Ryoke Belt Research Articles

Thermal Modeling of the Sanbagawa and Ryoke Belts

The Sanbagawa and Ryoke belts were formed in a convergent plate boundary along the eastern margin of Eurasia. Thermal modeling using the geological records of these belts as constraints allows quantitative estimates of both shear heating along the Wadati-Benioff zone and magma fluxes beneath the volcanic arc. In contrast to real-time observations of crustal movement and heat flow, rocks record changes in pressures and temperatures that occur over periods of several million years and can be used to examine conditions from the surface to the mantle. Thermal modeling combined with such geological records helps to bridge the gap in our knowledge between real-time observations of ongoing geological processes and the development of orogenies in convergent plate margins over geological time.

Paired Metamorphism of SW Japan and Implications for Tectonics of Convergent Margins

The Sanbagawa-Ryoke pair of geological units in southwest Japan is the classic example of paired metamorphism originally identified by Akiho Miyashiro. Together these belts represent an important study area for developing and testing ideas about how convergent margins behave over geological time based on studies of the rock record including petrology, geochemistry, deformation, and geochronology. The two sides of the pair represent ancient examples of a subduction zone in the Sanbagawa belt and an associated volcanic arc in the Ryoke belt. This issue of Elements brings together the results of a wide range of different approaches summarizing the current state of knowledge about the Sanbagawa-Ryoke pair and how this informs our understanding of convergent margins in general.

Inside the Ryoke Magmatic Arc: Crustal Deformation, High-T Metamorphism, and Magmatic Pulses

The Ryoke belt represents the root of a volcanic arc exposed across SW Japan. It records successive deformation phases, high-temperature metamorphism, and several magmatic pulses that occurred during the Late Cretaceous. Successive magma intrusions at different crustal levels raised the overall geothermal gradient of the arc crust, and their thermal influence was contrastingly recorded in metamorphic zircon and monazite. Despite a broadly similar duration of magmatic activity (20–30 My) along the belt, the timing and periodicity of magma pulses varied. An along-arc variation in lower crustal magma generation together with a fluctuating crustal stress regime likely controlled the formation and evolution of this magmatic arc section.

Dominant slip systems of quartz under lower amphibolite-facies conditions identified from microstructures and CPOs in quartz phenocrysts

Identifying the dominant slip systems in naturally deformed quartz is crucial for understanding continental crust rheology. Although the basal <a> slip system is considered dominant in quartz under upper to middle crustal conditions, its activity is controversial because an oriented nucleation and growth model has been proposed for the c-axis distribution near the minimum strain axis. This study examined the crystallographic orientation and shape of quartz phenocrysts in a deformed granitic porphyry in the Ryoke belt, SW Japan, to clarify the dominant slip systems in naturally deformed quartz at temperature conditions of ∼400–500 °C utilizing optical and electron backscatter diffraction (EBSD) observations. Identified active slip systems include prism <a>, basal <a>, prism [c], and rhomb <a> through misorientation analyses. The aspect ratios of phenocrysts with dominant prism <a> and basal <a> slip systems are higher than prism [c] and rhomb <a> slip systems, indicating similar strengths between prism <a> and basal <a> slip systems, which are weaker than prism [c] and rhomb <a> slip systems under lower amphibolite-facies conditions. The c-axis of phenocrysts with dominant basal <a> distribute at pole figures peripheries, indicating basal <a> activation over the proposed oriented nucleation and growth model.

Zircon U–Pb ages of granitic and mafic dikes associated with granitoids and metamorphic rocks of the Ryoke Belt in the eastern Kii Peninsula, southwest Japan

AbstractZircon U–Pb dates for felsic and intermediate to mafic dikes intruding into the Ryoke granitoids and metamorphic rocks at selected outcrops in the Takamiyama area of the eastern Kii Peninsula, southwest Japan, were determined along with their geology and petrography to reveal the history of Cretaceous magmatism. At each outcrop, the felsic and intermediate to mafic dikes exhibit specific structures that are indicative of magma intermingling and have coeval intrusion ages of ca. 81–77 Ma. Our zircon U–Pb data complement previously published data, suggesting that the mafic magmatism continued intermittently from 83 to 76 Ma in the Takamiyama area and that magmatism migrated eastward within the Ryoke Belt. A comparison of intrusion ages between a dike and a host Ryoke granitoid at one outcrop indicates that the host rock experienced ductile deformation at ~88 to ~83 Ma. Judging from the small number of zircons and the concordant date distributions, we didn't recognize the evidence suggesting the partial melting of the host rocks, as Nakajima et al. (Journal of the Geological Society of Japan, 2021, 127, 69–78) reported.

