Raman spectroscopy of carbonaceous material (RSCM) thermometry is an effective method to determine the thermal structure of metasedimentary domains with high spatial resolution. The mean value of numerous measurements in a single sample is used for temperature estimation. A potential complicating factor is the presence of detrital carbonaceous material (CM) that can introduce bias in the results. Detrital CM generally shows a high degree of crystallinity making it difficult to discriminate detrital and metamorphic CM in high-grade metamorphic domains. Application of the RSCM thermometry to 100 metamudstone samples from the Hongusan area of the Ryoke metamorphic belt reveals multimodal degrees of crystallinity of CM in many samples indicating the common presence of detrital CM grains. Application of a new procedure to filter out the effects of detrital CM reveals a thermal structure that can be explained as the result of the overprinting of two distinct thermal regimes related to regional and subsequent contact metamorphism. The ability to recognize the presence of detrital CM and measure its crystallinity can be used to infer the metamorphic grade present in the hinterland from which the detrital material is derived. A combination of this information on metamorphic temperature with geochemical and radiometric age data reported in the literature indicates that the detrital CM was derived from basement rocks of the Korean Peninsula.

Raman spectroscopy of carbonaceous material (RSCM) thermometry allows peak temperatures (T) of metasedimentary rocks to be estimated based on systematic and thermodynamically irreversible changes of Raman spectra of carbonaceous material (CM) that can be correlated with peak experienced temperature (e.g., [14]). This approach can be applied to a wide range of sedimentary lithologies and over a wide temperature range of 150–655°C. The utility of this method opens the possibility of determining the thermal structure of metasedimentary rock domains with very high spatial resolution largely controlled by sampling density (e.g., [5]). RSCM thermometry is based on averages of numerous Raman spectroscopy measurements in a single sample [1], and a potential complicating factor is the presence of detrital CM that can bias the average values. Such detrital grains are commonly recognized as grains of highly crystalline CM (close to graphite) in populations of grains with lower crystallinity and excluded from the final analysis. It is more challenging to identify detrital CM that has a degree of crystallinity less than graphite and is present in moderate metamorphic grade zones and is not generally addressed or considered in temperature estimates.

Recent studies focusing on CM in modern sediments have shown that the material cycling due to erosion and deposition in convergent plate margins leads to the storage of relatively well-crystallized CM in sediments. The preferential concentration of well-crystallized CM over weakly crystallized material is due to its greater resistance to weathering and erosion processes [610]. The presence of any detrital well-crystallized material in metamorphic rocks will potentially skew estimates of metamorphic temperatures using RSCM thermometry unless their effect is filtered out. In this study, we distinguish between detrital CM grains that have relatively high crystallinities unchanged from the sedimentary protolith and metamorphic CM grains that have varying crystallinities depending on the metamorphic temperature. The proportion of CM grains identified as detrital decreases as the metamorphic temperature increases.

Detrital CM can be readily identified in thin sections of low-grade metamorphic rocks (<~400°C) due to its relatively high reflectivity clearly contrasting with weakly crystallized metamorphic CM. Other techniques such as X-ray diffraction analysis have also been used to identify the presence of detrital CM in such rocks [11, 12]. However, distinguishing between metamorphic and detrital types of CM becomes more difficult as the metamorphic grade increases because the crystallinities of the two types of CM converge.

In this study, we report the results of Raman analyses of CM in metamudstone of the Hongusan area in Central Japan, which is part of the high-T/P-type Ryoke metamorphic belt (Figure 1). Knowledge of the general thermal structure of an area can be combined with Raman spectra of CM to distinguish between detrital and metamorphic CM grains. Our results suggest that the metamudstone in the area contains significant amounts of detrital CM with two distinct peaks in crystallinity. The presence of the detrital material needs to be taken into account to obtain a reliable estimate of the metamorphic T. Here, we propose a quantitative procedure to separate the effects of detrital and metamorphic CM, use our procedure to estimate the metamorphic thermal structure, and investigate the implications of the results for provenance studies of the region.

