Detrital zircon U-Pb dating and Raman spectral analysis of carbonaceous material along the boundary between the Sanbagawa metamorphic complex and Chichibu accretionary complex, central Kii Peninsula, southwest Japan

Abstract

We conducted detrital zircon U-Pb dating and Raman spectral analysis of carbonaceous material (CM) around the Takihara thrust, which is thought to be the boundary between the Sanbagawa metamorphic complex and Chichibu accretionary complex in the central Kii Peninsula, southwest Japan. The Mikabu greenstones, which are normally used to define the boundary, are missing from this area. U-Pb dating of psammitic rocks yields a youngest age of ∼ 118 Ma on the northern side of the Takihara thrust and ∼ 158 Ma on the southern side. The youngest age and age distribution obtained for the sample from the northern side is consistent with data for the Sanbagawa metamorphic complex in other regions, and the age data obtained for the sample from the southern side are consistent with the Chichibu accretionary complex, demonstrating that the Takihara thrust is the geological boundary between these complexes. We examined two parameters of the Raman spectra from CM: the temperature estimated from the full width at half maximum of the D1 band (T_D1) and the ratio of the intensities of the D4 and D1 bands (D4/D1_int). We obtained T_D1 values of ∼ 275-290 °C on the northern side of the Takihara thrust and ∼ 275-280 °C on the southern side, with no clear difference across the thrust. In contrast, there is a clear difference in D4/D1_int values across the thrust: ∼ 0.30-0.33 on the northern side and ∼ 0.47-0.50 on the southern side. This suggests that the D4/D1_int value can not only be used to distinguish between the Sanbagawa and Chichibu complexes in the central Kii Peninsula, but it is also useful for detecting differences in metamorphic grade in low-grade metamorphosed rocks that cannot be detected by the geothermometer that uses the full width at half maximum of the D1 band.

INTRODUCTION

The high-pressure Sanbagawa metamorphic complex (SMC), which originally formed as an accretionary complex during the Cretaceous, and the Chichibu accretionary complex (CAC), which formed mainly during the Jurassic, run parallel to each and extend for >800 km in southwest Japan (Fig. 1a). These complexes contain clues to the development of a Mesozoic subduction zone on the Asian margin. The Mikabu greenstones mark the boundary between these complexes; however, the greenstones are absent in the central Kii Peninsula (Fig. 1b; Nishioka et al., 2010). Furthermore, low-grade metamorphic rocks (greenschist facies or lower) are distributed throughout the central Kii Peninsula (Takeuchi, 1996; Shimura et al., 2019b, 2021), and pelitic rocks are dominant on both sides of the boundary in this region, making it difficult to determine whether the rocks belong to the SMC or CAC based solely on lithology (Aoya, 2010). This makes it difficult to determine the boundary between these complexes, which is crucial for elucidating the structural relationship between the geological bodies and their broader lateral continuity.

Figure 1. (a) Map of the Outer Zone of Southwest Japan (modified from Wallis et al., 2020; Mori et al., 2021). ISTL, Itoigawa-Shizuoka Tectonic Line; MTL, Median Tectonic Line. (b) Geological map of the central and eastern parts of the Kii Peninsula (modified from Nishioka et al., 2010; Geological Survey of Japan, 2022). (c) Geological map of the study area, showing the Takihara thrust (modified from Geological Survey of Japan, 2022) and sampling locations. The hillshade maps in (b) and (c) were produced by the Geospatial Information Authority of Japan (GSI). (d) Outcrop of the Takihara thrust. The dashed orange line marks the thrust, which forms the boundary between cataclastic pelitic phyllite (hanging wall) and meta-chert (footwall).

