolites in this study for the AD method affects the whole-rock Zr and Hf concentrations (Senda et al., 2014). Hence, we use the Zr concentration measured by XRF, and Hf is not included in the discussion. Generally, the trace element concentrations measured by ICP-MS are used for the discussion.
Whole-rock Sr-Nd isotopes were measured by thermal ionization mass spectrometer (TIMS) system of the Finnigan MAT262 at Niigata University. Sr and Nd were extracted from sample powders following Takahashi et al. (2009) and Hamamoto et al. (2000), respectively. Procedures for measurement were following Takahashi et al. (2009; Sr isotope), and Miyazaki and Shuto (1998; Nd isotope). Obtained 87Sr/86Sr and 143Nd/144Nd ratios were normalized by 88Sr/86Sr = 0.1194 and 146Nd/144Nd = 0.7219, and then, calibrated by using NIST987 (87Sr/86Sr = 0.710245) and JNdi-1 (143Nd/144Nd = 0.512106), respectively. In this study, 0.710222 ± 0.000043 (2SD; n = 9) and 0.512118 ± 0.000156 (2SD; n = 9) were obtained as means of 87Sr/86Sr of NIST987 and 143Nd/144Nd of JNdi-1, respectively.
In this study, we collected samples of volcanic rocks (lava and pyroclastic rocks) from the Tori, Iwaine, and Iozen Formations in the Nanto area of the central Toyama basin for geochemical analyses. The sampling locations are shown in Figure 2. The volcanic rocks can be distinguished into five rock types on the basis of mineral assemblages and textures. Mineral assemblages of each rock type are summarized in Table 1. Data on chemical composition of feldspar are shown in Supplementary Table A1 (Table A1 is available online from https://doi.org/10.2465/jmps.221219a).
| Formation | Rock type | Mineralogy | |||||
| Olivine | Clino- pyroxene |
Ortho- pyroxene |
Amphibole | Biotite | Ground mass (%) |
||
| Iozen | Type 5 | ++ | ++ | ++ | ∼ 80-95 | ||
| Iwaine | Type 4 | ++ | ++ | ∼ 55-95 | |||
| Type 3 | + | +++ | ∼ 80 | ||||
| Type 2 | ++ | ++ | ++ | ∼ 55-80 | |||
| Tori | Type 1 | + | ∼ 65-80 | ||||
| Formation | Rock type | |||||
| Plagioclase | Alkali feldspar |
Quartz | Opaque minerals |
Ground mass (%) |
||
| Iozen | Type 5 | +++ | ++ | ∼ 80-95 | ||
| Iwaine | Type 4 | +++ | + | ∼ 55-95 | ||
| Type 3 | ++* | +* | ++ | ∼ 80 | ||
| Type 2 | +++ | ++ | ∼ 55-80 | |||
| Tori | Type 1 | +++ | +++ | ++ | ∼ 65-80 | |
*, xenocryst; +, <0.1%; ++, 0.1-10%; +++, >10%.
All samples from the Tori Formation are weakly to densely welded tuff without lithic clasts, described as Type 1 rhyolite (Fig. 3a). Type 1 rhyolite (biotite-bearing rhyolite) is defined by mineral assemblage of quartz, alkali feldspar (hecatolite; Or = 45-48 mol%) and opaque minerals with rare biotite grains in descending order of their contents (Table 1). Xenoliths of paragneiss containing alkali-feldspar, quartz and muscovite are rarely found (Fig. 3b). In some samples, chalcedony fills vesicles.
Andesitic lavas and pyroclastic rocks in the Iwaine Formation are composed of olivine two-pyroxene andesite, two-pyroxene amphibole andesite and two-pyroxene andesite in stratigraphic ascending order. Olivine two-pyroxene andesite and two-pyroxene amphibole andesite are distributed in the lowermost part of the Iwaine Formation, whereas two-pyroxene andesite occupies the middle to upper part of the Iwaine Formation. Type 2 andesite (olivine two-pyroxene andesite) is defined by contained phenocrysts of plagioclase, clinopyroxene, orthopyroxene, olivine, and opaque minerals in descending order of their contents (Table 1 and Fig. 3c). Olivine phenocrysts in all samples are completely altered to clay minerals. Type 3 andesite (two-pyroxene amphibole andesitic lava and pyroclastic rocks; Fig. 3d) is defined by abundant amphibole phenocrysts in addition to clinopyroxene, orthopyroxene, and opaque minerals (Table 1). However, some samples contain plagioclase phenocrysts too. Xenoliths and xenocrysts of granitoids are commonly observed (e.g., alkali feldspar in Fig. 3d). Tsuchihashi and Ishiwatari (2006) also reported xenolith of quartzite, granitoids, pyroxenite, and skarn. Type 4 andesite (Two-pyroxene andesite; Fig. 3e) is defined by the presence of phenocrysts of plagioclase, clinopyroxene, orthopyroxene, and opaque minerals in descending order of their contents.
All samples from the Iozen Formation are classified as aphyric rhyolitic lava (Type 5 rhyolite; Fig. 3f). This type contains a small number of phenocrysts of plagioclase (An = 57-70 mol%), amphibole, orthopyroxene, clinopyroxene, and opaque minerals (Table 1). Type 5 rhyolite (aphyric rhyolite) is defined by the high-proportion of ground mass (∼ 80-95%). Some samples of Type 5 rhyolite are slightly to moderately altered, showing pale green color.
Whole-rock major and trace element, and Sr-Nd isotopic compositions of the Cenozoic volcanic rocks from the Nanto area of the central Toyama basin are shown in Figures 4-8 and Tables 2 and 3.
| Formation | Standard | Standard | Tori | Tori | Tori | Tori | Tori | Tori |
| Sample | BHVO-2 | BHVO-2 | NT01 | NT05 | NT07 | NT09 | NT16 | NT49 |
| Type | Type 1 | Type 1 | Type 1 | Type 1 | Type 1 | Type 1 | ||
| Rock type | Basalt | Basalt | Welded tuff | Welded tuff | Welded tuff | Welded tuff | Welded tuff | Welded tuff |
| Major elements measured by XRF (wt%) | ||||||||
| SiO2 | - | - | 76.72 | 76.41 | 77.93 | 76.23 | 75.01 | 77.65 |
| TiO2 | - | - | 0.09 | 0.17 | 0.14 | 0.14 | 0.20 | 0.12 |
| Al2O3 | - | - | 13.57 | 13.00 | 11.25 | 12.69 | 13.64 | 14.11 |
| FeO* | - | - | 1.93 | 1.70 | 2.00 | 1.53 | 1.20 | 1.75 |
| MnO | - | - | 0.01 | 0.003 | 0.02 | 0.02 | 0.003 | 0.02 |
| MgO | - | - | 0.17 | 0.16 | 0.10 | 0.19 | 0.16 | 0.19 |
| CaO | - | - | 0.06 | 0.11 | 0.11 | 0.17 | 0.19 | 0.31 |
| Na2O | - | - | 1.19 | 2.82 | 2.80 | 4.11 | 3.21 | 1.86 |
| K2O | - | - | 5.36 | 4.90 | 5.45 | 4.83 | 5.74 | 4.19 |
| P2O5 | - | - | 0.01 | 0.01 | 0.02 | 0.01 | 0.02 | 0.01 |
| Total | - | - | 99.11 | 99.29 | 99.80 | 99.93 | 99.37 | 100.21 |
| L.O.I. | 2.46 | 1.68 | 0.87 | 1.58 | 0.54 | 3.34 | ||
| Mg# | 14 | 14 | 8 | 18 | 19 | 16 | ||
| Trace elements measured by XRF (ppm) | ||||||||
| Ba | - | - | 481 | 164 | 95 | 387 | 324 | 345 |
| Ni | - | - | 3 | n.d. | 7 | 4 | 8 | 10 |
| Pb | - | - | 10.5 | 12.0 | 16.5 | 13.4 | 14.4 | 42.2 |
| Th | - | - | 19.5 | 17.7 | 23.3 | 13.8 | 17.6 | 15.2 |
| Rb | - | - | 176 | 169 | 191 | 150 | 181 | 117 |
| Sr | - | - | 11 | 12 | 16 | 23 | 22 | 46 |
| Y | - | - | 38 | 22 | 53 | 35 | 43 | 64 |
| Zr | - | - | 271 | 409 | 356 | 246 | 432 | 266 |
| Nb | - | - | 15.5 | 13.7 | 20.2 | 11.2 | 13.4 | 13.7 |
| Cr | - | - | 5 | 3 | 5 | 5 | 5 | 5 |
| V | - | - | 11 | 7 | 10 | 13 | 5 | 12 |
| Trace elements measured by ICP-MS (ppm) | ||||||||
| Method | ADAF | AD | ADAF | ADAF | ADAF | AD | ||
| Rb | 8.30 | 9.70 | - | - | 196 | 152 | 171 | 116 |
| Sr | 376 | 395 | - | - | 15.0 | 21.9 | 21.7 | 41.6 |
| Y | 28.0 | 28.2 | - | - | 53.0 | 33.0 | 46.0 | 63.8 |
| Zr | 173 | 183 | - | - | 343 | 236 | 444 | 192 |
| Nb | 19.2 | 19.6 | - | - | 22.2 | 12.0 | 14.9 | 12.6 |
| Cs | 0.064 | 0.092 | - | - | 1.69 | 2.77 | 2.12 | 2.38 |
| Ba | 133 | 134 | - | - | 104 | 389 | 323 | 331 |
| La | 15.4 | 15.5 | - | - | 64.5 | 43.8 | 76.0 | 42.6 |
| Ce | 37.3 | 38.0 | - | - | 125 | 81.4 | 134 | 73.9 |
| Pr | 5.37 | 5.44 | - | - | 14.2 | 9.47 | 16.1 | 11.2 |
| Nd | 23.8 | 25.0 | - | - | 48.0 | 33.4 | 56.8 | 39.2 |
| Sm | 6.24 | 5.99 | - | - | 10.6 | 7.03 | 11.6 | 8.47 |
| Eu | 2.12 | 2.00 | - | - | 0.407 | 0.549 | 0.750 | 1.03 |
| Gd | 5.89 | 6.28 | - | - | 8.14 | 5.55 | 8.49 | 9.14 |
| Tb | 0.969 | 0.912 | - | - | 1.43 | 0.915 | 1.34 | 1.59 |
| Dy | 4.88 | 5.16 | - | - | 8.09 | 5.07 | 7.33 | 9.38 |
| Ho | 0.988 | 1.04 | - | - | 1.68 | 1.11 | 1.55 | 2.03 |
| Er | 2.64 | 2.46 | - | - | 5.18 | 3.40 | 4.53 | 6.03 |
| Tm | 0.325 | 0.355 | - | - | 0.790 | 0.500 | 0.690 | 0.905 |
| Yb | 1.82 | 2.07 | - | - | 4.59 | 3.07 | 3.97 | 5.18 |
| Lu | 0.280 | 0.278 | - | - | 0.764 | 0.503 | 0.661 | 0.800 |
| Hf | 4.05 | 4.38 | - | - | 8.60 | 5.80 | 9.20 | 5.22 |
| Ta | 1.13 | 1.19 | - | - | 1.37 | 0.745 | 0.918 | 0.885 |
| Pb | 2.55 | 1.48 | - | - | 24.1 | 19.3 | 19.6 | 42.1 |
| Th | 1.35 | 1.23 | - | - | 20.6 | 13.2 | 18.0 | 14.4 |
| U | 0.411 | 0.382 | - | - | 2.87 | 2.74 | 2.57 | 2.55 |
n.d., not detected; L.O.I., Loss on ignition.