2.1. Geological Relationships between Intrusive and Country Rocks

The Hongusan area is a well-studied region of the Ryoke metamorphic belt that consists of granitoid bodies and metamorphic rocks (Figures 1 and 2; [1317]). The Ryoke belt is a domain of metamorphism that affects part of a Jurassic to early Cretaceous accretionary complex, the Mino-Tanba belt, which is one of the most widely developed basement units of the Japanese island arc. The Mino-Tanba belt can be correlated with the Chichibu and North Kitakami-Oshima belts (Figure 1; [1821]). The metamorphic rocks in the study area are dominantly composed of metamudstone, metasandstone, and metachert, all of which show a monoclinic north-dipping schistosity generally subparallel to bedding [13, 14, 22].

In the Hongusan area, two intrusions, the Shinshiro tonalite and the Busetsu granite, crosscut the tectonic foliation of the Ryoke metamorphic rocks (e.g., [17, 23]). The lithological contact of the Shinshiro tonalite is subperpendicular to lithological layering of the Ryoke metamorphic rocks in this area, whereas the Busetsu granite has a contact roughly parallel to the same layering (Figure 2(a)).

The results of CHIME monazite and U–Pb zircon dating suggest that the age of the Ryoke metamorphism is around 100–90 Ma [2427]. U–Pb dating of zircon yields intrusion ages of 72–70 Ma for both the Shinshiro and Busetsu plutons [17]. However, the monazite dating suggests that parts of the Shinshiro tonalite may be as old as 85 Ma [28].

2.2. Metamorphism of the Hongusan Area

Previous studies using silicate mineral thermobarometry provide good estimates of T in key locations, and the regional trends of the thermal structure in the study area are well established. These results provide a framework to interpret our new RSCM thermometry, and a combination with our new RSCM thermometry data allows a much more detailed thermal structure to be determined.

The study area shows a metamorphic thermal structure formed by a combination of the Ryoke regional metamorphism overprinted by contact metamorphisms around both the Shinshiro and Busetsu intrusions [13, 15, 22, 29]. The Ryoke metamorphism can be separated into biotite (Bt), K-feldspar-sillimanite (Kfs–Sil), and garnet-cordierite (Grt–Crd) zones from north to south in order of increasing grade [22] with temperatures ranging ~500–800°C [16, 30, 31]. A distinct Kfs–Crd metamorphic zone can be defined as the product of the contact metamorphism around both the Shinshiro and Busetsu intrusions (Figure 2(b); [22]). The Kfs–Crd and Kfs–Sil isograds occur at ~600°C in the area based on intersecting univariant mineral reactions [13] and pseudosection analysis [30].

The thermal effect of the intrusion of the Shinshiro tonalite is also reflected in changes in rock microstructure such as changes in quartz grain size due to thermally driven static recrystallization [32] and growth of posttectonic porphyroblasts including andalusite (C-type andalusite) [13, 15]. The observation that C-type andalusite occurs up to ~7 km from the contact—a distance which is similar to the diameter of the whole Shinshiro pluton—means this contact aureole is unusually broad compared to most documented domains of contact metamorphism (Figure 2(b); [33]).

3.1. Sampling Strategy

In the Hongusan area, the thermal structure of the Ryoke metamorphism increases in grade from north to south. CM shows complete graphitization at T>655°C, and the area south of the Kfs–Sil zone is beyond the temperature range applicable to the RSCM thermometry. In this study, a total of 100 metamudstone samples were collected at distances of up to 14 km from the Shinshiro tonalite (Figure 3(a)) with 60 from the Bt zone, 5 from the Kfs–Sil zone, and 35 from the Kfs–Crd zone. Most samples were collected from the area of Bt zone regional metamorphism where metamudstone is common (Figure 3(a) and Table 1). The mineral parageneses observed in thin sections of all the collected samples show good agreement with the metamorphic zones indicated by Miyazaki et al. [22] (Table 1). All thin sections contain CM suitable for Raman analysis (Figure 3(b)). In thin sections, CM grains show both inter- and intragranular occurrences of the major mineral grains such as quartz, feldspar, andalusite, biotite, and muscovite. The CM grains are all less than ~10 μm in diameter. Some of the intergranular CM grains occur as clusters along grain boundaries, while intergranular CM grains are generally distinct from and unconnected to neighboring CM grains.