We focused on clastic sedimentary rocks and their metamorphic equivalents near the Takihara thrust, which is thought to mark the boundary between the SMC to the north and the CAC to the south in the central Kii Peninsula (Figs. 1c and 1d; Kimura, 1954). Although Kimura (1954) proposed that the Takihara thrust represents the boundary between the complexes based on lithology, a quantitative evaluation based on a geochronological approach that can determine the protolith age, which is the primary criteria for distinguishing between the SMC and CAC, and petrological approaches has not yet been conducted around the Takihara thrust. Therefore, our research had two main aims: 1) to confirm whether the Takihara thrust is the boundary between the SMC and CAC using detrital zircon U-Pb dating, and 2) to explore the potential of Raman spectral analysis of carbonaceous material (CM) as a simple, useful method for quantitatively distinguishing between the complexes.

Recent studies in the central Kii Peninsula have revealed structural, petrological, and chronological similarities between the SMC and the Shimanto accretionary complex (SAC; e.g., Shimura et al., 2021), and Takeuchi et al. (2025) proposed that distinguishing between these two complexes in the central Kii Peninsula is unnecessary. Consequently, while we employ the term ‘Sanbagawa metamorphic complex’ in this paper, following previous broad geological classifications (e.g., Nishioka et al., 2010), the following discussion assumes that the SMC and SAC in the central Kii Peninsula are the same complex. Furthermore, detrital zircon U-Pb ages from the SMC and SAC in the central Kii Peninsula indicate depositional ages younger than ∼ 120 Ma, whereas those from the CAC are older than ∼ 150 Ma, suggesting a distinct age gap between these complexes in the region. We use this age gap as the main criteria for distinguishing between the two complexes in the study area.

GEOLOGICAL SETTING AND SAMPLES

The Takihara thrust strikes ESE-WNW and dips shallowly to the north (Figs. 1c and 1d). The area to the north of the thrust is mainly pelitic phyllite with minor meta-sandstone (Fig. 1c). The area to the south of the thrust is dominated by pelitic phyllite and meta-chert, with minor meta-sandstone, basalt, and marble. The schistosity strikes mainly ENE-WSW to NE-SW and dips moderately to the north (Fig. 1c). There is no clear change in the orientation of the schistosity across the thrust. In an exposure of the Takihara thrust, it forms a boundary between cataclastic pelitic phyllite and meta-chert that contains a foliation oriented N76°W, 12°N (Fig. 1c and 1d).

We collected eight samples of weakly metamorphosed sedimentary rocks along a traverse across the Takihara thrust (Fig. 1c): five samples (TK1-5) from the north side of the thrust and three samples (TK6-8) from the south side. Samples TK1, TK4, TK5, TK7, and TK8 are pelitic phyllite and TK2, TK3, and TK6 are meta-sandstone. We selected two samples (TK2 and TK6) for U-Pb dating, and we carried out Raman spectral analyses of CM from all samples. The lithological characteristics of the samples from the northern and southern sides of the Takihara thrust are similar; the meta-sandstone has a weak schistosity and consists mainly of quartz, feldspar, mica, lithic fragments, and CM (Figs. 2a and 2d), and the pelitic phyllites consists mainly of quartz, plagioclase, muscovite, chlorite, and CM (Figs. 2b, 2c, and 2e). There are subtle variations between the pelitic phyllite samples to the north of the Takihara thrust and those to the south, including a slightly more distinct foliation, slightly larger muscovite grains, and the development of crenulations in the former rocks.

Figure 2. Photomicrographs of the pelitic phyllite and meta-sandstone samples subjected to analysis (cross-polarized light). (a) Meta-sandstone (TK2), (b) pelitic phyllite with a distinct foliation (TK4), and (c) pelitic phyllite with crenulations (TK1) collected north of the Takihara thrust. (d) Meta-sandstone (TK6) and (e) pelitic phyllite (TK7) collected south of the Takihara thrust. The yellow arrows indicate the orientation of schistosity and the dashed yellow line in (c) indicates the fold axial plane. Qtz, quartz; Pl, plagioclase; Ms, muscovite; Afs, alkali feldspar; Cal, calcite; Zr, zircon; VRF, volcanic rock fragment; PRF, pelitic rock fragment; CM, carbonaceous material.