Low-temperature thermochronology of Ryoke belt granitoids, SW Japan: New insights into the recent cooling history from monazite fission-track dating

Traditional low-temperature thermochronology techniques such as apatite fission track (AFT) and apatite (U-Th-Sm)/He (AHe) have so far produced limited information on the neotectonic (late Miocene and younger) evolution of Japan. However, the development of monazite fission track (MFT) dating, which has an ultra-low temperature sensitivity (<∼25 – 46°C), provides a fresh opportunity to directly analyse recent denudation histories. Low-temperature thermochronology methods have been applied to granitoid samples from the Ryoke belt, located in eastern Yamaguchi and Nara Prefectures, SW Japan. Zircon (U-Th)/He (ZHe), AFT and AHe data and modelled thermal histories reveal Late Cretaceous - Pliocene cooling related to granitoid thermal relaxation combined with paleo-Izanagi and Pacific plate subduction along the eastern Eurasian continental margin. MFT dating reveals Plio – Pleistocene cooling ages, coincident with elastic loading caused by Philippine Sea plate subduction since the Middle - Late Miocene, along with the Quaternary collision of NE and SW Japan at the Itoigawa-Shizuoka Tectonic Line. Silica concentration, non-thermal “radiation assisted” annealing, and changing climatic conditions are proposed as possible caveats on MFT annealing kinetics since the samples are residing in an “open” system. Taking all these factors into consideration, the apparent MFT ages are interpreted as minimum cooling ages, being slightly younger than the thermal events that cooled the samples through the MFT nominal closure temperature. Estimated denudation rates based on MFT dating are higher in Nara relative to eastern Yamaguchi Prefecture, with the difference likely reflecting variations in the tectonic regime, timing of uplift and uplift mechanisms of the two regions.

Oxygen isotope data of quartz from San-yo and Ryoke belt granites, schists, and siliceous veins: constraining the effects of 18O-rich fluids on granitic magma

Oxygen isotope data of quartz from San-yo and Ryoke belt granites, schists, and siliceous veins: constraining the effects of ¹⁸O-rich fluids on granitic magma

A shape-change model for isolated K-feldspar inclusions within a shear zone developed in the Teshima granite, Ryoke metamorphic belt, Japan: Estimation of the duration of deformation in a natural shear zone

A shear zone developed in the Teshima granite in the Ryoke metamorphic belt of Japan contains isolated K-feldspar inclusions showing various shapes within deformed host quartz grains. We present a shape-change model for isolated K-feldspar inclusions, which is a mathematical equation that considers stress, inclusion size, temperature, and the duration of deformation. To calculate the shape-change pattern (i.e., the aspect ratio and size of inclusions) with respect to deformation duration using the model, we used values for deformation temperature (~500–600 °C) and paleo-stress (90 ± 10 MPa) estimated from deformed quartz microstructures within the shear zone. Shape-change patterns were obtained by calculating the model at the estimated range of temperatures with an interval of 25 °C and at the estimated paleo-stress. Shape-change patterns were subsequently corrected by compensating for the effect of post-deformation annealing during exhumation of the shear zone. Deformation duration was estimated by identifying the shape-change pattern calculated by the model that most closely corresponded to the observed shape-change pattern for the shear zone and considering the cooling rate of the Ryoke belt. The resultant deformation duration of the shear zone is estimated as 16,500–8400 yr under conditions of T = 550 °C and σ = 90 ± 10 MPa. Shear strain in the shear zone is calculated as 50–260 by multiplying the shear strain rate by the deformation duration.

Multistage zircon growth recording polyphase metamorphic evolution caused by pulsed granitoid intrusions into a low‐P/T type metamorphic belt: P–T–D–t evolution of migmatites in the Ryoke belt, southwest Japan