Previous studies have shown that time scales of metamorphism greater than 104–105 years are sufficient for the degree of crystallization of CM to achieve a steady state [3436]. The Shinshiro and Busetsu bodies both have diameters of the order of 104 meters implying conductive cooling time scales t (t=h2κ1; h is a half width of the intrusion, and κ is a thermal diffusivity (~10−6 m2 s−1); [37]) of at least 105 years, sufficient to assume that a steady state has been attained (e.g., [34]). The study area is also affected by the Ryoke regional metamorphism which is associated with time scales at least one order of magnitude greater [17, 25]. The time scales involved imply that all metamorphic CM in the region can be considered to have attained a steady state.

3.2. RSCM Thermometry

To estimate temperatures, we used the calibration and procedure of Aoya et al. [1], which can be applied to medium- to high-grade CM (340–655°C) samples. The geothermometer uses Raman spectra of CM in the first order (1000–1800 cm-1) decomposed into three (or fewer) peaks referred to as the G-, D1-, and D2-bands. The proxy for temperature is the R2 ratio, which is the area ratio of the three peaks (R2=D1/G+D1+D2). The relationship between the R2 and the peak experienced temperature is
(1)T°C=221.0R22637.1R2+672.3.

The associated error is ±30°C [1]. Raman measurements were carried out using a Nicolet Almega XR (Thermo Scientific) with a 532 nm Nd-YAG Laser at Nagoya University; light backscattered from the target CM grain is detected using a Peltier cooled CCD with a 2,400 lines/mm diffraction grating. The analysis uses a 100x objective (NA=0.9), a laser power of ~3 mW at the thin section surface, and an acquisition time of 30 seconds. We measured 30–60 randomly selected grains of CM all located beneath the surface of the thin section. The presence of the grains beneath the surface was verified for all samples by comparing images obtained using transmitted and reflected light microscopy. The effect of measuring grain surfaces with different crystallographic orientations in the same samples was examined, but in accordance with the results of Aoya et al. [1], no significant differences were recognized. The edges of the platy CM grains were avoided as much as possible in the analysis. Similarly, the results showed no significant correlation with either CM grain size or grain shape. To assess any possible in situ crystal structure changes due to laser-induced heating [38], we monitored the peak position of the G-band (~1580 cm-1) throughout the measurements. However, no significant changes were recognized for the measurement conditions used in this study. The obtained Raman spectra were decomposed into G-, D1- (1350 cm-1), and D2- (1620 cm-1) bands using PeakFit 4.12 ver. (Systat Software, Inc., USA).

3.3. Results of the Raman Analysis

The Bt zone is the lowest grade part of the studied area. However, 13% of the measured CM grains in samples from this zone are fully recrystallized graphite (R2=0). There is also considerable variation in R2 values from single samples (0–~0.5) and between samples in this metamorphic zone (R2mean~0.1–~0.3) (Table 1, Figures 4 and 5). Histograms of the R2 values for these samples show multimodal distributions including a peak at R2=0 and a variable number of other peaks with R2>0 (Figure 4). The Kfs–Sil and Kfs–Crd zones are higher grade and mainly occur around the Shinshiro body. The CM grains from these metamorphic zones are well-crystallized graphite with 53% showing R2=0. R2 values range from ~0 to ~0.2 within a single sample (Table 1, Figure 4), and the R2mean values show a similar scatter (Table 1; Figures 4 and 5). Histograms of R2 values are bimodal with one peak at R2=0. The other at higher R2 corresponds to less-crystallized CM. The only exceptions are the samples nearest to the Shinshiro tonalite in which all the CM show R2=0 (e.g., 89120601 in Figure 4). Four samples were collected in the Kfs–Crd zone around the Busetsu granite. Of these, the CM characteristics including the R2 ranges, proportion of graphite grains, and R2mean values of three samples (SSB-21, SSB-23, and 89101603) correspond to those of the Bt zone. The remaining sample (SSB-22) shows characteristics comparable to those of the Kfs–Crd and Kfs–Sil zones.

