Shimanto Supergroup Research Articles

Geochemistry of Neogene Nansatsu volcanic rocks in South Kyushu, Japan

ABSTRACT We report the whole-rock major and trace element and Sr – Nd – Pb isotopic compositions of volcanic rocks from the Nansatsu area in South Kyushu. We analysed samples from the Oldest stage (middle Miocene), Older stage (late Miocene), and Middle stage (early Pliocene) Nansatsu volcanic rocks, which are andesite to dacite in composition, with an island-arc trace element signature. The compositional variabilities within the Older and the Middle stage rocks are relatively small compared to those in the Oldest stage and Quaternary volcanic rocks. Isotopic characteristics suggested that the Oldest stage volcanic rocks had magma sources similar to regional ilmenite-series granitoids of later ages (Outer Zone granitoids), which incorporated a substantial amount of graphite-bearing accretionary sediments (Shimanto Supergroup). The sources for the Older and Middle stage magmas were similar to those of primary Quaternary basalts in South Kyushu, indicating that Pliocene and Quaternary volcanic rocks in South Kyushu share a common mantle source, despite the change in the nature of volcanic activity from andesite-dominated to more variable volcanism of basalt to rhyolite.

Mineralogical fingerprinting to characterize spatial distribution of coastal and riverine sediments in southern Japan

Knowledge of the provenance and transport processes of coastal sediments is central to safeguarding against coastal erosion. However, to date coastal sediments have been poorly investigated without considering complex artificial and natural attributes such as coastal structures and watershed geology of potential source rivers. In this study, mineralogical fingerprinting technique was used to track the sources of sediments along a coastline in southwest Japan with reference to sediments of outflowing rivers. The sediment samples were collected from 65 beach sites along the Miyazaki Coast and its surrounding coastal area, and five river sites nearby. To determine the size distribution and geochemical and mineralogical compositions of the sediments, grain size, X-ray fluorescence, and X-ray diffraction (XRD) analyses were performed. Subsequently, cluster analysis was used to classify mineralogically similar groups in terms of XRD peak intensity ratio. The coastal sediment clusters corresponded well to nearby riverine clusters, suggesting that the coastal sediments were primarily transported from these river watersheds, although uncertainty as contributions of oceanic flows might be involved. The concordance ascribes to the major geological classifications in the watersheds of major rivers, as well as smaller rivers flowing in the northern area (e.g., the Shimanto Supergroup and Osuzuyama Acid Rocks). In addition to the major tendency, we also observed potential effects of manmade structures on the coastal sediment prevalence; the mineralogical similarities decreased across jetties in the Hitotsuse River mouth. The vertical mineralogical distribution of the core sediment samples observed on the left and right banks of the Hitotsuse River mouth supported the general trend of uniform cluster patterns in each core sample with horizontally different clusters across the river mouth. Mineralogical analysis targeting both coastal and riverine sediments is effective for understanding the characteristics of sediment distribution, which in turn is useful for efforts devoted to protection against coastal erosion.

Interpretation of gravity data to delineate the geothermal reservoir extent and assess the geothermal resource from low-temperature fluids in the Municipality of Isa, Southern Kyushu, Japan

The municipality of Isa is located in the southern part of Kyushu Island, and it is considered one of the wealthiest places in Japan because of the existence of precious minerals. This study attempts to analyze and interpret the gravity data for delineating the geothermal reservoir boundary and the assessment of geothermal resource. Accordingly, the corresponding authors selected an area of 10 × 10 km and analyzed considering a Bouguer Density of 2.46 g/cm3. The Bouguer Anomaly result shows a high value at the center, which extends 5.61 km2. The 2-D gravity inversion was carried out and the profiles showed the depth of the Shimanto Supergroup. Also, the authors carried out 3-D gravity modeling. The result shows an area which has density values below 2.40 g/cm3 and the zone of NE-SW direction that starts at the Uonogoe area and ends at the Yamada area. Besides, a large anomaly located in the central area that has density values above 2.50 g/cm3 is evident. This anomaly represents the hydrothermal alteration zone that hosts the propylitic altered rocks having precious minerals. Between these evidenced zones, we could infer the location of the probable geothermal reserve which has an area of 4.006 km2. Integrating past research results, using the volumetric method, and a Monte Carlo simulation, we estimated with 90% of confidence the geothermal power potential, which is greater than or equal to 4.27 MWe and 121.9 kWe for the inferred and probable geothermal resource respectively for 20 years.