| Formation | Tori | Tori | Tori | Tori | Tori | Tori | Tori | Iwaine | Iwaine |
| Sample | NT63a | NT63c | NT65 | NT66 | NT67 | NT71 | NT74 | NT48b | NT51 |
| Type | Type 1 | Type 1 | Type 1 | Type 1 | Type 1 | Type 1 | Type 1 | Type 2 | Type 2 |
| Rock type | Welded tuff | Welded tuff | Welded tuff | Welded tuff | Welded tuff | Welded tuff | Welded tuff | Lava | Lava |
| Major elements measured by XRF (wt%) | |||||||||
| SiO2 | 79.20 | 79.51 | 79.32 | 75.77 | 76.56 | 78.45 | 75.83 | 56.78 | 55.63 |
| TiO2 | 0.14 | 0.14 | 0.15 | 0.19 | 0.18 | 0.07 | 0.19 | 1.05 | 1.03 |
| Al2O3 | 11.75 | 11.34 | 10.28 | 13.53 | 12.78 | 11.43 | 12.67 | 15.87 | 16.82 |
| FeO* | 1.75 | 1.78 | 2.34 | 1.96 | 1.65 | 1.36 | 1.89 | 8.33 | 8.05 |
| MnO | 0.003 | 0.02 | 0.01 | 0.01 | 0.01 | 0.004 | 0.07 | 0.14 | 0.21 |
| MgO | 0.10 | 0.05 | n.d. | 0.14 | 0.18 | 0.04 | 0.03 | 5.66 | 6.04 |
| CaO | 0.12 | 0.12 | 0.05 | 0.10 | 0.11 | 0.05 | 0.53 | 8.01 | 8.04 |
| Na2O | 2.70 | 2.76 | 2.31 | 3.13 | 2.50 | 3.44 | 4.46 | 3.01 | 3.47 |
| K2O | 4.71 | 4.80 | 5.49 | 5.09 | 5.46 | 4.82 | 3.61 | 0.90 | 1.02 |
| P2O5 | 0.01 | 0.01 | 0.02 | 0.02 | 0.02 | 0.01 | 0.02 | 0.19 | 0.19 |
| Total | 100.48 | 100.52 | 99.97 | 99.93 | 99.45 | 99.68 | 99.28 | 99.94 | 100.49 |
| L.O.I. | 1.26 | 1.07 | 0.52 | 1.79 | 1.93 | 0.65 | 3.70 | 0.66 | 0.92 |
| Mg# | 9 | 4 | n.d. | 11 | 16 | 5 | 3 | 55 | 57 |
| Trace elements measured by XRF (ppm) | |||||||||
| Ba | 133 | 159 | 161 | 323 | 237 | 56 | 326 | 4 | 6 |
| Ni | 5 | 2 | 4 | 4 | 7 | 11 | 8 | 32 | 27 |
| Pb | 17.2 | 14.6 | 20.1 | 21.5 | 17.2 | 15.5 | 21.4 | 5.0 | 5.6 |
| Th | 16.8 | 14.7 | 13.2 | 18.9 | 17.4 | 20.9 | 17.5 | 2.0 | 2.6 |
| Rb | 153 | 142 | 176 | 190 | 186 | 202 | 198 | 17 | 19 |
| Sr | 15 | 22 | 11 | 23 | 17 | 7 | 39 | 615 | 634 |
| Y | 32 | 25 | 44 | 51 | 51 | 57 | 43 | 22 | 22 |
| Zr | 393 | 370 | 336 | 424 | 426 | 266 | 419 | 107 | 106 |
| Nb | 13.3 | 11.8 | 12.4 | 17.6 | 15.6 | 16.5 | 12.5 | 4.4 | 5.7 |
| Cr | 3 | 3 | 7 | 5 | 3 | 3 | 4 | 152 | 143 |
| V | 3 | 8 | 9 | 4 | 8 | 4 | 4 | 215 | 222 |
| Trace elements measured by ICP-MS (ppm) | |||||||||
| Method | AD | AD | AD | AD | |||||
| Rb | - | 149 | 180 | 195 | - | - | - | - | 20.1 |
| Sr | - | 20.4 | 9.0 | 20.4 | - | - | - | - | 610 |
| Y | - | 23.0 | 42.8 | 51.1 | - | - | - | - | 22.5 |
| Zr | - | 136 | 179 | 286 | - | - | - | - | 108 |
| Nb | - | 12.4 | 10.8 | 16.7 | - | - | - | - | 4.4 |
| Cs | - | 1.60 | 2.26 | 2.30 | - | - | - | - | 0.416 |
| Ba | - | 160 | 154 | 312 | - | - | - | - | 401 |
| La | - | 54.3 | 51.0 | 61.5 | - | - | - | - | 11.1 |
| Ce | - | 104 | 95.4 | 118 | - | - | - | - | 25.3 |
| Pr | - | 12.0 | 11.1 | 13.4 | - | - | - | - | 3.49 |
| Nd | - | 41.8 | 38.9 | 47.7 | - | - | - | - | 15.0 |
| Sm | - | 7.26 | 7.35 | 9.25 | - | - | - | - | 3.60 |
| Eu | - | 0.322 | 0.481 | 0.533 | - | - | - | - | 1.12 |
| Gd | - | 5.75 | 6.85 | 8.71 | - | - | - | - | 3.68 |
| Tb | - | 0.806 | 0.977 | 1.32 | - | - | - | - | 0.567 |
| Dy | - | 4.35 | 6.01 | 7.71 | - | - | - | - | 3.46 |
| Ho | - | 0.840 | 1.27 | 1.73 | - | - | - | - | 0.768 |
| Er | - | 2.45 | 3.51 | 4.88 | - | - | - | - | 2.22 |
| Tm | - | 0.295 | 0.515 | 0.733 | - | - | - | - | 0.331 |
| Yb | - | 1.72 | 3.29 | 4.41 | - | - | - | - | 2.04 |
| Lu | - | 0.253 | 0.477 | 0.652 | - | - | - | - | 0.302 |
| Hf | - | 3.49 | 4.73 | 8.11 | - | - | - | - | 2.62 |
| Ta | - | 0.839 | 0.757 | 1.17 | - | - | - | - | 0.251 |
| Pb | - | 12.7 | 21.2 | 23.0 | - | - | - | - | 4.98 |
| Th | - | 14.1 | 12.6 | 18.5 | - | - | - | - | 1.92 |
| U | - | 1.49 | 2.10 | 2.73 | - | - | - | - | 0.472 |
n.d., not detected; L.O.I., Loss on ignition.