AbstractWe report contrasting pressure–temperature–time (P–T–t) paths of migmatites developed in the highest‐grade metamorphic zone (Grt–Crd zone) and the contact metamorphic zone (Crd–Kfs zone) of the Mikawa area, Ryoke belt, southwest Japan to discuss the complex P–T–D–t evolution of the middle crust that experienced pulsed granitoid intrusions. In the Grt–Crd zone, sillimanite‐grade high‐T metamorphic condition prevailed from ca. 97 to 87 Ma, followed by cooling to ~500 °C, ~4 kbar. The intrusion of gneissose granitoids below the Grt–Crd zone isobarically reheated the Grt–Crd zone rocks again to the sillimanite‐grade high‐T condition at ca. 84 Ma. This was followed by ca. 71–70 Ma contact metamorphism. Ductile deformation that formed and folded the foliation of migmatites started before ca. 89 Ma and continued at least until ca. 84 Ma in the Grt–Crd zone. On the other hand, ca. 74 Ma age of the Crd–Kfs zone migmatite developed around the Inagawa Granodiorite in addition to ca. 70 Ma age of a syn‐tectonic pegmatite vein revealed that the intrusion of “75–69 Ma granitoids” caused partial melting and locally triggered low‐strain ductile deformation in their contact aureoles. Comparison with other areas of the Ryoke belt suggests that plutono‐metamorphic evolution of the Mikawa and Aoyama areas are similar with each other in that ca. 80 Ma reheating events (i.e., contact metamorphism) are observed, while absence of separate reheating event postdating peak metamorphism in the Yanai area is a rather uncommon feature in the Ryoke belt.

First- and second-order Raman spectra of carbonaceous material through successive contact and regional metamorphic events (Ryoke belt, SW Japan)

The evolution of both the first- and second-order Raman spectra of carbonaceous material (CM) through successive contact and regional metamorphic events is explored in the western part of the Ryoke belt (Iwakuni-Yanai area, SW Japan). Thirty-two metasedimentary rock samples were collected along a N-S gradient locally affected by contact metamorphism before and after the main regional tectono-metamorphic event (DP1). First-order spectra document a decreasing peak area ratio R2 and an increasing temperature TCM towards the south and the surrounding granitoids. Domains with intermediate (535–600 °C) TCM values match the extent of the pre-DP1 contact aureole but also image a so far unclear post-DP1 aureole. The axial part of the belt, likely unaffected by granite intrusions, preserves southward increasing TCM from 425 to 660 °C. Second-order spectra show a single S1 band that splits into two peaks (S1- and S1+) whose frequency difference ΔS1 increases stepwise towards the south. The spatial distribution of ΔS1 follows that of the E–W trending regional metamorphic zones. The splitting of S1 indicates a transition from two-dimensional to three-dimensional CM and occurs at ~500 °C, which seems to be common to all metamorphic belts worldwide. Despite regional metamorphism CM was able to record the post-DP1 contact overprint and there is no clear observation of delayed CM recrystallization, which likely depends on the crystallinity of the CM precursor. A discrepancy between first- and second-order Raman parameters suggests that they partly record the influence of different factors; R2 gives an account of thermal events, particularly those related to localized contact metamorphism, whereas ΔS1 potentially yields information on regional variations in heating duration and pressure. This demonstrates the potential of the full Raman spectrum of CM for deciphering the complex thermal history of orogenic systems.

Alteration of magmatic monazite in granitoids from the Ryoke belt (SW Japan): Processes and consequences

AbstractThe alteration of magmatic monazite and its consequences for monazite geochronology are explored in granitoids from the western part of the Ryoke belt (Iwakuni-Yanai area, SW Japan). Biotite-granite samples were collected in two plutons emplaced slightly before the main tectono-metamorphic event: the first one, a massive granite (Shimokuhara) adjoins schistose rocks affected by greenschist facies metamorphism; and the second, a gneissose granite (Namera) adjoins migmatitic gneiss that experienced upper-amphibolite facies conditions. Despite contrasting textures, the granite samples have similar mineral modes and compositions. Monazite in the massive granite is dominated by primary domains with limited secondary recrystallization along cracks and veinlets. It is variably replaced by allanite+apatite±xenotime±Th-U-rich phases. The outermost rims of primary domains yield a weighted average 206Pb/238U date of 102 ± 2 Ma while the Th-U phases show Th-U-Pb dates of 58 ± 5 and 15 to 14 ± 2–3 Ma. Monazite in the gneissose granite preserves sector- or oscillatory-zoned primary domains cross-cut by secondary domains enriched in Ca, Y, U, P, and containing numerous inclusions. The secondary domains preserve concordant 206Pb/238U dates spreading from 102 ± 3 to 91 ± 2 Ma while primary domain analyses are commonly discordant and range from 116 to 101 Ma.Monazite alteration textures in the two granites chiefly reflect differences in their post-magmatic histories. In the massive granite, monazite replacement occurred via a nearly stoichiometrically balanced reaction reflecting interaction with an aqueous fluid enriched in Ca+Al+Si±F during hydrothermal alteration of the granitic assemblage, likely below 500 °C. In the gneissose granite, a small amount of anatectic melt, probably derived from the neighboring metasedimentary rocks, was responsible for pseudomorphic recrystallization of monazite by dissolution-reprecipitation above 600 °C. Regardless of whether monazite underwent replacement or recrystallization, primary monazite domains preserve the age of magmatic crystallization for both plutons (102 ± 2 and 106 ± 5 Ma). Conversely, the age of monazite alteration is not easily resolved. Monazite replacement in the massive granite might be constrained using the Th-U-rich alteration products; with due caution and despite probable radiogenic Pb loss, the oldest date of 58 ± 5 Ma could be ascribed to chloritization during final exhumation of the granite. The spread in apparently concordant 206Pb/238U dates for secondary domains in the gneissose granite is attributed to incomplete isotopic resetting during dissolution-reprecipitation, and the youngest date of 91 ± 2 Ma is considered as the age of monazite recrystallization during a suprasolidus metamorphic event. These results reveal a diachronous, ca. 10 Ma-long high-temperature (HT) history and an overall duration of about 15 Ma for the metamorphic evolution of the western part of the Ryoke belt.