Characteristics and Behavior of Hydrothermal Fluids for Gold Mineralization at the Hishikari Deposits, Kyushu, Japan

AbstractThe Hishikari gold–silver deposits in southern Kyushu, Japan, are low‐sulfidation vein‐type epithermal deposits. Bulk composition analysis, fluid inclusion microthermometry and quantitative gas composition analysis on the basis of the crush‐fast scan (CFS) method were conducted to elucidate characteristics and behavior of hydrothermal fluids responsible for the gold mineralization of the Hishikari deposits. Quartz vein samples were collected from widely scattered locations of ore veins: Zuisen‐6, ‐3, ‐1 (Main deposit), Keisen‐2 and Shosen‐5 (Sanjin deposit), and Seisen‐8 (Yamada deposit) between −70 and 64 ML (mine meter level). The samples are composed mainly of quartz and adularia, and ore minerals of electrum, pyrargyrite, pyrite and/or stibnite, having an average bulk Ag/Au weight ratio of 0.54. Fluid inclusions in quartz and adularia are two‐phase liquid–vapor, and their trapping temperatures and salinities are ca. 190–220 °C and 1.5–2.7 wt.% NaCl eq., respectively. The salinity is highest with an average 2.4 wt.% NaCl eq. of the fluid inclusions from the deepest level (Zuisen‐6 vein at −70 ML), while the fluid inclusion salinities from other samples are relatively low less than ca. 2.0 wt.% NaCl eq. Parallel‐ and lattice‐bladed quartz occur in a number of veins, suggesting fluid boiling during the vein formation.Gases of fluid inclusions are composed mainly of H2O (93.4–99.8 mol%) and CO2 (0.15–6.2 mol%), N2 (0.007–0.96 mol%), Ar (0.0001–0.0047 mol%), CH4 (0.002–0.048 mol%), H2S (0.0004–0.0028 mol%), H2 (0–0.074 mol%) and He (0.0002–0.0011 mol%). CO2, N2 and Ar contents of the fluid inclusions are elevated in the samples collected from deep levels, while H2O, H2S and He contents are elevated in those from shallow levels. The Ar/He, CO2/CH4 and N2/Ar ratios of fluid inclusion gas indicate a systematic transition from magmatic to meteoric signatures in order from deep to shallow levels. The CO2/N2 ratio and volatiles contents are positively correlated suggesting condensation in relatively high Au‐grade quartz vein samples (19.5–27.7 ppm Au), while negative correlations suggest boiling with volatile contents less than 1% are indicative of low Au‐grade quartz vein samples (0.14 to 0.18 ppm Au). The elevated CO2 contents observed in the Zuisen veins are signatures of the deeply circulating hydrothermal fluid with a magmatic contribution. Due to intermittent boiling of hydrothermal fluid, CO2 and H2S were partitioned into the vapor phase in deeper portions and condensed into the steam‐heated meteoric water in shallower portions. The signatures of both meteoric and magmatic fluids in fluid inclusion gas compositions were detected in the samples collected near the unconformity between Shimanto Supergroup (shale and sandstone) and Hishikari Lower Andesite. This suggests that a mixing of the deeply circulating hydrothermal fluids with the steam‐heated meteoric water took place near the unconformity, and it was one of the mechanisms to have precipitated gold.

Imbricate structure in the Ikenodan Formation of the Shimanto supergroup in the Hokusatsu district, southern Kyushu, Japan.

Imbricate structure in the Ikenodan Formation of the Shimanto supergroup in the Hokusatsu district, southern Kyushu, Japan.

Variation of Arsenic Contents in Pyrite from the Shimanto Supergroup: Framboidal vs. Massive Crystalline Pyrites.

Variation of Arsenic Contents in Pyrite from the Shimanto Supergroup: Framboidal vs. Massive Crystalline Pyrites.