DETRITAL ZIRCON U-Pb DATING

Detrital zircon U-Pb analyses were conducted by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) at Nagoya University, Japan, following the procedure of Kouchi et al. (2015) (Supplementary Table S1; Supplementary Tables S1-S2 are available online from https://doi.org/10.2465/jmps.241123L). The accuracy was monitored by analyzing the Plešovice zircon standard (²³⁸U/²⁰⁶Pb age = 337.13 ± 0.37 Ma; Sláma et al., 2008). The weighted mean ²³⁸U/²⁰⁶Pb age of Plešovice zircon measured during this study was 337.4 ± 2.4 Ma, which is consistent with the published age. Concordant zircon U-Pb zircon ages were defined as those with a U-Pb age ratio (²³⁸U/²⁰⁶Pb age over ²³⁵U/²⁰⁷Pb age) of 90-110% (Figs. 3a, 3b, 3e, and 3f) to improve the accuracy of the ages and interpretations, following previous studies (e.g., Tokiwa et al., 2021). ²³⁸U/²⁰⁶Pb ages are used for zircon grains younger than 1000 Ma, and ²⁰⁷Pb/²⁰⁶Pb ages for older grains. All uncertainties are 2σ. Ages were calculated using Isoplot/Ex 4.15 (Ludwig, 2012). To determine the youngest detrital zircon age, we used the weighted mean age of the youngest two or more grains whose ages overlapped at the 1σ level (YC1σ), as proposed by Dickinson and Gehrels (2009). We treated this youngest cluster age as the depositional age, as the youngest Jurassic and Cretaceous cluster ages of detrital zircon grains from trench-fill deposits in the Kii Peninsula are consistent with depositional ages inferred from radiolarian fossils (e.g., Tokiwa et al., 2019, 2021).

Figure 3. U-Pb dating results for samples [(a)-(d)] TK2 and [(e)-(h)] TK6. [(a), (b), (e), and (f)] Concordia diagrams with 2σ error ellipses for [(a) and (e)] all data and [(b) and (f)] 500-0 Ma data. [(c), (d), (g), and (h)] Histograms and probability density plots for [(c) and (g)] all concordant data, and [(d) and (h)] 500-0 Ma data.

Sample TK2, collected from north of the Takihara thrust, yielded a total of 127 concordant zircon ages (Figs. 3a-3d; Supplementary Table S2), including Precambrian (52.8%), Silurian (0.8%), Permian (2.4%), Triassic (16.5%), Jurassic (22.8%), and Cretaceous (4.7%) grains (Figs. 3c and 3d). The youngest single grain (YSG) age is 116.8 ± 3.8 Ma, and the age of the highest peak on a probability density plot is ∼ 179 Ma (Fig. 3d). The youngest cluster consists of three zircon grains, with a YC1σ age of 118.0 ± 2.6 Ma (MSWD = 0.59).

Sample TK6, collected from south of the Takihara thrust, yielded a total of 125 concordant zircon ages (Figs. 3e-3h; Supplementary Table S2), including Precambrian (45.6%), Permian (3.2%), Triassic (6.4%), and Jurassic (44.8%) zircon grains but no Cretaceous grains (Figs. 3g and 3h). The YSG age is 156.0 ± 5.1 Ma, and the age of the highest peak in the probability density plot is ∼ 170 Ma (Fig. 3h). The youngest cluster consists of two zircon grains, with a YC1σ age of 157.5 ± 4.4 Ma (MSWD = 1.2).

In both analyzed samples (TK2 and TK6), Mesozoic zircon grains are euhedral and show oscillatory zoning (Supplementary Figures S1a-S1c, S1e-S1g; Supplementary Fig. S1 is available online from https://doi.org/10.2465/jmps.241123L). Precambrian grains are rounded and have distinct cores and rims or are homogeneous (Supplementary Figures S1d and S1h).