Graphite grains with R2=0 are detected in 96 out of 100 samples throughout the area. However, the R2max values of the same group of samples (corresponding to the least crystallized CM) show a good general correlation with metamorphic grade (Figures 6(a) and 6(b)). This observation implies that the highly crystalline CM in samples with moderate metamorphic temperatures, less than about 600°C, is not related to metamorphism and is of detrital origin.

When plotted along an E–W trend in areas>5km from the Shinshiro tonalite, the R2max values scatter in the range 0.2–0.5. However, in areas<5km from the tonalite, the R2max values show a clear decreasing trend from ~0.4 to 0 (Figure 6(a)). In contrast, R2max values plotted along a N–S trend at distances>5km from the tonalite show a decreasing trend from ~0.5 to ~0.2 towards the south (Figure 6(b)). This distribution of R2max is consistent with the thermal structures of the regional and contact metamorphism recognized by observing changes of the mineral assemblages. In addition, samples with larger values of R2max tend to show a broader range of the R2 values. These features can be shown using a plot of the R2mean (R2=0 excluded) and the associated 1σ values that shows a clear positive correlation (Figure 5).

3.4. Presence of Detrital CM

The distributions of R2 values (R2>0) show clear multimodal distributions in many samples, and almost all metamudstone samples in the area contain at least some fully crystallized graphite CM grains. Processes such as late-stage deformation and crystallization from fluids may cause local development of CM with crystallinities unrelated to the regional metamorphic temperature (e.g., [35, 3941]). However, no microstructural evidence was observed for either late-stage deformation or crystallization of minerals related to infiltration by fluids in any of the measured samples. In addition, short time scales (<105 yr) of recrystallization can result in CM that has not reached a steady state and shows a lower degree of crystallinity than expected from the metamorphic temperature [34]. In this study, the time scales are at least an order of magnitude greater than what is required for a steady state to be achieved. The good correlation between the R2max values and the overall changes in metamorphic grade is confirmation that the R2 peak representing the least degree of crystallization is related to the metamorphic temperature. This result also implies that peaks in the R2 spectra that correspond to more highly crystallized CM reflect the presence of detrital grains.

In several earlier studies, the presence of high-crystallinity CM that does not match the regional metamorphic degree has been attributed to the presence of detrital high-grade CM (e.g., [4, 10, 4244]). This interpretation is supported by studies of modern sediment detrital material showing that high-crystallinity CM grains are relatively resistant to weathering and erosion compared to low-crystallinity CM grains and can be stored as sediment [6, 10]. In this study, we follow the above reasoning and interpret polymodal distributions of R2 as indicating the presence of detrital CM. This implies that metamudstone in the Hongusan area contains a large amount of detrital CM, and as the maximum temperature rises, the low-grade CM progressively changes to graphite (Figure 7).

3.5. Temperature Estimates

In order to obtain reliable estimates of temperature from samples with polymodal distributions of R2 values, it is necessary to filter out the higher-grade detrital CM. The need to filter out the influence of detrital CM is a common problem, but there has been little attention to ways in which this can be approached in a systematic way. The most quantitative approach [1, 45] is to treat CM with R2 values greater than 2σ away from the overall mean as detrital origin and eliminate these data from the final analysis. However, this procedure is applicable only to low-grade metamorphic rock where there are clear differences between the crystallinities of detrital and metamorphic grains. In addition, applying a simple cutoff in the data used for temperature estimates could result in removing data that should ideally be included in the final analysis.

In the Hongusan area, we recognize complex patterns of R2 values with multiple peaks. To account for the effects of the detrital CM, first we treated all the graphite grains (R2=0) in regions with expected metamorphic temperatures<655°C as detrital and removed these data. The remaining data need to be analyzed to determine if they can be treated as a single distribution. If not, further filtering is required.