South-Vergent Imbricate Fan in the Ikenodan Formation of the Shimanto Supergroup, Southern Kyushu, Japan

The Noda area in Hokusatsu District, southwest Kyushu is underlain by Imanokoba, Ikenodan and Ogakura Formations of Cretaceous Shimanto Supergroup. The Imanokoba Formation comprises beds of sandstone and mudstone. The beds generally trend NE-SW and dip in a moderate to steep angles to east or west. The Ikenodan Formation comprises beds of sandstone, mudstone, siliceous mudstone and chert and basaltic lavas. The beds generally strike between NE and NW, and dip toward west in moderate to high angles with a westerly way-up. The chert and basaltic lava beds appear in a left-stepping en echelon arrangement. The contact between Imanokoba and Ikenodan Formations is in a N-S trending fault known as Noda Fault. The Ogakura Formation is composed of alternating beds of sandstone and mudstone. The beds generally strike between NE and NNE and dip toward west in moderate to high angles with westerly way-up. The contact relationship between Ikenodan and Ogakura Formations is not distinct. The westward disappearance of basaltic lava marks western limit of Ikenodan Formation. The en echelon array in Ikenodan Formation is interpreted as an imbricate fan with Noda Fault on its bottom as a decollement and is called the Ikenodan imbricate fan. The Ikenodan imbricate fan is considered to have been formed in an accretionary complex developed in a northbound subduction zone. The primary attitude of Ikenodan imbricate fan was probably south-vergent then turned over with its west side down. It was previously reported that strata of Shimanto Supergroup in Hokusatsu area form a regional bending structure called the Hokustasu Bend. The Noda area is situated in a hinge zone of Hokusatsu Bend, although formations in Noda area have not been involved in bending structure. The Ikenodan Formation and Noda Fault presumably extend to north without bending, otherwise bending structure occurs anywhere outside Noda area. In either case, Hokusatsu Bend is not present as interpreted in previous studies.

Sedimentary environments of the Upper Shimanto Supergroup in South Kyushu and the examination of forearc models

Sedimentary environments of the Upper Shimanto Supergroup in South Kyushu and the examination of forearc models

Variations in Chemical Composition of Clay Minerals and Magnetic Susceptibility of Hydrothermally Altered Rocks in the Hishikari Epithermal Gold Deposit, SW Kyushu, Japan

AbstractHydrothermal alteration, involving chiefly chlorite and illite, is extensively distributed within host rocks of the Pleistocene Hishikari Lower Andesites (HLA) and the Cretaceous Shimanto Supergroup (SSG) in the underground mining area of the Hishikari epithermal gold deposit, Kagoshima, Japan. Approximately 60% of the mineable auriferous quartz‐adularia veins in the Honko vein system occur in sedimentary rocks of the SSG, whereas all the veins of the Yamada vein system occur in volcanic rocks of the HLA. Variations in the abundance and chemical composition of hydrothermal minerals and magnetic susceptibility of the hydrothermally altered rocks of the HLA and SSG were analyzed. In volcanic rocks of the HLA, hydrothermal minerals such as quartz, chlorite, adularia, illite, and pyrite replaced primary minerals. The amount of hydrothermal minerals in the volcanic rocks including chlorite, adularia, illite, and pyrite as well as the altered and/or replaced pyroxenes and plagioclase phenocrysts increases toward the veins in the Honko vein system. The vein‐centered variation in mineral assemblage is pronounced within up to 25 m from the veins in the peripheral area of the Honko vein system, whereas it is not as apparent in the Yamada vein system. The hydrothermal minerals in sandstone of the SSG occur mainly as seams less than a few millimeters thick and are sporadically observed in halos along the veins and/or the seams. The alteration halos in sandstone of the SSG are restricted to within 1 m of the veins. In the peripheral area of the Honko vein system, chlorite in volcanic rocks is characterized by increasing in Al in its tetrahedral layer and the Fe/Fe + Mg ratio toward the veins, while illite in volcanic rocks has relatively low K and a restricted range of Fe/Fe + Mg ratios. Temperature estimates derived from chlorite geothermometry rise toward the veins within the volcanic rocks. The magnetic susceptibility of tuff breccia of the HLA varies from 21 to less than 0.01 × 10−3 SI within a span of 40 m from the veins and has significant variation relative to that of andesite (27–0.06 × 10−3 SI). The variation peripheral to the Honko vein system correlates with an increase in the abundance of hematite pseudomorphs after magnetite, the percentage of adularia and chlorite with high Fe/Fe + Mg ratios, and the degree of plagioclase alteration with decreasing distance to the veins. In contrast, sedimentary rocks of the SSG maintain a consistent magnetic susceptibility across the alteration zone, within a narrow range from 0.3 to 0.2 × 10−3 SI. Magnetic susceptibility of volcanic rocks of the HLA, especially tuff breccia, could serve as an effective exploration tool for identifying altered volcanic rocks.