RAMAN SPECTRAL ANALYSIS OF CARBONACEOUS MATERIAL

Raman spectral analysis of CM was conducted using a confocal Raman microscope at Shinshu University, Japan, with analytical conditions based on previous studies (Supplementary Table S1; Aoya et al., 2010; Kouketsu et al., 2014; Kaneki and Kouketsu, 2022). The spectra obtained in this study corresponds to that at ∼ 270 °C, as shown in fig. S5 of Kaneki and Kouketsu (2022). In this temperature range, five bands can be identified: G, D1, D2, D3, and D4 (e.g., Kouketsu et al., 2014). Therefore, we used the Fitting E Python script of Kaneki and Kouketsu (2022), which decomposes the Raman spectra into the five bands (Figs. 4a and 4b).

Figure 4. Results of Raman spectral analyses of carbonaceous materials. [(a) and (b)] Peak separation in Raman spectra for samples (a) TK2 and (b) TK7. T_D1 and D4/D1_int values from each spectrum are shown at upper left. Note that the T_D1 values in (a) and (b) are the same, whereas D4/D1_int values are different. (c) T_D1 and (d) D4/D1_int values versus latitude. Red circles and error bars represent the mean and SE, respectively. Orange vertical lines represent the location of the Takihara thrust. n is number of data points for FWHM_D1 or D4/D1_int within two standard deviations of the mean, with the total number of measurements in parentheses.

To examine the spatial variation in spectral characteristics, we first used the metamorphic peak temperature (T_D1), which has an applicable range of 150-400 °C and is estimated using Raman CM geothermometry as follows (Kaneki and Kouketsu, 2022):

  

\begin{equation} T_{\text{D1}}\ ({{}^{\circ}\text{C}}) = -2.30 \times (\mathit{FWHM}_{\text{D1}}) + 486 \end{equation} (1),

where FWHM_D1 is the full width at half maximum of the D1 band. As the samples generally exhibit a unimodal distribution, we followed the approach of Kaneki and Kouketsu (2022) to consider the variation in the data. Firstly, data more than two standard deviations from the mean FWHM_D1 were excluded. The mean value was then used as the temperature estimate, and the standard error [SE; see text S1 in Kaneki and Kouketsu, (2022) for details] was adopted as the temperature error (Fig. 4c).

One of the key features of Raman spectra of CM that experienced temperatures below ∼ 340 °C is the presence of the D4 band. This band decreases gradually with increasing metamorphic temperature (e.g., Kouketsu et al., 2014, 2017) and is associated with organic carbon structures, including disordered graphite lattices and vibrations such as C-C and C=C stretching in polyene-like structures, as well as C_aromatic-C_alkyl vibrations (e.g., Aramendia et al., 2023). Additionally, it has been linked to ionic impurities (e.g., Sadezky et al., 2005; Aramendia et al., 2023). While its exact origin remains debated, its systematic variation with metamorphic conditions suggests that it can serve as a useful thermal indicator. In the Kanto Mountains, where the Mikabu greenstones occur along the boundary between the SMC and CAC, a systematic change in the ratio of the intensities of the D1 and D4 bands (D4/D1 intensity ratio; D4/D1_int) with changes in T_D1 has been observed across the boundary (Hara et al., 2021). We identified a similar pattern, with a more pronounced D4 band in the CAC than in the SMC (Figs. 4a and 4b). Therefore, we also investigated the spatial variation in this indicator, which represents another thermal parameter. D4/D1_int generally exhibits a unimodal distribution; therefore, data that deviated from the sample mean by more than two standard deviations were excluded. The mean value was used as the representative value, and the SE was adopted as the error in the intensity (Fig. 4d).

The value of T_D1 is ∼ 275-290 °C on the northern side of the Takihara thrust and ∼ 275-280 °C on the southern side (Fig. 4c), indicating no discernible difference in temperature across the thrust. The D4/D1_int ratios are ∼ 0.30-0.33 on the northern side of the thrust and ∼ 0.47-0.50 on the southern side, showing a clear difference (Fig. 4d).