| Formation | Iwaine | Iwaine | Iwaine | Iwaine | Iwaine | Iwaine | Iwaine | Iwaine | Iwaine |
| Sample | NT69 | NT03 | NT20 | NT21 | NT22 | NT23 | NT24 | NT25 | NT26 |
| Type | Type 2 | Type 3 | Type 3 | Type 3 | Type 3 | Type 3 | Type 3 | Type 3 | Type 3 |
| Rock type | Lava | Lava | Lava | Lava | Lava | Lava | Lava | Lava | Lava |
| Major elements measured by XRF (wt%) | |||||||||
| SiO2 | 55.89 | 56.92 | 58.26 | 54.42 | 56.03 | 57.38 | 55.97 | 56.34 | 56.77 |
| TiO2 | 1.02 | 1.10 | 1.05 | 1.14 | 1.09 | 1.05 | 1.11 | 1.07 | 0.93 |
| Al2O3 | 14.43 | 15.78 | 16.29 | 16.83 | 16.34 | 15.86 | 16.52 | 15.93 | 16.23 |
| FeO* | 7.32 | 7.24 | 6.78 | 7.09 | 6.90 | 6.47 | 6.94 | 6.84 | 7.12 |
| MnO | 0.13 | 0.16 | 0.09 | 0.12 | 0.12 | 0.15 | 0.19 | 0.19 | 0.17 |
| MgO | 7.69 | 4.15 | 2.94 | 4.32 | 4.07 | 4.16 | 4.39 | 3.72 | 3.91 |
| CaO | 8.19 | 6.85 | 6.52 | 6.81 | 6.72 | 6.30 | 7.05 | 7.02 | 6.78 |
| Na2O | 2.61 | 3.02 | 3.08 | 3.37 | 3.19 | 3.14 | 3.12 | 3.22 | 3.42 |
| K2O | 1.05 | 2.73 | 2.92 | 2.93 | 2.96 | 3.08 | 2.78 | 2.72 | 1.75 |
| P2O5 | 0.20 | 0.35 | 0.34 | 0.35 | 0.35 | 0.32 | 0.34 | 0.34 | 0.18 |
| Total | 98.53 | 98.29 | 98.27 | 97.38 | 97.76 | 97.91 | 98.41 | 97.37 | 97.27 |
| L.O.I. | 1.91 | 1.33 | 1.22 | 2.38 | 2.18 | 2.29 | 2.07 | 1.50 | 0.72 |
| Mg# | 65 | 51 | 44 | 52 | 51 | 53 | 53 | 49 | 49 |
| Trace elements measured by XRF (ppm) | |||||||||
| Ba | 11 | 7 | 7 | 8 | 7 | 10 | 7 | 7 | 7 |
| Ni | 131 | 55 | 51 | 48 | 48 | 45 | 50 | 50 | 34 |
| Pb | 4.6 | 8.2 | 9.8 | 5.7 | 7.0 | 7.5 | 7.6 | 7.9 | 6.5 |
| Th | 1.5 | 7.2 | 6.8 | 7.1 | 10.8 | 8.3 | 9.4 | 9.5 | 5.1 |
| Rb | 26 | 33 | 37 | 37 | 44 | 51 | 33 | 36 | 36 |
| Sr | 476 | 2355 | 2311 | 2301 | 2244 | 2278 | 2372 | 2424 | 791 |
| Y | 21 | 21 | 21 | 24 | 21 | 25 | 22 | 22 | 24 |
| Zr | 118 | 173 | 175 | 183 | 175 | 180 | 180 | 175 | 147 |
| Nb | 11.4 | 7.4 | 7.5 | 7.8 | 6.8 | 9.7 | 6.7 | 6.6 | 6.9 |
| Cr | 367 | 82 | 72 | 80 | 80 | 80 | 77 | 77 | 81 |
| V | 172 | 174 | 160 | 165 | 165 | 134 | 191 | 177 | 160 |
| Trace elements measured by ICP-MS (ppm) | |||||||||
| Method | AD | AD | AD | AD | |||||
| Rb | 29.1 | 38.6 | - | - | 40.6 | - | - | 41.3 | - |
| Sr | 468 | 2571 | - | - | 2256 | - | - | 2579 | - |
| Y | 20.7 | 21.5 | - | - | 21.9 | - | - | 21.1 | - |
| Zr | 126 | 197 | - | - | 175 | - | - | 188 | - |
| Nb | 12.3 | 6.9 | - | - | 6.3 | - | - | 7.0 | - |
| Cs | 0.338 | 0.280 | - | - | 0.398 | - | - | 0.458 | - |
| Ba | 370 | 1181 | - | - | 1117 | - | - | 1136 | - |
| La | 18.3 | 44.2 | - | - | 38.4 | - | - | 44.2 | - |
| Ce | 39.2 | 93.0 | - | - | 80.3 | - | - | 91.8 | - |
| Pr | 4.84 | 11.3 | - | - | 10.0 | - | - | 11.2 | - |
| Nd | 18.7 | 41.6 | - | - | 37.4 | - | - | 41.0 | - |
| Sm | 3.95 | 6.75 | - | - | 6.24 | - | - | 6.73 | - |
| Eu | 1.22 | 2.00 | - | - | 1.79 | - | - | 1.93 | - |
| Gd | 3.55 | 5.32 | - | - | 4.89 | - | - | 5.03 | - |
| Tb | 0.559 | 0.658 | - | - | 0.659 | - | - | 0.646 | - |
| Dy | 3.19 | 3.74 | - | - | 3.50 | - | - | 3.61 | - |
| Ho | 0.704 | 0.768 | - | - | 0.716 | - | - | 0.745 | - |
| Er | 1.78 | 2.16 | - | - | 2.04 | - | - | 2.01 | - |
| Tm | 0.275 | 0.285 | - | - | 0.280 | - | - | 0.292 | - |
| Yb | 1.60 | 1.84 | - | - | 1.68 | - | - | 1.81 | - |
| Lu | 0.261 | 0.287 | - | - | 0.273 | - | - | 0.269 | - |
| Hf | 2.99 | 5.15 | - | - | 4.70 | - | - | 5.05 | - |
| Ta | 0.743 | 0.345 | - | - | 0.295 | - | - | 0.357 | - |
| Pb | 6.04 | 12.5 | - | - | 12.6 | - | - | 12.5 | - |
| Th | 3.06 | 8.36 | - | - | 7.81 | - | - | 10.2 | - |
| U | 0.561 | 1.53 | - | - | 1.32 | - | - | 2.13 | - |
n.d., not detected; L.O.I., Loss on ignition.
| Formation | Iwaine | Iwaine | Iwaine | Iwaine | Iwaine | Iwaine | Iwaine | Iwaine | Iwaine |
| Sample | NT73 | NT28 | NT33b | NT34 | NT35a | NT35b | NT36 | NT10 | NT11 |
| Type | Type 3 | Type 3 | Type 3 | Type 3 | Type 3 | Type 3 | Type 3 | Type 4 | Type 4 |
| Rock type | Lava | Pumice tuff | Pumice tuff | Pumice tuff | Pumice tuff | Pumice tuff | Pumice tuff | Lava | Lava |
| Major elements measured by XRF (wt%) | |||||||||
| SiO2 | 57.90 | 59.35 | 54.63 | 56.40 | 54.29 | 58.48 | 58.57 | 54.97 | 54.65 |
| TiO2 | 1.11 | 1.03 | 1.05 | 1.50 | 1.31 | 1.14 | 0.99 | 0.93 | 0.93 |
| Al2O3 | 15.34 | 17.90 | 20.31 | 15.46 | 20.32 | 16.48 | 16.84 | 18.80 | 18.25 |
| FeO* | 6.22 | 7.12 | 6.15 | 7.16 | 6.61 | 6.64 | 6.10 | 7.57 | 8.33 |
| MnO | 0.14 | 0.13 | 0.14 | 0.11 | 0.16 | 0.11 | 0.09 | 0.21 | 0.13 |
| MgO | 3.64 | 2.83 | 2.18 | 4.54 | 2.70 | 4.40 | 3.55 | 3.15 | 3.31 |
| CaO | 6.74 | 4.94 | 9.68 | 7.21 | 6.23 | 6.67 | 6.41 | 8.94 | 8.35 |
| Na2O | 2.81 | 4.15 | 4.50 | 2.22 | 3.42 | 2.43 | 2.64 | 2.82 | 2.84 |
| K2O | 2.15 | 1.91 | 2.52 | 3.98 | 3.46 | 1.96 | 2.57 | 1.39 | 1.35 |
| P2O5 | 0.43 | 0.20 | 0.25 | 0.57 | 0.28 | 0.30 | 0.29 | 0.14 | 0.16 |
| Total | 96.48 | 99.55 | 101.40 | 99.14 | 98.78 | 98.62 | 98.03 | 98.90 | 98.30 |
| L.O.I. | 1.18 | 4.54 | 4.48 | 2.00 | 2.52 | 4.57 | 3.66 | 1.49 | 1.51 |
| Mg# | 51 | 41 | 39 | 53 | 42 | 54 | 51 | 43 | 41 |
| Trace elements measured by XRF (ppm) | |||||||||
| Ba | 7 | 9 | 9 | 10 | 9 | 6 | 8 | 6 | 6 |
| Ni | 45 | 1 | 11 | 48 | 23 | 11 | 21 | 8 | 5 |
| Pb | 7.9 | 6.5 | 6.8 | 19.3 | 8.6 | 7.4 | 10.2 | 3.7 | 3.8 |
| Th | 7.7 | 6.2 | 4.2 | 11.1 | 6.8 | 5.0 | 8.7 | 4.5 | 3.3 |
| Rb | 30 | 55 | 61 | 60 | 89 | 46 | 48 | 40 | 32 |
| Sr | 2388 | 438 | 593 | 2222 | 875 | 1091 | 950 | 437 | 431 |
| Y | 21 | 24 | 21 | 29 | 30 | 25 | 21 | 22 | 22 |
| Zr | 165 | 162 | 188 | 307 | 206 | 166 | 205 | 117 | 122 |
| Nb | 6.7 | 8.9 | 8.7 | 10.0 | 8.5 | 6.5 | 7.7 | 6.0 | 6.4 |
| Cr | 76 | 18 | 79 | 104 | 126 | 36 | 51 | 35 | 19 |
| V | 173 | 140 | 188 | 209 | 235 | 199 | 142 | 233 | 226 |
| Trace elements measured by ICP-MS (ppm) | |||||||||
| Method | AD | AD | AD | AD | AD | ||||
| Rb | 38.3 | - | - | 72.9 | 97.7 | - | - | 37.1 | 33.3 |
| Sr | 2893 | - | - | 2442 | 846 | - | - | 407 | 422 |
| Y | 22.1 | - | - | 29.4 | 29.5 | - | - | 22.0 | 23.6 |
| Zr | 204 | - | - | 399 | 223 | - | - | 119 | 129 |
| Nb | 7.0 | - | - | 9.9 | 7.9 | - | - | 5.2 | 5.7 |
| Cs | 0.523 | - | - | 0.289 | 1.04 | - | - | 0.895 | 0.485 |
| Ba | 1230 | - | - | 1631 | 1036 | - | - | 430 | 437 |
| La | 45.7 | - | - | 82.4 | 44.5 | - | - | 19.9 | 21.0 |
| Ce | 96.0 | - | - | 171 | 78.8 | - | - | 40.2 | 43.5 |
| Pr | 11.9 | - | - | 21.8 | 11.9 | - | - | 4.92 | 5.43 |
| Nd | 41.8 | - | - | 83.4 | 44.3 | - | - | 19.2 | 20.3 |
| Sm | 6.58 | - | - | 14.0 | 8.09 | - | - | 3.94 | 4.09 |
| Eu | 2.13 | - | - | 3.65 | 2.20 | - | - | 1.18 | 1.26 |
| Gd | 4.68 | - | - | 9.46 | 6.77 | - | - | 3.85 | 3.94 |
| Tb | 0.623 | - | - | 1.06 | 0.955 | - | - | 0.581 | 0.601 |
| Dy | 3.52 | - | - | 5.03 | 5.22 | - | - | 3.35 | 3.52 |
| Ho | 0.739 | - | - | 1.01 | 1.09 | - | - | 0.742 | 0.785 |
| Er | 1.94 | - | - | 2.66 | 2.95 | - | - | 2.21 | 2.07 |
| Tm | 0.264 | - | - | 0.370 | 0.421 | - | - | 0.319 | 0.299 |
| Yb | 1.72 | - | - | 2.12 | 2.64 | - | - | 2.01 | 1.84 |
| Lu | 0.293 | - | - | 0.334 | 0.363 | - | - | 0.293 | 0.296 |
| Hf | 5.48 | - | - | 9.46 | 5.25 | - | - | 2.77 | 2.99 |
| Ta | 0.390 | - | - | 0.538 | 0.435 | - | - | 0.195 | 0.263 |
| Pb | 12.4 | - | - | 28.9 | 11.6 | - | - | 6.24 | 7.46 |
| Th | 8.21 | - | - | 18.1 | 7.77 | - | - | 3.29 | 3.55 |
| U | 1.45 | - | - | 3.13 | 1.42 | - | - | 0.322 | 0.426 |
n.d., not detected; L.O.I., Loss on ignition.