Major and trace element geochemistry and Os isotopic composition of metalliferous umbers from the Late Cretaceous Japanese accretionary complex

Metalliferous umbers and red shales occur as unique products of the Kula‐Pacific ridge‐forearc collision in the Late Cretaceous Shimanto Supergroup, an accretionary complex in Japan. These umbers are closely associated with greenstones of mid‐ocean ridge basalt (MORB) origin and are regarded as hydrothermal metalliferous precipitates related to MOR‐type volcanism. The umbers and red shales were deposited in the trench area where both terrigenous detritus from land and hydrothermal metalliferous particulates from a MOR were supplied simultaneously. Besides a predominance of Fe and Mn, the umbers exhibit remarkable enrichments in P, V, Co, Ni, Zn, Y, Mo, rare earth elements (REEs), and Os relative to continental crustal abundances. The X/Fe (X = Mn, P, V, Co, Ni, Zn, Y, and REEs) ratios and PAAS‐normalized REE patterns of the umbers are very similar to those of modern hydrothermal plume fallout precipitates deposited on flanks of MOR. This indicates that the umbers preserve primary geochemical signatures of hydrothermal metalliferous sediments that scavenged seawater‐derived elements and thus can be used as a proxy for Late Cretaceous seawater. The marine 187Os/188Os ratios reconstructed from the late Maastrichtian umbers range from 0.42 to 0.56 and are very consistent with recent data obtained from the Pacific and Atlantic pelagic carbonates that record an abrupt decline from 0.55 to 0.4 during the period between 67.0 Ma and 65.7 Ma.

Lower Cretaceous radiolarian bedded chert from the Mineoka Belt, Boso Peninsula, Japan

A meter-scale, bedded red radiolarian chert block was found on the Yo-oka Beach at the eastern edge of the Mineoka Belt, Boso Peninsula, central Japan. The block is included in the area composed of fragments of the Mineoka ophiolite and other sedimentary and igneous rocks, including a Paleocene to Miocene pelagic limestone/chert sequence. The radiolarian fauna indicates an assemblage zone of middle to late Albian (Early Cretaceous), partly corresponding to the age of the Tethyan fauna. The tectonic significance of this Cretaceous radiolarian chert block is not clear: it could be from either the Mineoka ophiolite and related oceanic plate stratigraphy or from the Shimanto Supergroup.

紀伊半島四万十累帯白亜系寺杣層の砂岩組成変化とその意義

Both heavy mineral analysis and modal composition analysis were carried out for the same sandstones in the Late Cretaceous Terasoma Formation of the Shimanto Supergroup in the Kii Peninsula, Southwest Japan. Heavy mineral assemblages of the lower part of the Terasoma Formation are rich in zircon and garnet associated with epidote, titanite and allanite. In the upper part, Zircon is also abundant, but only minor amounts of garnet and anatase are contained. The modal composition of the sandstones from the lower Terasoma Formation are situated in the boundary zone between feldspathic and lithic wacke, and rich in K-feldper and granitic rock fragments. The sandstones from upper formation are more dominant in rock fragments, and belong to lithic wacke without exception. The provenances deduced from the heavy mineral composition are in accordance with the those from the modal composition; the provenance of the sandstones in the lower Terasoma Formation was composed chiefly of granitic rocks, whereas in the upper part of the formation felsic volcanic rocks became predominant. Similar transition of sandstone composition in the middle Late Cretaceous is also recognized in the Soto-Izumi Group, which is situated in the north and correlated with the Terasoma Formation in age. Judging from the modal and chemical composition of sandstones from the Cretaceous Shimanto Supergroup in Japan, violent felsic magmatism took place in the provenance areas where granitic rocks were mainly exposed associated with minor andesitic vol-canism.