DISCUSSION

The depositional age of TK2, north of the Takihara thrust, is estimated to be ∼ 118 Ma based on the youngest ages, whereas that of TK6, south of the Takihara thrust, is estimated to be ∼ 158 Ma. Given the depositional ages presented by previous studies of the central Kii Peninsula, the former corresponds to the SMC, and the latter to the CAC. In addition, zircon grains from the SMC (including the SAC) samples in the central Kii Peninsula show a predominance of Precambrian, Permian-Triassic, and Cretaceous grains. Although the relative proportions of these three groups vary with the depositional age (Shimura et al., 2019a; Jia and Takeuchi, 2020), samples with depositional ages older than ∼ 105 Ma commonly contain <10% Cretaceous grains, consistent with the data for TK2. In contrast, the age distributions of zircon from the CAC in the central Kii Peninsula are characterized by a predominance of Precambrian (∼ 29-53%) and Permian-Jurassic grains (∼ 47-69%; Shimura et al., 2019b; Tokiwa et al., 2019), which is consistent with the data for TK6. The similarities of the youngest ages and age distributions with those of previous studies support the finding of Kimura (1954) that the Takihara thrust represents the geological boundary between the SMC and CAC.

Based on this identification, the SMC is positioned structurally above the CAC. However, in other areas, including the Kohtaki outcrop (Fig. 1b), where the boundary of the SMC and CAC can be identified, the opposite structural relationship is reported (e.g., Aoya, 2010; Endo and Wallis, 2017). Aoya (2010) pointed out that the boundary between the SMC and CAC in the Kohtaki outcrop was formed during the early exhumation of the SMC, preserving the primary structural relationship and potentially forming a broad ductile shear zone. In contrast, a sharp lithological boundary is seen in the outcrop of the Takihara thrust, with a narrow brittle shear zone. Given these observations, the inverted structural relationship at the Takihara thrust might have formed through the secondary effect of brittle deformation at shallower depths during a later stage of exhumation.

T_D1 values are similar on the northern and southern sides of the Takihara thrust, suggesting that they cannot be used to distinguish between the two complexes. In contrast, there is a clear difference in the D4/D1_int ratio either side of the thrust, suggesting that this parameter can be used to determine the location of the boundary between the SMC and CAC in the central Kii Peninsula where the Mikabu greenstones are absent. The critical D4/D1_int value for distinguishing between the belts is ∼ 0.4 (Fig. 4d). In addition, our results suggest that the D4/D1_int value is useful for detecting differences in metamorphic grade in low-grade metamorphosed rocks.

In contrast, in the Kanto Mountains and central Shikoku, T_D1 values of ∼ 270 °C have been obtained within the CAC rather than at the boundary between the SMC and CAC (Endo and Wallis, 2017; Hara et al., 2021). In the Kanto Mountains, D4/D1_int ratios of ∼ 0.4 have also been reported in roughly the same areas (Hara et al., 2021). These locations correspond roughly to the southernmost extent of the Sanbagawa metamorphism; therefore, the D4/D1_int ratio may be a valuable tool for determining the southern limit of the Sanbagawa metamorphism.

ACKNOWLEDGMENTS

The authors thank Y. Shimura and K. Miyazaki for their comments on an earlier version of this manuscript, and Y. Kouketsu for her careful editorial assistance. We also thank K. Yamamoto and Y. Asahara for their assistance with U-Pb dating at Nagoya University. We thank M. Aoya and K. Yamaoka for their helpful advice. Field studies were conducted during the 1:50000 quadrangle geological map project of GSJ, AIST. This work was supported by grants from JSPS KAKENHI (JP21K14012, JP19K04013, and JP21H01188) and the Institute of Mountain Science and the Faculty of Science, Shinshu University.

SUPPLEMENTARY MATERIALS

Supplementary Tables S1-S2 and Figure S1 are available online from https://doi.org/10.2465/jmps.241123L.

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