We applied the Shapiro–Wilk test to datasets of R2>0 to examine if the data are significantly different from a unimodal Gaussian distribution (Table 1). The R2 values cannot be negative, and it is possible to consider alternative non-Gaussian distributions. However, we focus on data with intermediate values, and the Gaussian distribution is a good approximation for many distributions if the sample data are large. The null hypothesis is that the sample data were obtained from a single normal distribution. If this probability was more than 5%, i.e., with a p value greater than 0.05, we considered the data distribution indistinguishable from unimodal and estimated the peak experienced temperature using its mean R2 and equation (1). In contrast, for samples which fall below the significance level, we reject the null hypothesis and the samples are interpreted as having a bimodal distribution. To derive the peak experienced temperature, two best fit Gaussian functions are determined by a least square method and the greater mean value for R2 is used in equation (1) (Figures 8(a) and 8(b)). For some samples, applying the above procedure drastically reduced the number of CM grains suitable for temperature estimates. We excluded samples from the final analysis if this number fell below 10 (Table 1). We use a statistical analysis to filter the data and cannot exclude the possibility that some data for detrital grains will be included in the final analysis. In this case, the peak experienced temperature will be somewhat higher than the true value. A comparison with independent temperature estimates from the same area can help identify outliers that yield anomalous results. In our case, we did not identify any that gave clearly anomalous results.

In our study, we identified thirteen samples that have p values less than 0.05. These all exhibit multimodal R2 values with peaks at 0 and ~0.1–0.2 and a third range that correlates with the metamorphic grade. These samples with clear trimodal distributions were collected from the northwestern part of the biotite zone (Figures 6(c) and 6(d)). A compilation of the data from these thirteen samples yields mean values for the first two peaks of R2=0 and 0.138 and corresponding temperatures of >655°C and 589°C (Figure 8(c)) based on the calibration of Aoya et al. [1]. The third peak reflects the peak experienced temperature, and its value depends on the location of the sample.

3.6. Thermal Structure in the Hongusan Area

Our approach allows us to refine our understanding of the thermal structure of the region and define the two-dimensional thermal structure with a much-increased spatial resolution compared to previous studies (Figure 9). In particular, our new analysis allows us to make a much clearer definition of the thermal structure within the zone of contact metamorphism. Note that the distance in this study is based on the horizontal distance on the map. The temperature ranges for each metamorphic zone are ~460–610°C for the Bt zone and >570°C for the Kfs–Sil and Kfs–Crd zones around the Shinshiro tonalite, and these are consistent with other temperature estimates in the study area using chemical compositions of silicate minerals [16, 30, 31]. The boundaries between the Bt zone and Kfs–Sil zone and Bt zone and Kfs–Crd zones occur at ~600°C [13, 22, 30]. The regional metamorphism is most clearly seen in domains>5km from the Shinshiro tonalite and shows a north to south temperature increase from ~460 to 610°C. The contact metamorphic aureole around the Shinshiro tonalite shows a west to east temperature increase from ~460°C to ~650°C within the domain up to 5 km from the tonalite (Figure 6(c)). Samples that contain fully crystallized CM (graphite) indicate T>655°C and are observed in a domain within 1 km of the tonalite.

A narrow zone with high temperatures (~550°C and ~600°C) is also observed close to the Busetsu granite (Figure 6(d)). Except for these data, the lower limit of the metamorphic temperature of the regional Ryoke metamorphism occurs in the northwest of the sampled domain with a value of around 460°C. In addition, the peak temperature of the contact metamorphism due to the Busetsu granite intrusion is constrained at ~600°C close to the contact between the granite and the host metamudstone and <460°C at a distance of 130 m from the contact (Figure 6(d)).

4.1. Inferred Provenance Characteristics from Geochemistry of Metamudstone

Our investigations of CM crystallinity in the Hongusan area reveal not only the thermal structure but also the extensive presence of large amounts of detrital CM. Most of the detrital CM is fully crystallized suggesting that it was derived from an area that had at least locally undergone heating in excess of 655°C. The presence of significant amounts of slightly less highly crystallized detrital CM suggests that there was a broad region of metamorphism in the provenance with domains at different grades that were sufficiently widely distributed to contribute significantly to the detrital input. This result shows that the Raman study on the detrital CM can be a useful tool in provenance analysis and help expand the type of rocks used in provenance studies, which generally focus on relatively coarse and easily analyzed detrital grains preserved in sandstone and conglomerates. Raman CM analyses of units with different stratigraphic ages and from different geographic locations have the potential to contribute to estimating the spatiotemporal distribution of metamorphic belts and the associated grades in the hinterland of sedimentary units. In the following, we combine our approach with the results of other methods that have been applied as tools in provenance analysis to the study area and consider how far the detrital CM study can further constrain the provenance.