| Formation | Iwaine | Iwaine | Iwaine | Iwaine | Iwaine | Iozen | Iozen | Iozen | Iozen |
| Sample | NT12 | NT13 | NT52 | NT62 | NT68 | NT04 | NT17 | NT18 | NT41 |
| Type | Type 4 | Type 4 | Type 4 | Type 4 | Type 4 | Type 5 | Type 5 | Type 5 | Type 5 |
| Rock type | Lava | Lava | Lava | Lava | Lava | Lava | Lava | Lava | Lava |
| Major elements measured by XRF (wt%) | |||||||||
| SiO2 | 55.04 | 53.24 | 54.47 | 57.72 | 54.63 | 75.00 | 75.99 | 75.46 | 70.64 |
| TiO2 | 0.93 | 0.92 | 1.58 | 1.15 | 1.63 | 0.16 | 0.15 | 0.16 | 0.48 |
| Al2O3 | 18.32 | 17.49 | 15.73 | 19.94 | 16.36 | 13.08 | 13.63 | 13.48 | 15.26 |
| FeO* | 8.00 | 9.35 | 11.36 | 5.74 | 10.66 | 2.09 | 1.02 | 1.74 | 3.15 |
| MnO | 0.13 | 0.16 | 0.30 | 0.11 | 0.20 | 0.03 | 0.01 | 0.02 | 0.03 |
| MgO | 3.19 | 5.01 | 4.36 | 1.44 | 4.11 | 0.23 | 0.24 | 0.16 | 0.60 |
| CaO | 8.45 | 8.71 | 7.42 | 8.12 | 8.22 | 1.19 | 0.71 | 0.99 | 3.11 |
| Na2O | 2.81 | 2.54 | 2.88 | 3.19 | 2.62 | 3.67 | 2.58 | 3.50 | 3.63 |
| K2O | 1.35 | 0.96 | 1.62 | 1.68 | 0.77 | 3.73 | 3.71 | 3.81 | 3.29 |
| P2O5 | 0.16 | 0.15 | 0.21 | 0.22 | 0.21 | 0.01 | 0.02 | 0.01 | 0.10 |
| Total | 98.38 | 98.53 | 99.93 | 99.30 | 99.39 | 99.18 | 98.06 | 99.31 | 100.29 |
| L.O.I. | 1.19 | 2.48 | 0.80 | 1.11 | 1.58 | 0.62 | 1.23 | 0.90 | 1.02 |
| Mg# | 42 | 49 | 41 | 31 | 41 | 16 | 30 | 14 | 25 |
| Trace elements measured by XRF (ppm) | |||||||||
| Ba | 6 | 6 | 7 | 8 | 6 | 833 | 1075 | 821 | 747 |
| Ni | 7 | 11 | 5 | 5 | n.d. | 4 | 11 | 3 | 3 |
| Pb | 4.2 | 3.2 | 4.6 | 6.2 | 3.8 | 11.9 | 16.9 | 13.8 | 13.8 |
| Th | 3.6 | 3.2 | 2.9 | 4.6 | 2.4 | 11.9 | 13.7 | 12.7 | 9.3 |
| Rb | 31 | 13 | 51 | 46 | 10 | 115 | 127 | 116 | 91 |
| Sr | 439 | 432 | 419 | 474 | 464 | 140 | 102 | 123 | 259 |
| Y | 27 | 30 | 25 | 23 | 26 | 21 | 26 | 30 | 23 |
| Zr | 120 | 119 | 114 | 130 | 119 | 168 | 159 | 161 | 178 |
| Nb | 5.7 | 6.1 | 7.0 | 8.0 | 6.4 | 8.3 | 8.9 | 9.5 | 8.1 |
| Cr | 18 | 49 | 11 | 22 | 7 | 4 | 4 | 7 | 8 |
| V | 219 | 227 | 404 | 227 | 414 | 9 | 11 | 5 | 60 |
| Trace elements measured by ICP-MS (ppm) | |||||||||
| Method | AD | AD | AD | AD | ADAF | ADAF | ADAF | AD | |
| Rb | 34.1 | - | 54.4 | 48.3 | 9.76 | 114 | 124 | 118 | 92.6 |
| Sr | 423 | - | 393 | 440 | 460 | 132 | 94.6 | 115 | 241 |
| Y | 26.9 | - | 23.2 | 22.9 | 26.2 | 22.0 | 25.0 | 30.0 | 21.5 |
| Zr | 126 | - | 113 | 135 | 126 | 167 | 153 | 161 | 136 |
| Nb | 5.7 | - | 6.1 | 6.7 | 6.6 | 9.1 | 9.1 | 9.3 | 8.2 |
| Cs | 0.704 | - | 1.35 | 0.640 | 0.251 | 2.38 | 3.42 | 2.23 | 1.18 |
| Ba | 439 | - | 321 | 375 | 361 | 830 | 1069 | 818 | 722 |
| La | 24.5 | - | 15.7 | 17.9 | 16.6 | 35.0 | 42.3 | 51.7 | 30.7 |
| Ce | 42.8 | - | 34.3 | 38.1 | 36.2 | 59.1 | 60.4 | 70.0 | 55.8 |
| Pr | 5.96 | - | 4.37 | 4.85 | 4.83 | 6.77 | 8.03 | 10.2 | 6.14 |
| Nd | 23.0 | - | 17.7 | 19.2 | 19.1 | 22.2 | 26.1 | 34.1 | 21.8 |
| Sm | 4.60 | - | 4.14 | 4.11 | 4.22 | 4.17 | 5.13 | 6.79 | 3.70 |
| Eu | 1.43 | - | 1.26 | 1.28 | 1.40 | 0.877 | 1.08 | 1.30 | 0.929 |
| Gd | 4.46 | - | 4.27 | 4.28 | 4.40 | 3.39 | 3.91 | 4.95 | 3.49 |
| Tb | 0.686 | - | 0.659 | 0.631 | 0.677 | 0.555 | 0.638 | 0.800 | 0.514 |
| Dy | 3.99 | - | 3.98 | 3.74 | 4.17 | 3.06 | 3.52 | 4.32 | 3.23 |
| Ho | 0.854 | - | 0.860 | 0.782 | 0.870 | 0.681 | 0.776 | 0.933 | 0.701 |
| Er | 2.26 | - | 2.33 | 2.11 | 2.41 | 2.16 | 2.49 | 2.90 | 2.04 |
| Tm | 0.363 | - | 0.352 | 0.299 | 0.355 | 0.320 | 0.370 | 0.450 | 0.313 |
| Yb | 2.07 | - | 2.16 | 1.81 | 2.04 | 2.07 | 2.30 | 2.55 | 1.96 |
| Lu | 0.335 | - | 0.320 | 0.254 | 0.339 | 0.355 | 0.402 | 0.433 | 0.314 |
| Hf | 2.96 | - | 2.81 | 3.15 | 3.20 | 4.30 | 4.10 | 4.30 | 3.55 |
| Ta | 0.269 | - | 0.357 | 0.385 | 0.403 | 0.635 | 0.662 | 0.689 | 0.594 |
| Pb | 7.31 | - | 6.17 | 8.40 | 6.57 | 18.4 | 20.8 | 16.5 | 13.4 |
| Th | 3.44 | - | 3.57 | 4.36 | 3.80 | 13.6 | 11.6 | 13.3 | 9.89 |
| U | 0.469 | - | 0.577 | 0.806 | 0.649 | 1.97 | 2.28 | 1.88 | 1.66 |
n.d., not detected; L.O.I., Loss on ignition.