Disequilibrium REE compositions of garnet and zircon in migmatites reflecting different growth timings during single metamorphism (Aoyama area, Ryoke belt, Japan)

Chemical disequilibrium of coexisting garnet and zircon in pelitic migmatites (Aoyama area, Ryoke belt, SW Japan) is shown by microtextural evidence and their heavy rare earth element (HREE) patterns. In zircon, two stages of metamorphic rim growth is observed under cathodoluminescence image, although their SHRIMP UPb zircon ages are similar at ca. 92 Ma. Inner and outer rims of zircon tend to show steep HREE patterns irrespective of the UPb age. The inner rims tend to give higher U content than the outer rims; some rim analyses give various Th/U ratios of 0.02–0.07 compared to the very low (<0.02) values seen in the rest of rim analyses. The higher-Th/U values are ascribed to the mixed analyses between thin prograde domains and thick retrograde overgrowths. Zircon grains with inclusions similar to previously-reported melt inclusions are further enclosed in garnet, supporting the growth of thin zircon domains coexisting with garnet during the prograde metamorphism.Garnet rims are commonly replaced by biotite-plagioclase intergrowths, indicating a back reaction with partial melts. Garnet exhibits decrease in HREE and Y concentrations towards the rim, pointing to its prograde growth. The garnet cores have prograde xenotime inclusions, show steep HREE patterns, and yield growth temperature of ~530–570 °C by a YAG-xenotime thermometer. On the other hand, the garnet rims have no xenotime inclusion and show flat HREE patterns. Rare garnet domains including sillimanite needles also show flat HREE patterns and low Y concentrations, which is interpreted as a product of dehydration melting consuming biotite and sillimanite at near-peak P-T conditions (~800 °C and ~0.5 GPa). One such garnet domain gives nearly-equilibrium REE distribution pattern when paired with the matrix zircon rims.Retrograde xenotime is present in the cracks in garnet and in the biotite-plagioclase intergrowths, suggesting that retrograde breakdown of garnet released HREE and Y to form it. Considering the availability of HREE and Zr and presence of melt inclusions in zircon rims, most part of the zircon rims with positive HREE patterns likely grew during the melt crystallization stage, meaning that the zircon rims and presently-preserved garnet domains did not grow in equilibrium. The above scenario was tested by the array plot analysis and it gave a result consistent with microtextural and traditional REE distribution constraints. Combination of microtextural and the array plot analyses may become a powerful tool to reliably correlate the zircon ages to the P-T evolution of the high-grade metamorphic rocks.

LA-ICP-MS zircon U-Pb age and Hf isotope data from the granitic rocks in the Iwakuni area, Southwest Japan: re-evaluation of emplacement order and the source magma