Petrological studies of siliciclastic sedimentary rocks have been widely used in provenance analyses. In strongly recrystallized metamudstone, it is difficult to identify the original grain types with confidence. However, bulk rock chemistry is less likely to undergo significant change during metamorphism and can be used to characterize the provenance [46, 47] and nature of chemical weathering [4850]. We used bulk rock geochemical data of metamudstone from the area available in published articles [13, 14, 30]. Graphs of SiO2 vs. K2O/Na2O can be used to distinguish different tectonic environments [46]. The data from the Hongusan area show metamudstone data plot along and around a bisector between the passive margin and active continental margin fields (Figure 10(a)). Using the F1 vs. F2 graph, where F1 and F2 are discriminant functions defined in Roser & Korsch [47], suggests that the sediments were derived from a combination of a felsic igneous provenance and a mature continental provenance (Figure 10(b)).

The Chemical Index of Alteration (CIA; [48]) measures the degree of chemical weathering of feldspars relative to unweathered protoliths, which is calculated using the following formula:
(2)CIA=Al2O3Al2O3+CaO+Na2O+K2O×100inmolecularproportions.

Higher CIA values suggest higher degrees of weathering. CIA values of the metamudstones in the study area are in the range of 57–70 with an average of 65 (Table 2). This indicates low-to-intermediate chemical weathering and is generally interpreted as due to a relatively small transport distance from the provenance. CIA values can be affected by climatic conditions so that the value provides only a qualitative indication but does suggest that large-distance transport of the detrital particles by large river systems within the Asian continent, such as the modern Yangtze River system, can be excluded. An alternative plausible provenance is around the eastern margin of the Asian continent.

The type of provenance that is consistent with these geochemical signatures is a continental margin where the exposed basement consists of regional high-T/P-type metamorphic rocks and felsic volcanic or plutonic rocks.

4.2. Possible Geological Terranes as the Provenance

A comparison of ages of detrital zircon and monazite in the trench-fill sediments of the Jurassic accretionary complex in SW Japan with ages of basement rocks in the Korean Peninsula and its surroundings suggests that the eastern margin of the North China Craton may be the main source of detrital sediments in the Ryoke belt with smaller amounts derived from the South China Craton (e.g., [25, 5153]). The main characteristic of the detrital grain age spectrum used in this comparison is the presence of Permian-Jurassic and Paleoproterozoic ages. These ages correspond to the ages of phases of igneous activity in the basement of the Korean Peninsula (e.g., [54, 55]). Conglomerates and sandstones of the Jurassic accretionary complex in Japan representing trench-fill deposits show characteristics suggesting that they were derived from a deeply dissected provenance (e.g., [53, 5658]). This indicates that basement metamorphic rocks and felsic plutonic granitoids associated with the post-Permian igneous phases of magmatism were already exposed and being eroded in the Jurassic. Quartzofeldspathic gneisses, felsic intrusions, and siliciclastic sedimentary rocks metamorphosed to different degrees make up a significant fraction of the pre-Cretaceous Korean basement (e.g., [5962]). This aspect is consistent with the chemical features shown in the Hongusan area (Figure 10(b)), and other terranes that have characteristics matching the above features have not been recognized.

The crystallinity of the detrital CM observed in the Hongusan area can contribute to constraining the source belts or terranes in the Korean Peninsula. The Ogcheon metamorphic belt—a regional Permian-Triassic fold-and-thrust belt of the Korean Peninsula (Figure 1)—is a potential source for detrital CM with R2>0. The Ogcheon belt was metamorphosed during the Ogcheon orogeny at ~260 Ma with peak temperatures of 500–630°C (e.g., [54, 60]). This temperature range corresponds well to the temperature range estimated from detrital CM with moderate crystallinity (mode 2 in Figure 8(c)). It is also noteworthy that CM-rich metamudstone layers have been documented in the Ogcheon metamorphic belt [6365] and the presence of these layers can help account for the unusually large proportion of detrital CM observed in the Hongusan area.