| Formation | Iozen | Iozen | Iozen | Iozen | Iozen | Iozen | Iozen |
| Sample | NT43 | NT53 | NT55 | NT56 | NT57 | NT59 | NT60 |
| Type | Type 5 | Type 5 | Type 5 | Type 5 | Type 5 | Type 5 | Type 5 |
| Rock type | Lava | Lava | Lava | Lava | Lava | Lava | Lava |
| Major elements measured by XRF (wt%) | |||||||
| SiO2 | 70.97 | 75.58 | 66.82 | 74.51 | 75.76 | 78.31 | 71.74 |
| TiO2 | 0.46 | 0.17 | 0.63 | 0.18 | 0.15 | 0.16 | 0.45 |
| Al2O3 | 15.15 | 13.65 | 15.34 | 14.00 | 13.16 | 12.42 | 14.63 |
| FeO* | 3.21 | 1.64 | 3.99 | 1.80 | 1.40 | 1.14 | 2.53 |
| MnO | 0.03 | 0.02 | 0.10 | 0.01 | 0.01 | 0.02 | 0.03 |
| MgO | 0.82 | 0.51 | 1.17 | 0.20 | 0.14 | 0.13 | 0.29 |
| CaO | 2.69 | 1.00 | 3.60 | 1.29 | 0.88 | 1.62 | 3.17 |
| Na2O | 3.55 | 2.68 | 3.89 | 3.49 | 3.20 | 3.08 | 3.57 |
| K2O | 3.21 | 4.05 | 2.81 | 3.99 | 3.97 | 3.26 | 2.97 |
| P2O5 | 0.07 | 0.02 | 0.14 | 0.02 | 0.02 | 0.02 | 0.09 |
| Total | 100.17 | 99.31 | 98.48 | 99.47 | 98.68 | 100.14 | 99.48 |
| L.O.I. | 1.58 | 3.28 | 2.17 | 1.90 | 1.60 | 1.86 | 0.35 |
| Mg# | 31 | 36 | 34 | 16 | 15 | 17 | 17 |
| Trace elements measured by XRF (ppm) | |||||||
| Ba | 756 | 923 | 628 | 829 | 874 | 876 | 714 |
| Ni | n.d. | 1 | 3 | 2 | 2 | n.d. | n.d. |
| Pb | 14.5 | 16.1 | 12.2 | 18.6 | 18.2 | 13.7 | 14.0 |
| Th | 10.0 | 13.2 | 9.7 | 11.6 | 13.5 | 12.9 | 9.5 |
| Rb | 93 | 119 | 81 | 119 | 124 | 98 | 84 |
| Sr | 240 | 131 | 291 | 151 | 120 | 281 | 259 |
| Y | 22 | 24 | 37 | 22 | 23 | 20 | 20 |
| Zr | 184 | 177 | 210 | 177 | 149 | 147 | 171 |
| Nb | 8.5 | 8.2 | 10.4 | 9.3 | 11.0 | 8.5 | 8.2 |
| Cr | 7 | 2 | 7 | 3 | 3 | 3 | 8 |
| V | 58 | 7 | 48 | 6 | 6 | 7 | 53 |
| Trace elements measured by ICP-MS (ppm) | |||||||
| Method | AD | AD | AD | AD | AD | ||
| Rb | 91.2 | - | 82.9 | 120 | 126 | - | 85.6 |
| Sr | 221 | - | 266 | 138 | 108 | - | 243 |
| Y | 19.5 | - | 34.8 | 18.3 | 21.0 | - | 18.9 |
| Zr | 134 | - | 219 | 94.6 | 92.6 | - | 131 |
| Nb | 8.5 | - | 10.1 | 9.0 | 8.9 | - | 7.9 |
| Cs | 1.43 | - | 2.15 | 1.47 | 2.15 | - | 1.42 |
| Ba | 746 | - | 579 | 813 | 840 | - | 714 |
| La | 30.7 | - | 34.4 | 34.6 | 57.9 | - | 28.5 |
| Ce | 54.9 | - | 58.9 | 58.5 | 61.7 | - | 53.1 |
| Pr | 6.26 | - | 8.51 | 7.12 | 12.1 | - | 5.74 |
| Nd | 21.5 | - | 31.4 | 23.8 | 38.9 | - | 19.9 |
| Sm | 3.66 | - | 6.49 | 4.10 | 6.20 | - | 3.48 |
| Eu | 0.893 | - | 1.42 | 0.814 | 1.10 | - | 0.868 |
| Gd | 3.39 | - | 6.17 | 3.28 | 4.62 | - | 3.23 |
| Tb | 0.473 | - | 0.931 | 0.483 | 0.654 | - | 0.498 |
| Dy | 2.87 | - | 5.55 | 2.83 | 3.57 | - | 2.88 |
| Ho | 0.658 | - | 1.25 | 0.614 | 0.704 | - | 0.642 |
| Er | 1.92 | - | 3.45 | 1.76 | 1.94 | - | 1.86 |
| Tm | 0.292 | - | 0.524 | 0.281 | 0.294 | - | 0.280 |
| Yb | 1.88 | - | 3.37 | 1.91 | 1.95 | - | 1.77 |
| Lu | 0.295 | - | 0.534 | 0.285 | 0.299 | - | 0.269 |
| Hf | 3.53 | - | 5.27 | 2.88 | 2.65 | - | 3.41 |
| Ta | 0.545 | - | 0.637 | 0.696 | 0.668 | - | 0.522 |
| Pb | 14.6 | - | 12.8 | 16.4 | 16.4 | - | 12.5 |
| Th | 9.80 | - | 8.57 | 11.6 | 12.2 | - | 9.14 |
| U | 1.59 | - | 1.68 | 1.46 | 1.72 | - | 1.62 |
n.d., not detected; L.O.I., Loss on ignition.
| Formation | Sample | Rock type | Age (Ma) | 87Sr/86Sr | Error (2SE) | Rb/Sr | 87Rb/86Sr | SrI |
| Tori | NT07 | Type 1 | 22.5 | 0.720885 | 0.000013 | 13.03 | 37.76 | 0.70855 |
| Tori | NT09 | Type 1 | 22.5 | 0.715993 | 0.000014 | 6.93 | 20.06 | 0.70944 |
| Tori | NT16 | Type 1 | 22.5 | 0.716441 | 0.000014 | 7.89 | 22.86 | 0.70897 |
| Tori | NT49 | Type 1 | 22.5 | 0.711244 | 0.000014 | 2.79 | 8.08 | 0.70860 |
| Tori | NT63c | Type 1 | 22.5 | 0.714648 | 0.000014 | 7.34 | 21.24 | 0.70771 |
| Tori | NT66 | Type 1 | 22.5 | 0.716730 | 0.000014 | 9.55 | 27.67 | 0.70769 |
| Iwaine | NT51 | Type 2 | 17.3 | 0.704836 | 0.000014 | 0.03 | 0.10 | 0.70481 |
| Iwaine | NT03 | Type 3 | 17.3 | 0.704168 | 0.000014 | 0.02 | 0.04 | 0.70416 |
| Iwaine | NT22 | Type 3 | 17.3 | 0.704334 | 0.000014 | 0.02 | 0.05 | 0.70432 |
| Iwaine | NT25 | Type 3 | 17.3 | 0.704071 | 0.000014 | 0.02 | 0.05 | 0.70406 |
| Iwaine | NT34 | Type 3 | 17.3 | 0.703755 | 0.000014 | 0.03 | 0.09 | 0.70373 |
| Iwaine | NT52 | Type 4 | 17.3 | 0.707228 | 0.000014 | 0.14 | 0.40 | 0.70713 |
| Iwaine | NT62 | Type 4 | 17.3 | 0.707651 | 0.000014 | 0.11 | 0.32 | 0.70757 |
| Iozen | NT41 | Type 5 | 17.0 | 0.708750 | 0.000014 | 0.38 | 1.11 | 0.70848 |
| Iozen | NT43 | Type 5 | 17.0 | 0.708761 | 0.000014 | 0.41 | 1.19 | 0.70847 |
| Iozen | NT55 | Type 5 | 17.0 | 0.708420 | 0.000013 | 0.31 | 0.90 | 0.70820 |
| Iozen | NT56 | Type 5 | 17.0 | 0.708887 | 0.000014 | 0.87 | 2.52 | 0.70828 |
| Iozen | NT57 | Type 5 | 17.0 | 0.709084 | 0.000014 | 1.16 | 3.37 | 0.70827 |
| Iozen | NT60 | Type 5 | 17.0 | 0.708701 | 0.000014 | 0.35 | 1.02 | 0.70846 |
| Formation | Sample | Rock type | Age (Ma) | 143Nd/144Nd | Error (2SE) | Sm/Nd | 147Sm/144Nd | NdI |
| Tori | NT07 | Type 1 | 22.5 | 0.512207 | 0.000013 | 0.220 | 0.133 | 0.51219 |
| Tori | NT09 | Type 1 | 22.5 | 0.512048 | 0.000012 | 0.210 | 0.127 | 0.51203 |
| Tori | NT16 | Type 1 | 22.5 | 0.512260 | 0.000017 | 0.203 | 0.123 | 0.51224 |
| Tori | NT49 | Type 1 | 22.5 | 0.512092 | 0.000018 | 0.216 | 0.131 | 0.51207 |
| Tori | NT63c | Type 1 | 22.5 | 0.512244 | 0.000014 | 0.174 | 0.105 | 0.51223 |
| Tori | NT66 | Type 1 | 22.5 | 0.512252 | 0.000013 | 0.194 | 0.117 | 0.51223 |
| Iwaine | NT51 | Type 2 | 17.3 | 0.512810 | 0.000014 | 0.241 | 0.145 | 0.51279 |
| Iwaine | NT03 | Type 3 | 17.3 | 0.512658 | 0.000014 | 0.162 | 0.098 | 0.51265 |
| Iwaine | NT22 | Type 3 | 17.3 | 0.512665 | 0.000014 | 0.167 | 0.101 | 0.51265 |
| Iwaine | NT25 | Type 3 | 17.3 | 0.512630 | 0.000022 | 0.164 | 0.099 | 0.51262 |
| Iwaine | NT34 | Type 3 | 17.3 | 0.512852 | 0.000021 | 0.167 | 0.101 | 0.51284 |
| Iwaine | NT52 | Type 4 | 17.3 | 0.512425 | 0.000016 | 0.234 | 0.141 | 0.51241 |
| Iwaine | NT62 | Type 4 | 17.3 | 0.512381 | 0.000020 | 0.214 | 0.129 | 0.51237 |
| Iozen | NT41 | Type 5 | 17.0 | 0.512292 | 0.000013 | 0.170 | 0.103 | 0.51228 |
| Iozen | NT43 | Type 5 | 17.0 | 0.512298 | 0.000014 | 0.170 | 0.103 | 0.51229 |
| Iozen | NT55 | Type 5 | 17.0 | 0.512342 | 0.000014 | 0.206 | 0.125 | 0.51233 |
| Iozen | NT56 | Type 5 | 17.0 | 0.512290 | 0.000014 | 0.172 | 0.104 | 0.51228 |
| Iozen | NT57 | Type 5 | 17.0 | 0.512368 | 0.000014 | 0.160 | 0.096 | 0.51236 |
| Iozen | NT60 | Type 5 | 17.0 | 0.512218 | 0.000014 | 0.175 | 0.106 | 0.51221 |
Type 1 rhyolite is classified as rhyolite in SiO2-Na2O+K2O diagram (Fig. 4a). This type is characterized by high K2O (4.18-5.78 wt%; Fig. 4b) and low CaO (0.06-0.31 wt%; Fig. 5) contents. Type 1 rhyolite shows decreasing pattern from Cs to Lu, with negative spikes of Ba, Sr, and Eu, and negative anomaly of Nb and Ta in multi-element spider diagram (Fig. 6a). In REE pattern, Type 1 rhyolite shows decreasing pattern from LREE (light rare earth elements) to HREE (heavy rare earth elements), with flat pattern between MREE (middle rare earth elements) and HREE (Fig. 6b). This type has more enriched trace element compositions than those of Type 5 rhyolite from the Iozen Formation, showing strongly negative anomalies of Ba, Nb, Ta, Sr, and Eu and weakly positive anomaly of Pb. The initial Sr and Nd isotopic ratios are calculated as 0.70769-0.70944 and 0.51203-0.51224, respectively. Type 1 rhyolite has the most enriched Sr-Nd isotopic compositions, showing much more enriched than the bulk earth, in all types of the volcanic rocks (Fig. 7a).