U-Pb-Hf isotopes of zircons from the granitic intrusive rocks in the Iwakuni area of Southwest Japan are determined by LA-ICP-MS analysis. The Gamano Granodiorite belonging to the Older Ryoke Granites of the Ryoke Belt yielded U-Pb mean age of 92.3 ± 2.6 Ma (MSWD = 1.8). The Namera Granite belonging to the Younger Ryoke Granites of the Ryoke Belt yielded U-Pb mean age of 106.3 ± 1.6 Ma (MSWD = 2.3). The Shimokuhara Granite belonging to the Hiroshima Granites of the San-yo Belt yielded U-Pb mean age of 103.8 ± 1.5 Ma (MSWD = 0.32). A granite porphyry sample taken from about 15 m thick dike discordantly intruded into pelitic schist of the Ryoke metamorphic rocks yielded U-Pb mean age of 92.5 ± 1.6 Ma (MSWD = 0.63). Hf isotope data, obtained from the same dated zircon grains, show initial 176Hf/177Hf isotopic ratios referred to as “eHf(t)” generally ranging between–4.8 and +1.1 except an anomalously low value (–15.0) from an inherited core. The eHf(t) values, calculated with average crustal 176Lu/177Hf ratio of 0.015, correspond to Hf isotope crustal model ages between 1474 to 1087 Ma (Mesoproterozoic). The Gamano Graodiorite has long been regarded to be a syn-tectonic intrusion, but the zircon ages and eHf(t) values are similar to those of the post-tectonic granite porphyry dike. The results of this study together with available U-Pb zircon ages of granitic rocks in the Iwakuni area reported elsewhere reveal that the order of emplacement indicated by the U-Pb zircon ages are much discrepant with the traditional classification, the Older Ryoke, the Younger Ryoke. It is better to refrain from the use of “Older” and “Younger” for the collective names of the granitic intrusive bodies.

山口県東部，柳井地域に産する領家帯蒲野花崗閃緑岩のマグマ過程

山口県東部，柳井地域に産する領家帯蒲野花崗閃緑岩のマグマ過程

Monazite Behaviour and Time-scale of Metamorphic Processes along a Low-pressure/High-temperature Field Gradient (Ryoke Belt, SW Japan)

Low-pressure/high-temperature metamorphic rocks exposed in the western part of the Ryoke belt (Iwakuni–Yanai area, SW Japan) include a section with increasing temperature conditions from ∼425 to 880°C. We use this setting to explore the evolution of monazite grain size, texture and composition, and variations in the whole-rock composition of 11 metapelite, metapsammite or metachert samples collected along the metamorphic field gradient. Monazite grain size increases with rising metamorphic grade, regardless of the whole-rock composition. From low- to high-grade conditions we infer: (1) the initial nucleation of monazite aggregates after allanite (∼425°C); (2) monazite coarsening and coalescence driven by incipient monazite recycling; that is, dissolution of small grains to grow larger ones by Ostwald ripening (500–600°C); (3) a first major recycling stage enhanced by fluid liberation owing to muscovite breakdown (600–630°C); (4) a second recycling stage assisted by an increase in the proportion of anatectic melt owing to biotite breakdown (> 850°C). A succession of four compositional domains is recognized in monazite. We emphasize the usefulness of comparing their Ce/ThMnz, Ce/YMnz and Th/UMnz molar ratios with those derived from whole-rock analyses to constrain the origin of each domain. Domain I, with variable ratios, reflects the progressive transfer of Th ± U from allanite to monazite at low-grade conditions. Domain II, with Ce/ThMnz matching the whole-rock values, indicates growth under rock(decimetre)-scale equilibrium conditions. Domains II and III, with Th/UMnz and Ce/YMnz departing from the whole-rock values, record the competition with zircon (for U) and garnet (for Y) during growth at peak P–T conditions. Domain IV points to Y supply by garnet resorption during retrograde chloritization (< 550°C). In the highest-grade sample, zircon grains included in garnet or cordierite show metamorphic rims with sillimanite and Si-rich inclusions. These rims formed at suprasolidus conditions (650–880°C) and yield 206Pb/238U ages of 103–97 Ma (± 5 Ma), which bracket the timing of high-temperature metamorphism. Monazite dating by electron microprobe and laser ablation inductively coupled plasma mass spectrometry reveals two age groups. For domains I–III, some relatively old 206Pb/238U ages (99–95 ± 3–5 Ma) represent minimum estimates for the timing of prograde to peak metamorphism, whereas the similar oldest 206Pb/238U age for domain IV (93 ± 7 Ma) points to rapid cooling after the temperature peak. A more dominant population of younger Th–U–Pb and 206Pb/238U dates is ascribed to age resetting by heterogeneous annealing of the monazite crystal lattice. The youngest concordant 206Pb/238U age of 86 ± 4 Ma is correlated with the end of intermediate-temperature conditions (< 200–300°C) below which monazite, in metamorphic rocks as well as in the surrounding granitoids, eventually behaved as a closed isotopic system. This contribution is dedicated to the memory of Professor Kazuhiro Suzuki, for his contribution to geochronology and Japanese geology.