A significant proportion of the detrital CM recognized in the Hongusan area is associated with R2=0, which indicates peak metamorphic temperatures655°C. Such high-T conditions are not seen in the Ogcheon belt but are widely recognized in metasedimentary rocks of the Imjingang belt and Precambrian basement units of the Korean Peninsula. The Imjingang belt mainly consists of Devonian to Carboniferous sedimentary rock, recording ~260–250 Ma tectonothermal events similar to those recorded in the Ogcheon metamorphic belt (Figure 1; [66]) although higher metamorphic T conditions are estimated from metamudstones (~700°C; [67]). The main Precambrian basement units of the Korean Peninsula are the Nangrim, Gyeonggi, and Yeongnam massifs (Figure 1). While most researchers are in agreement that the Nangrim massif correlates with the North China Craton, the affinities of the Gyeonggi and Yeongnam massifs are disputed: both North China and South China craton origins have been proposed and other more complex histories including microcontinental slivers are also possible (e.g., [54, 6871]). Parts of these basement massifs have experienced metamorphic temperatures up to ~850°C [54]. The detrital CM in the Hongusan area with R2=0 may have been sourced from these high-grade basement domains.

Pyrope-rich detrital garnet is common in Jurassic accretionary complexes of Japan including the Mino-Tanba, Chichubu, and North Kitakami-Oshima belts and is interpreted to represent eroded remnants of amphibolite to granulite facies rocks now present in the Korean Peninsula (e.g., [7275]). These findings are consistent with the proposed source regions of the Korean Peninsula.

It is likely that other pre-Jurassic terranes of the Korean Peninsula that include low-grade metasedimentary rocks (e.g., Taean Formation and Taebaeksan Basin) also contributed to the trench-fill sediments of the Jurassic accretionary complexes in SW Japan. However, in the Hongusan area, such material cannot be identified due to the subsequent high-temperature metamorphism that overprinted any earlier low-temperature history. Better spatial constraints on the provenance could potentially be achieved by application of our method to less strongly metamorphosed sedimentary domains within the Mino-Tanba belt and other Jurassic accretionary complexes.

  • (1)

    Raman studies of the crystallinity of carbonaceous material (CM) in the Hongusan metamorphic area of SW Japan reveal the presence of multiple R2 peaks in individual samples implying the presence of a mixture of CM grains with contrasting crystallinities and geological histories. The long time scales of metamorphism in this area imply that these peaks cannot be attributed to kinetic differences for different grains. In addition, the lack of microstructural evidence for secondary effects of deformation or crystallization from fluids implies that the multiple R2 peaks reflect a combination of primary sedimentary differences, i.e., detrital CM grains, and the subsequent metamorphic history. Such detrital CM grains are a common component of metamudstone in the Hongusan study area

  • (2)

    A compilation of data from samples in the study area displaying multiple R2 peaks shows the presence of two groups of detrital grains with different mean R2 values and a third group with mean R2 values that decrease with the increasing regional metamorphic grade. Statistical analysis can be used to filter out the effects of the detrital material and obtain a reliable estimate of peak temperature

  • (3)

    The results of Raman studies of detrital CM in lithologies of orogenic belts yield information that is useful to constrain the metamorphic grade of metasedimentary rocks exposed in the hinterland and can assist in reconstructing paleogeography

  • (4)

    A combination of the crystallinity of detrital CM grains with other provenance signals and in particular zircon age spectra shows that the trench-fill deposits of the Jurassic accretionary complex of SW Japan likely originated in a suite of geological terranes in the Korean Peninsula: the Ogcheon metamorphic belt, the Imjingang belt, and adjacent Precambrian basement

All of the data presented in this paper are available from the tables and figures within the text, as well as from supplementary files (available here).

The authors confirm that they have no conflicts of interest.

We are grateful to the members of the Petrology-Tectonics Seminar in the University of Tokyo for many discussions.

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