Petrochemistry of Volcanic Rocks in the Hishikari Mining Area of Southern Japan, with Implications for the Relative Contribution of Lower Crust and Mantle‐derived Basalt

Abstract. This study presents the petrographical, mineralogical, and geochemical characteristics of Late Pliocene‐Pleistocene volcanic rocks distributed in the Hishikari gold mining area of southern Kyushu, Japan, and discusses their origin and evolution. The Hishikari volcanic rocks (HVR), on the basis of age and chemical compositions, are divided into the Kurosonsan (2.4–1.0 Ma) and Shishimano (1.7–0.5 Ma) Groups, which occur in the northern and southern part of the area, respectively. Each group is composed of three andesites and one rhyodacite. HVR are characterized by high concentrations of incompatible elements compared with other volcanic rocks in southern Kyushu, and have low Sr/Nd and high Th/U, Th/Pb, and U/Pb ratios compared with typical subduction‐related arc volcanic rocks.Modal and whole‐rock compositions of the HVR change systematically with the age of the rocks. Mafic mineral and augite/hypersthene ratios of the andesites decrease with decreasing age in the Kurosonsan Group, whereas in the Shishimano Group, these ratios are higher in the youngest andesite. Similarly, major and trace element compositions of the younger andesites in the former group are enriched in felsic components, whereas in the latter group the youngest andesite is more mafic than older andesites. Moreover, the crystallization temperature of phenocryst minerals decreases with younger age in the former group, whereas the opposite trend is seen in the latter group. Another significant feature is that rhyodacite in the Shishimano Group is enriched in felsic minerals and incompatible elements, and exhibits higher crystallization temperatures of phenocryst minerals than the rhyodacite of the Kurosonsan Group.Geochemical attributes of the HVR and other volcanic rocks in southern Kyushu indicate that a lower subcontinental crust, characterized by so‐called EMI‐type Sr‐Nd and DUPAL anomaly‐like Pb isotopic compositions, is distributed beneath the upper to middle crust of the Shimanto Supergroup. The HVR would be more enriched in felsic materials derived from the lower crust by high‐alumina basaltic magma from the mantle than volcanic rocks in other areas of southern Kyushu. The Kurosonsan Group advanced the degree of the lower crust contribution with decreasing age from 51 %, through 61 and 66 % to 77 %. In the Shishimano Group, the younger rhyodacite and andesite are derived from hotter magmas with smaller amounts of lower crust component (58 and 57 %) than the older two andesites (65 % and 68 %). We suggest that the Shishimano rhyodacite, which is considered to be responsible for gold mineralization, was formed by large degree of fractional crystallization of hot basaltic andesite magma with less lower crustal component.

Geochemical contrast between the Sanbagawa psammitic schists (Oboke unit) and the Cretaceous Shimanto sandstones in Shikoku, Southwest Japan and its geologic significance

Abstract A systematic geochemical study of sandstones from the Cretaceous Shimanto Supergroup and psammitic schists from the Oboke unit in Shikoku has been carried out in order to clarify the depositional age of the protoliths of the Oboke psammitic schists. The geochemical data, together with chronological and geologic data, led to the following conclusions. (i) It is inferred that Oboke psammitic schists are metamorphically equivalent to sandstones in the Hiwasa Formation of the Shimanto accretionary complex, deposited in a trench area during the Campanian, in eastern Shikoku. (ii) The protolith attained to maximum metamorphic conditions within 20 million years after the deposition. (iii) The accumulation of a large amount of coarse‐grained clastic sediments in the trench area induced offscraping and underplating of the sediments in the subduction zone, forming the Hiwasa Formation and Oboke unit, respectively.

Hydrogen Isotope Study of Fluid Inclusions in Vein Quartz of the Hishikari Gold Deposits, Japan

Abstract: The origin of mineralizing fluids responsible for the Hishikari vein‐type epithermal Au deposits was studied on the basis of the hydrogen isotopic ratio (δD) of the inclusion fluid from vein quartz and adularia. The origin of hydrothermal fluids was estimated by combination of the present δ values and the oxygen isotopic ratios (δ18O) previously reported by Shikazono and Nagayama (1993).The water in the fluid inclusions was extracted by means of decrepitation of quartz at 500°C. Hydrogen was obtained by reduction of the collected water with Zn shot at 450°C. The δD values were determined by mass spectrometer.The δD values of inclusion fluid obtained from quartz range from –61 to –114%. These are significantly lower than the δD value of the thermal water presently venting from the Hishikari deposits and that of local meteoric water.Hydrogen isotopic fractionation between water and amorphous silica, which might have initially precipitated from the hydrothermal fluids at least partly, is not a probable cause of this isotopic depletion, while some water might have been released from the initial hydrous amorphous silica during recrystallization to quartz observed presently. Thus, a part of ore fluids for the Hishikari deposits is supposed to have been originated from the water having anomalous δD values of lower than –100%.Such D depletion cannot be caused by simple oxygen‐shift of meteoric water or by contribution of magmatic volatiles. The δD values of water released from the shale samples of the Shimanto–Supergroup, a major host to the Hishikari veins range from –132 to –148%. Therefore, the anomalous δD values of inclusion fluids from some vein quartz and adularia suggest that the water released from hydrous minerals of the sedimentary basement rocks by dehydration or the groundwater isotopically exchanged with sedimentary rocks at elevated temperatures during circulation, partly contributed to the hydrothermal fluids responsible for the Hishikari deposits.