Iwaine FormationType 2 andesite is classified as basaltic andesite in SiO2-Na2O+K2O diagram (Fig. 4a). Type 2 andesite is characterized by higher MgO content (5.61-7.81 wt%; Fig. 5) than that of the other andesites from the Iwaine Formation and shows a calc-alkalic trend in FeO*/MgO-SiO2 diagram (Fig. 4c). Type 2 andesite shows decreasing pattern from Cs to Lu in REE pattern (Fig. 6c). This type exhibits negative anomalies of Nb and Ta and positive anomalies of Pb and Sr, but the degree of the negative anomalies of Nb and Ta is lower than the other andesites from the Iwaine Formation. In REE pattern, Type 2 andesite shows decreasing pattern from LREE to HREE, without negative spike of Eu (Fig. 6d). LREE of Type 2 andesite is the most depleted in the andesites from the Iwaine Formation, nevertheless, MREE and HREE have the almost same enrichment as the other andesites. The initial Sr and Nd isotopic ratios are calculated as 0.70482 and 0.51279, respectively. This type is more depleted in NdI than that of the bulk earth (Fig. 7a).
Type 3 andesite is classified as andesite, trachy-basaltic andesite and trachy-andesite in SiO2-Na2O+K2O diagram (Fig. 4a). Type 3 andesite is characterized by higher Na2O+K2O (4.46-6.96 wt%; Fig. 4a), K2O (1.80-4.02 wt%; Fig. 4b), P2O5 (0.19-0.57 wt%; Fig. 5), and Sr (846-2893 ppm; Fig. 5) contents than those of the other andesites from the Iwaine Formation. In Y-Sr/Y and Sr-(SiO2/MgO)×100-K/Rb diagrams (Fig. 8), Type 3 andesite is plotted near low-SiO2 adakite (LSA) of Martin et al. (2005). Type 3 andesite shows decreasing pattern from Cs to Lu, with positive spikes of Pb and Sr and negative anomalies of Nb and Ta (Fig. 6c). In REE pattern, Type 3 andesite shows decreasing pattern from LREE to HREE, without negative spike of Eu (Fig. 6d). This type has the most enriched REE compositions in the andesites from the Iwaine Formation. The initial Sr and Nd isotopic ratios are calculated as 0.70373-0.70432 and 0.51262-0.51284, respectively. This type is plotted near the bulk earth in SrI-NdI diagram (Fig. 7a).
Type 4 andesite is classified as basaltic andesite to andesite in SiO2-Na2O+K2O diagram (Fig. 4a). Type 4 andesite shows tholeiitic trend in FeO*/MgO-SiO2 diagram (Fig. 4c). This type is characterized by high FeO* (5.78-11.36 wt%) and CaO (7.43-9.04 wt%) contents than those of the other andesites from the Iwaine Formation (Fig. 5). Type 4 andesite shows decreasing pattern from Cs to Lu (Fig. 6c). This type exhibits negative anomalies of Nb and Ta and a positive anomaly of Pb. In REE pattern, Type 4 andesite shows decreasing pattern from LREE to HREE, without negative spike of Eu (Fig. 6d). Type 4 andesite has intermediate enrichment of REE between Type 2 and 3 andesites. The initial Sr and Nd isotopic ratios are calculated as 0.70713-0.70756 and 0.51237-0.51241, respectively. This type has most enriched Sr-Nd isotope in the andesites from the Iwaine Formation (Fig. 7a).
Iozen FormationType 5 rhyolite is classified as dacite to rhyolite in SiO2-Na2O+K2O diagram (Fig. 4a). Type 5 rhyolite shows decreasing pattern from Cs to Lu (Fig. 6a). Type 5 rhyolite exhibits negative anomalies of Nb, Ta, Sr, and Eu and a positive anomaly of Pb, but the degree of the negative anomalies of Sr and Eu is much lower than those of the Type 1 rhyolite. In REE pattern, Type 5 rhyolite shows decreasing pattern from LREE to HREE and flat pattern between MREE and HREE (Fig. 6b). REE of this type is more depleted than that of Type 1 rhyolite. The initial Sr and Nd isotopic ratios are calculated as 0.70750-0.70823 and 0.51221-0.51236, respectively. This type has more enriched Sr-Nd isotopic compositions than of the Iwaine Formation, and it shows less enriched than that of Type 1 rhyolite from the Tori Formation (Fig. 7a).
Highly-differentiated granitic and rhyolitic magmas are thought to be formed by (a) continuous melt extraction from previously generated dikes, (b) crustal host rock melting, or (c) high-degree crystallization of basaltic sills, based on petrological and geodynamic calculations (Rummel et al., 2020). Type 1 rhyolite shows highly enriched Sr-Nd isotopic composition (Fig. 7a; SrI = 0.70769-0.70944), suggesting that its genesis was involved largely by crustal material. Since Type 1 rhyolite accompanies contemporaneously active basaltic to andesitic magmatism (Kamiwazumi and Matsunagi Formations), it is considered that such andesitic magma involved the genesis of Type 1 rhyolite (assimilation or mixing between the andesitic magma and crustal material?).
Type 1 rhyolite is much more enriched in Sr-Nd isotope than basalts and andesites from the Kamiwazumi and Matsunagi Formations (Fig. 7a), strongly suggesting that enriched crustal material such as the Hida belt, one of the basement rocks, was assimilated or melted to generate the rhyolitic magma. Type 1 rhyolite is plotted in areas of the Hida gneiss and the Hida granites of the basement rocks in SrI-NdI and 147Sm/144Nd-143Nd/144Nd diagrams (Figs. 7a and 7b). In Fig. 7c, a mixing line and AFC (assimilation and fractional crystallization) lines between the most enriched andesite from the Kamiwazumi and Matsunagi Formations (initial magma) and the most enriched sample from the Hida gneiss of the Hida belt (wall rock) are shown. Sr-Nd isotopic mixing and AFC lines were calculated following the models by Langmuir et al. (1978) and DePaolo (1981), respectively, in this study. The bulk partition coefficients of Type 1 and 5 rhyolites for AFC lines, shown in Table 4, were extrapolated from the mineral-melt partition coefficients and the mineral assemblages (e.g., Rollinson and Pease, 2021). Sr-Nd concentrations and isotopic values of the end members are shown in Table 5. According to the results (Fig. 7c), Sr-Nd isotopic composition of Type 1 rhyolite can be explained by 40-70% of mixing rate or more than 50% of assimilation rate (r > 0.5). These indicate that the initial rhyolitic magma of Type 1 rhyolite was generated by high rate assimilation or high rate mixing between the andesitic magma and the crustal material (Hida belt). In any case, the crustal material contributed largely the genesis of Type 1 rhyolite. Ishida et al. (1998) considered that relatively dehydrated mantle material or crustal material (e.g., granulite) are involved in the genesis of rhyolites, containing hecatolite, in the Toyama basin, because chemical composition of alkali feldspar suggests that the magma was relatively dry. In fact, Type 1 rhyolite contains xenolith of gneiss probably derived from the Hida belt (Fig. 3b). Accordingly, it is considered that metamorphic rocks and granitoids in the Hida belt largely involved the genesis of the rhyolite from the Tori Formation (Type 1 rhyolite).
| Partition coefficient |
Mineralogy and bulk partition coefficients |
|||||
| Sr | Nd | KMF | Type 4 andesite | |||
| Olivine | 0.020 | 0.011 | ++* | |||
| Clinopyroxene | 0.280 | 0.420 | +++* | ++ | ||
| Orthopyroxene | 0.005 | 0.019 | ++ | |||
| Plagioclase | 2.625 | 0.065 | +++* | +++ | ||
| Sr | Nd | Sr | Nd | |||
| Bulk | 1.90 | 0.13 | 2.20 | 0.09 | ||
Data on mineral-melt partition coefficients are from Rollinson and Pease (2021).