Spatiotemporal evolution of magmatic pulses and regional metamorphism during a Cretaceous flare-up event: Constraints from the Ryoke belt (Mikawa area, central Japan)

The spatiotemporal relationship between granitoid intrusions and low-pressure/temperature type regional metamorphism in the Ryoke belt (Mikawa area) is investigated to understand the tectono-thermal evolution of the upper- to middle-crust during a Cretaceous flare-up event at the Eurasian active continental margin. Three plutono-metamorphic stages are recognized; (1) 99–84 Ma: intrusion of granitoids (99–95 Ma pulse) into the upper crust and high-T regional metamorphism reaching sillimanite-grade (97.0 ± 4.4 Ma to 88.5 ± 2.5 Ma) in the middle crust, (2) 81–75 Ma: intrusion of gneissose granitoids (81–75 Ma Ma pulse) into the middle crust at ~19–24 km depth, and (3) 75–69 Ma: voluminous intrusions of massive to weakly-foliated granitoids (75–69 Ma pulse) at ~9–13 km depth and formation of contact metamorphic aureoles. Cooling of the highest-grade metamorphic zone below the wet solidus of granitic rocks is estimated at 88.5 ± 2.5 Ma. At ca. 75 Ma, the upper-middle crustal section underwent northward tilting, resulting in the exhumation of regional metamorphic zones to ~9–13 km depth. Although the highest-grade metamorphic rocks and the 99–95 Ma pulse granitoids preserve similar U–Pb zircon ages, the absence of spatial association suggests that the regional metamorphic zones were mainly produced by a transient thermal anomaly in the mantle and thermal conduction through the crust, supplemented by localized advection due to granitoid intrusions. The successive emplacement of granitoids into shallow, deep and shallow levels of the crust was probably controlled by the combination of change in thermal structure of the crust and tectonics during granitoid intrusions.

Age gap between the intrusion of gneissose granitoids and regional high‐temperature metamorphism in the Ryoke belt (Mikawa area), central Japan

AbstractThe relationships between the intrusion of gneissose granitoids and the attainment of regional high‐T conditions recorded in metamorphic rocks from the Ryoke belt of the Mikawa area, central Japan, are explored. Seven gneissose granitoid samples (tonalite, granodiorite, granite) were collected from three distinct plutonic bodies that are mapped as the so‐called “Older Ryoke granitoids.” Based on bulk‐rock compositions and U–Pb zircon ages obtained by laser ablation inductively coupled plasma mass spectrometry, the analyzed granitoids can be separated into two groups. Gneissose granitoids from the northern part of the area give weighted mean 206Pb/238U ages of 99 ±1 Ma (two samples) and 95 ±1 Ma (one sample), whereas those from the southern part yield 81 ±1 Ma (two samples) and 78–77 ±1 Ma (two samples). Regional comparisons allow correlation of the northern granitoids (99–95 Ma) with the Kiyosaki granodiorite, and mostly with the Kamihara tonalite found to the east. The southern granitoids are tentatively renamed as “78–75 Ma (Hbl)−Bt granite” and “81–75 Ma Hbl−Bt tonalite” (Hbl, hornblende; Bt, biotite). and seem to be broadly coeval members of the same magmatic suite. With respect to available age data, no gneissose granitoid from the Mikawa area shows a U–Pb zircon age which matches that of high‐T metamorphism (ca 87 Ma). The southern gneissose granitoids (81–75 Ma), although they occur in the highest‐grade metamorphic zone, do not seem to represent the heat source which produced the metamorphic field gradient with a low dP/dT slope.

Geology of the Ryoke Belt in Iwakuni-Yanai area, Japan:

Geology of the Ryoke Belt in Iwakuni-Yanai area, Japan:

Garnet versus cummingtonite in the quartz diorites of the Mitsuhashi Granite pluton in the Ryoke Belt, southwest Japan

Garnet versus cummingtonite in the quartz diorites of the Mitsuhashi Granite pluton in the Ryoke Belt, southwest Japan

Discrepancy between U-Pb zircon and monazite dates in granitoids from the Ryoke Belt (Iwakuni-Yanai area, SW Japan)

Discrepancy between U-Pb zircon and monazite dates in granitoids from the Ryoke Belt (Iwakuni-Yanai area, SW Japan)