四国東部に分布する白亜系四万十累層群の砂岩組成と白亜紀火山活動との関連

四国東部に分布する白亜系四万十累層群の砂岩組成と白亜紀火山活動との関連

種子島南部島間の石英斑岩のＫ‐Ａｒ年代と岩石学的特徴および種子島北部のランプロファイアーのＫ‐Ａｒ年代 西南日本外帯中新世火成活動における位置付け

Tanegashima, an island situated south of the Kyushu island, in the Outer Zone of Southwest Japan, contains small amounts of Miocene igneous rocks which intruded into the Kumage Formation of the Shimanto Supergroup. The quartz-porphyry occurs near Shimama in the southern part of the island, and is peraluminous and rich in K2O, Rb and Sn. The geochemical characteristics indicate that the quartz-porphyry is derived from S-type magma. The lamprophyre is present in the northern part of the island. K-Ar ages of the Shimama quartz-porphyry and the lamprophyre are 15.6±0.8 Ma and 18.2±0.9 Ma, respectively. The ages indicate two episodes of igneous activity on the island. The age of the Shimama quartz-porphyry is similar to the ages of a quartz-porphyry and a small granitoid body in the northeastern part of Yakushima, an island situated 20 km west of Tanegashima, but is slightly older than the main body of the Yakushima granite. The ages and geochemical characteristics of quartz-porphyries from the two islands suggest that they could be derived from a common magmatic event. The magmatic activity is considered to be part of Miocene acidic magmatism of the Outer Zone of Southwest Japan.

紀伊半島四万十累層群の上部白亜系竜神累層に挟まれる酸性凝灰岩のフィッション・トラック年代とその意義

紀伊半島四万十累層群の上部白亜系竜神累層に挟まれる酸性凝灰岩のフィッション・トラック年代とその意義

Ridge collision and in situ greenstones in accretionary complexes: An example from the Late Cretaceous Ryukyu Islands and southwest Japan margin

Abstract Mélange units containing greenstones are common throughout the Cretaceous‐Miocene Shimanto Supergroup in the Ryukyu Is and southwest Japan. Most greenstones in the accretionary complex originated in oceanic spreading ridges and seamounts, and they formed far from the convergent margin. Some mélange‐like units in the supergroup, however, contain greenstones that were extruded upon and intruded into unconsolidated fine‐grained terrigenous clastic sediments. It is inferred that eruption of the in situ greenstones resulted from igneous activity in the trench area. Geochemical signatures indicate that the greenstone protoliths were similar to mafic lavas generated at spreading ridges. Fossil ages of the strata containing in situ greenstones become younger over a distance of 1300 km eastward from Amami‐Oshima (Cenomanian‐Turonian) in the Ryukyu Is to central Japan (Late Maestrichtian‐earliest Paleocene), implying that a site of igneous activity in the trench area migrated eastward along the Ryukyu Is and southwest Japan margin. Plate reconstructions of the northwest Pacific Ocean suggest the presence of the Kula‐Pacific ridge near Late Cretaceous to early Paleogene Japan. In this context, it is suggested that the greenstones formed in response to Kula‐Pacific ridge‐forearc collision.Ancient ridge‐forearc collisions are best recognized by the presence of mid‐ocean ridge basalt (MORB) extruded on sediments inferred to have accumulated in the trench area. Diachronous occurrences of the strata associated with these MORB in an orogenic belt are useful for documenting the ridge collision through time.

九州東部‐四国西部の四万十累層群中の砕屑性カリ長石および白雲母のＫ‐Ａｒ年代

九州東部‐四国西部の四万十累層群中の砕屑性カリ長石および白雲母のＫ‐Ａｒ年代