+, <0.1%; ++, 0.1-10%; +++, >10%; KMF, Kamiwazumi and Matsunagi Formations. * was extrapolated from the mineral assemblages by Okamura et al. (2016).
| Sr (ppm) | 87Sr/86Sr | Nd (ppm) | 143Nd/144Nd | Reference of data source | |
| End members for the Iozen Formation | |||||
| Initial magma (Type 4 andesite) | 439.8 | 0.70757 | 19.2 | 0.51237 | This study |
| Assimilant (Hida gneiss) | 210 | 0.72086 | 38.13 | 0.51171 | Arakawa and Shinmura (1995) |
| End members for the Iwaine Formation | |||||
| AOC melt | 1900 | 0.7040 | 47 | 0.51302 | Hanyu et al. (2006) |
| AOC fluid | 15000 | 0.7040 | 350 | 0.51302 | Hanyu et al. (2006) |
| MORB melt | 400a, b | 0.7026 | 25a, b | 0.51314 | Class and Lehnert (2012) |
| Sediment melt | 320 | 0.7096 | 53 | 0.51236 | Hanyu et al. (2006) |
| Sediment fluid | 1900 | 0.7096 | 80 | 0.51236 | Hanyu et al. (2006) |
| End members for the Tori Formation | |||||
| Initial magma (Kamiwazumi Formation) | 340.3 | 0.70633 | 10.01 | 0.51260 | Okamura et al. (2016) |
| Assimilant (Hida gneiss) | 210 | 0.72083 | 38.13 | 0.51171 | Arakawa and Shinmura (1995) |
AOC, Altered oceanic crust; MORB, Mid-ocean ridge basalt. a Calculated following procedures by Tatsumi and Hanyu (2003) and Hanyu et al. (2006). b Melting degree was set as 0.25.
A plausible scenario on the genesis of Type 1 rhyolite is as follows. Basaltic to andesitic magma (Kamiwazumi and Matsunagi Formations) ascent and reached the continental crust beneath the Toyama basin. The andesitic magma in the crust finally melted or assimilated the crustal material (Hida belt) to produce the initial magma of Type 1 rhyolite probably due to large heat flow of the basaltic to andesitic magma. The degree of contribution of crustal material is thought to be relatively high.
Petrogenesis of Type 2 andesite from the Iwaine Formation (18-17 Ma)The whole-rock Mg# of Type 2 andesite reaches as high as 65 (Fig. 5). Assuming the Fe/Mg exchange partition coefficient of olivine-melt (0.3; Roeder and Emslie, 1970), mantle olivine having Mg/(Mg + Fe) greater than 0.87, and an Fe2+/(Fe2+ + Fe3+) of 0.9 in the magma, then a mantle-derived andesitic magma should have Mg# greater than 64 (Tatsumi, 2006). Therefore, primary magma of Type 2 andesite is thought to be in equilibrium with the mantle peridotite, and Type 2 andesite can be classified as HMA derived directly from the mantle wedge. High Ni and Cr concentrations (131 and 367 ppm in the maximum, respectively; Fig. 5) support this idea. Some samples of Type 2 andesite have low Ni and Cr concentrations (Fig. 5). This is considered as results of fractionation of olivine and pyroxene, contained in Type 2 andesite, because Type 2 andesite is plotted on lines passing through the origin in Rb-Th and Nb-Th diagrams (Figs. 9a and 9b).
Tatsumi (2006) and Hanyu et al. (2006) explained petrogenesis of HMA from the Setouchi Volcanic Belt resulted from the interaction between slab melt, which is derived from AOC (altered oceanic crust) and sediments, and the mantle wedge. Although Th/Yb-Ba/La and (La/Sm)N-Ba/Th diagrams (Figs. 9c and 9d) suggest that petrogenesis of the andesites from the Iwaine Formation was involved by subducted sediment component, the degree of contribution of the sediment component to Type 2 andesite seems to be the lowest, compared to the other andesites. Besides, Tatsumi (1981) and Hirose (1997) conducted melting experiments and found that HMAs can be generated by partial melting of the mantle peridotite saturated by H2O. Since Hirose (1997) revealed that HMA magmas are produced between 1000 to 1050 °C at 1 GPa, HMAs can be generated also by partial melting of the upper mantle saturated by H2O due to slab fluids. In Figure 7e, Sr-Nd isotopic compositions of the andesites from the Iwaine Formation are compared to mixing lines among MORB (Mid-ocean ridge basalt) melt, AOC melt, AOC fluid, sediment melt, and sediment fluid. Although it is difficult to estimate the actual Sr-Nd isotopic composition of slab melt and fluid owing to its large variations in Sr-Nd isotopic composition (e.g., Fig. 7a), Sr-Nd concentrations and isotopic values by Hanyu et al. (2006) was selected as chemical compositions of melts and fluids of AOC and sediment, which was subducting beneath the Toyama basin during the Japan Sea opening, in this study. Type 2 andesite is plotted between MORB-AOC-sediment melt (slab melt) mixing lines and an AOC-sediment fluid (slab fluid) mixing line (Fig. 7e). This suggests that slab melt had little effect on the genesis of Type 2 andesite and that slab fluids are strongly involved in the magma genesis of Type 2 andesite. This is consistent with relatively low Th/Yb and high Ba/La ratios of Type 2 andesite in Figure 9c, suggesting addition of slab fluids. From the above discussion, it is considered that Type 2 andesite magma was generated by partial melting of the upper mantle peridotite saturated mainly by slab fluids (mixture between slab and sediment fluids).
Petrogenesis of Type 3 andesite from the Iwaine Formation (18-17 Ma)Although petrography and some geochemical signatures (e.g., high Sr content) of Type 3 andesite are similar to adakite (Defant and Drummond, 1990), Type 3 andesite is plotted outside the field of adakite in Y-Sr/Y diagram (Fig. 8a). Geochemical characteristics (e.g., low-SiO2, high-MgO, high-K2O, and high-Sr contents; Fig. 8b) of Type 3 andesite is similar to low-SiO2 adakite (LSA) of Martin et al. (2005) rather than adakite of Defant and Drummond (1990).
Martin et al. (2005) discussed that LSA magma is generated by partial melting of the mantle wedge metasomatized by slab-derived melt. High Th/Yb and high (La/Sm)N ratios of Type 3 andesite (Figs. 9c and 9d) suggest that sediment melt derived from the subducting slab are involved in the origin of Type 3 andesite. Although degree of contribution of AOC, sediment component and slab fluid cannot be quantitively estimated, using Th/Yb-Ba/La and (La/Sm)N-Ba/Th diagrams (Figs. 9a and 9b), the contribution of slab melt and sediment component to Type 3 andesite seems to be larger than that of Type 2 andesite (HMA) owing to larger Th/Yb and (La/Sm)N ratios of Type 3 andesite. Sr-Nd isotopic compositions of Type 3 andesite are compared to mixing lines among MORB-AOC-sediment melts and AOC-sediment fluids in Figure 7e. According to the results, Type 3 andesite is plotted near the areas among MORB, AOC and sediment melts rather than a mixing line between AOC and sediment fluids. This suggests that MORB-AOC-sediment melts (slab melt) largely affected the genesis of Type 3 andesite. Considering the genetic model of LSA by Martin et al. (2005), it is proposed that the mantle wedge metasomatized by slab melt was partially melted to produce Type 3 andesite magma. Therefore, Type 3 andesite is not pure adakite derived directly from slab melt. Oligocene to Miocene basaltic to andesitic volcanic rocks related to slab melt, including adakites, have been reported also from the eastern Toyama basin and the Noto peninsula (Uematsu et al., 1995; Takahashi and Shuto, 1999; Sato et al., 2013; Fig. 1). Accordingly, the mantle wedge beneath the Toyama basin is thought to have been metasomatized intermittently by slab melt since Oligocene epoch.
On the other hand, Rb-Th and Nb-Th diagrams (Figs. 9a and 9b) indicate that fractional crystallization was dominant in Type 3 andesite magma rather than crustal assimilation, because Th/Rb and Th/Nb ratios of Type 3 andesite seems to be constant. Moreover, Type 3 andesite are depleted in Sr-Nd isotopes, compared to gneisses of the Hida belt including high Sr rocks such as limestone. Therefore, the generation of high-Sr andesite by assimilation of mantle derived magma with the Sr rich crustal material seems to be difficult in this case.
Although both of Type 2 and 3 andesites are related to interaction between sediment melt and the mantle peridotite, Type 3 andesite has significantly higher P2O5, K2O and trace element contents than those of Type 2 andesite (Figs. 4a, 5, and 6c). As opposed to Type 2 andesite, it is considered that Type 3 andesite was formed by ‘partial melting of the relatively enriched mantle peridotite’ by metasomatism with sediment melt. Therefore, incompatible elements to the mantle peridotite such as phosphorus and potassium were concentrated into Type 3 andesite magma. As a result, Type 3 andesite has more enriched concentrations in many incompatible elements than Type 2 andesite.
Petrogenesis of Type 4 andesite from the Iwaine Formation (18-17 Ma)As compared in Figure 6c, Type 4 andesite (tholeiitic andesite) displays general geochemical features as andesites from subduction zone (e.g., Nb-Ta negative anomaly). Although Type 4 andesite exhibits similar trends to Type 2 andesite in many chemical variation diagrams (FeO*, CaO, and P2O5; Fig. 5), their trends in FeO*/MgO-SiO2 diagram (Fig. 4c) are clearly different with each other. In FeO*/MgO-SiO2 (Fig. 4c) and SiO2-K2O diagrams (Fig. 4b), it seems that Type 4 andesite was produced from relatively alkali-poor and tholeiitic basalt. Low Mg# of Type 4 andesite (Mg# = 31-49; Fig. 5) also suggests that Type 4 andesite is differentiated and not derived directly from the mantle wedge.
Sr-Nd isotope of Type 4 andesite is much more enriched than that of the bulk earth (Fig. 7a). Therefore, there is a possibility that the crustal material such as the Hida belt involved the origin of the parental basaltic magma producing Type 4 andesite by, for instance, AFC. Ishiwatari and Ohama (1997) also considered that the andesitic magma of the Iwaine Formation underwent assimilation with crustal material such as granitoids. From the above discussion, it is considered that mantle peridotite was melted to generate basaltic magma. The parental basaltic magma assimilated the continental crust to form Type 4 andesite magma.
It is difficult to specify the source mantle (i.e., either asthenospheric or lithospheric mantle) for the Type 4 magma, because the original chemical composition of parental basaltic magma has not been preserved due to AFC. However, unusually high rate AFC is required to produce enriched Type 4 andesite from the asthenospheric mantle-origin basalts such as isotopically depleted basalts similar to MORB in the NE Japan arc (e.g., Shuto et al., 2006, 2015). Therefore, the basaltic magma which is the original magma of Type 4 andesite might have been originated from the isotopically enriched lithospheric mantle.
Petrogenesis of Type 5 rhyolite from the Iozen Formation (17-16 Ma)Since Type 5 rhyolite was formed just after active andesitic magma (Iwaine Formations), there is a possibility that the andesitic magma melted or assimilated the crustal material. Type 5 rhyolite is more enriched in Sr-Nd isotope than andesites from the Iwaine Formation (Fig. 7a; SrI = 0.70750-0.70823), strongly suggesting that enriched crustal materials are involved in the genesis of the initial magma of Type 5 rhyolite. Type 5 rhyolite is plotted in areas of the Hida gneiss and the Hida granites of the basement rocks in SrI-NdI and 147Sm/144Nd-143Nd/144Nd diagrams (Figs. 7b and 7d). Tholeiitic andesites, including Type 4 andesite, in the Iwaine Formation occupies the middle to upper parts of the Iwaine Formation in the Toyama basin (this study; Takahashi and Shuto, 1999), indicating that tholeiitic andesite magmatism was dominant in the middle to late stages of andesitic volcanic activity of the Iwaine Formation. Accordingly, Type 4 andesite magma of the Iwaine Formation had been effused until dacitic to rhyolitic magmatism of Type 5 rhyolite initiated at ∼ 17 Ma. Additionally, Type 4 andesite seems to have a series of Sr-Nd isotopic composition to Type 5 rhyolite in SrI-NdI diagram (Fig. 7a). Assuming that the intermediate magma is the most enriched Type 4 andesite magma of the Iwaine Formation, AFC and mixing lines shown in Figure 7d were calculated (see Tables 4 and 5 for detailed conditions). AFC model between the andesitic initial magma and the assimilant (Hida gneiss) even with low assimilation rate (r = 0.1) is able to explain Sr-Nd isotopic composition of Type 5 rhyolite. However, mixing model may be more suitable to explain the Sr-Nd isotopic composition of Type 5 rhyolite, because the steeply decreasing trend of Type 5 rhyolite in SrI-NdI diagram (Fig. 7d) fits the mixing line more than AFC lines. Therefore, the initial magma of Type 5 rhyolite (probably dacite) is considered to have been generated by mixing between andesitic magma of the Iwaine Formation and the crustal melt. Contribution of the crustal material to the chemical composition of Type 5 rhyolite is thought to be relatively smaller than that of Type 1 rhyolite, because 10-30% is the most coinciding mixing rate. In case of Type 5 rhyolite, low-rate mixing between andesitic magma and crustal melt is considered to be enough to explain the genesis of Type 5 rhyolite, because the initial magma of Type 5 rhyolite is dacite (not rhyolite; Fig. 4a).
A plausible scenario on the genesis of Type 5 rhyolite is as follows. Type 4 andesite magma derived from the mantle wedge (Iwaine Formation) ascent and reached the continental crust beneath the Toyama basin. It is thought that the andesitic magma was still active when dacitic to rhyolitic magmatism of Type 5 rhyolite initiated in the stage of formation of the Iozen Formation. The crustal material (Hida belt) was melted and mixed with Type 4 andesite magma to produce Type 5 rhyolite magma because of large heat flow of the andesitic magma.
Temporal change of arc volcanism during the Japan Sea opening and implications for the mantle-crust dynamicsBecause the Tori Formation and equivalents are the stratigraphically lowest unit in the Toyama basin (Nakajima et al., 2019; Yamada and Takahashi, 2021), the Toyama basin is considered to have been formed at ∼ 23 Ma (stage III) due to initiation of rifting with rhyolitic pyroclastic flows. Especially, these rhyolites are rift-type rhyolite such as the Great Rift Valley (Ishida et al., 1998; Ayalew and Ishiwatari, 2011). Contemporaneous formation of basaltic oceanic crust at ∼ 24 Ma resulted into the extension of back-arc basin in the Japan Sea (Kaneoka et al., 1992, 1996). It is considered that the heat flow caused rifting and crustal melting (or assimilation) to generate the basaltic to andesitic and rhyolitic magmas of the Kamiwazumi, Matsunagi, and Tori Formations (Fig. 10a). Because these basalts and andesites (Kamiwazumi and Matsunagi Formations) exhibit Nb and Ta negative anomalies in a multi-element spider diagram (Okamura et al., 2016), they are derived probably from basaltic primary magma generated by partial melting of the hydrous mantle wedge. In addition, Sr-Nd isotopic compositions of the basalts and andesites seem to be not so depleted like MORB (Fig. 7a), suggesting the lithospheric mantle source. Contemporaneous rhyolitic magmatism has been found even from the forearc region of the SW Japan arc (Hachiya Formation; Shinjoe et al., 2018) and North Korea (Kilchu rhyolite; Ayalew and Ishiwatari, 2011). Therefore, basaltic to andesitic magma assimilated and/or melted the continental crust to produce large-volume rhyolitic magma in the circum-Japan Sea region during stage III.
The SW Japan arc was rapidly rotated clockwise in 18-16 Ma of stage IV (Hoshi et al., 2015; Hoshi, 2018), while the Iwaine and Iozen Formations were formed in 18-16 Ma. Contemporaneously, the second stage of spreading in the Yamato basin occurred near the Toyama basin in 18-15 Ma (Nohda, 2009; Nakajima, 2013). The Toyama basin has rifted rapidly in this period (Nakajima et al., 2021). Large-scaled andesitic magmatism of the Iwaine Formation was followed by large-volume rhyolitic magmatism of the Iozen Formation. Since the old and cold Pacific plate was subducting to the SW Japan arc during the Japan Sea opening (Hall, 2002; Miller and Kennett, 2006; Kimura et al., 2014), additional heat source such as upwelling of the asthenospheric mantle into the mantle wedge (Tatsumi et al., 1989; Okamura et al., 1998; Shuto et al., 2006; Zadeh et al., 2013; Shuto et al., 2015) and/or slab tearing (slab window; Martynov et al., 2017) below the margin of the Eastern Asian continent is required to cause melting of the subducting slab. The slab fluids generated HMA magma, and the slab melt metasomatized the mantle wedge to produce high-Sr andesite magma (LSA; Fig. 10b). Therefore, andesitic magmatism related to the slab melting (HMA and high-Sr andesite) indicate that back-arc spreading in the Japan Sea was driven by upwelling of the asthenospheric mantle into the mantle wedge. Contemporaneously, the mantle wedge (lithospheric mantle?) was partially melted to produce tholeiitic basaltic magma, and this basaltic magma assimilated the continental crust to generate tholeiitic andesite. The generated basaltic to andesitic magma formed a large batholith beneath the Toyama basin (Ishiyama et al., 2017) and melted the continental crust to generate dacitic to rhyolitic magma of the Iozen Formation (Fig. 10b). The arc volcanism in the Toyama basin during the Japan Sea opening terminated with the rhyolitic magmatism of the Iozen Formation at ∼ 16 Ma.
In this study, we carried out whole-rock geochemical analyses (major element, trace element, and Sr-Nd isotope) for Oligocene to Middle Miocene volcanic rocks from the Tori, Iwaine, and Iozen Formations in the Toyama basin of the SW Japan arc and discussed the petrogenesis. Our conclusions based on results and discussion in this paper are as follows.
This paper is based mainly on the results and discussions in the Master and Ph.D. theses of R.Y. We are grateful to professors and students of the petrology seminar of Niigata University for their critical and constructive comments. We appreciate Syun Watanabe (Niigata University), Wataru Ouchi (Suncoh Consultants Co., Ltd.), and Dr. A.S. Silpa (Shimane University) for helps during the field work. Helpful and constructive comments from two anonymous reviewers and the editors (Dr. Masao Ban and Dr. Shumpei Yoshimura) are greatly appreciated. Our sincere gratitude goes to R.I. Nohara of Niigata University for helping us during the analyses. We especially thank members of Tono Geoscience Center, Japan Atomic Energy Agency (JAEA) for helping us a lot during analysis. Proofreading of the English manuscript by Dr. M. Satish-Kumar of Niigata University is appreciated. We also thank some public offices of Toyama Prefecture for their advice and permission to collect samples from prefectural natural parks.
Supplementary Table A1 is available online from https://doi.org/10.2465/jmps.221219a.
