Late Cenozoic volcanic activity in the Chugoku area, southwest Japan arc during back-arc basin opening and reinitiation of subduction

Abstract Temporal–spatial variations in Late Cenozoic volcanic activity in the Chugoku area, southwest Japan, have been examined based on 108 newly obtained K–Ar ages. Lava samples were collected from eight Quaternary volcanic provinces (Daisen, Hiruzen, Yokota, Daikonjima, Sambe, Ooe–Takayama, Abu and Oki) and a Tertiary volcanic cluster (Kibi Province) to cover almost all geological units in the province. Including published age data, a total of 442 Cenozoic radiometric ages are now available. Across‐arc volcanic activity in an area approximately 500 km long and 150 km wide can be examined over 26 million years. The period corresponds to syn‐ and post‐back‐arc basin opening stages of the island arc. Volcanic activity began in the central part of the rear‐arc ca 26 Ma. This was followed by arc‐wide expansion at 20 Ma by eruption at two rear‐arc centers located at the eastern and western ends. Expansion to the fore‐arc occurred between 20 and 12 Ma. This Tertiary volcanic arc was maintained until 4 Ma with predominant alkali basalt centers. The foremost‐arc zone activity ceased at 4 Ma, followed by quiescence over the whole arc between 4 and 3 Ma. Volcanic activity resumed at 3 Ma, covering the entire rear‐arc area, and continued until the present to form a Quaternary volcanic arc. Adakitic dacite first occurred at 1.7 Ma in the middle of the arc, and spread out in the center part of the Quaternary volcanic arc. Alkali basalt activities ceased in the area where adakite volcanism occurred. Fore‐arc expansion of the volcanic arc could be related to the upwelling and expansion of the asthenosphere, which caused opening of the Japan Sea. Narrowing of the volcanic zone could have been caused by progressive Philippine Sea Plate subduction. Deeper penetration could have caused melting of the slab and resulted in adakites. Volcanic history in the Late Cenozoic was probably controlled by the history of evolution of the upper mantle structure, coinciding with back‐arc basin opening and subsequent reinitiation of subduction.     

Hydrothermal alteration of green tuff belt, Japan: Implications for the influence of seawater/volcanic rock interaction on the seawater chemistry at a back-arc basin

Abstract Chemical data on hydrothermally altered volcanic rocks from a green tuff belt in Japan indicate that the average rate of Mg removal from seawater due to seawater cycling through back-arc basins in the circum-Pacific region during the early to middle Miocene (25–15 Ma) is estimated to be 2.6±1 × 10¹³ g/year. This is similar to that through present-day mid-ocean ridges (2.4 × 10¹³ g/year). Hydrothermal fluxes of K, Ca and Si are calculated to be 4.2±1.6 × 10¹³ g/year, 4.3±1.7×10¹³ g/year and 1.0±0.4 × 10¹⁴ g/year, respectively. These calculated results indicate that the seawater/volcanic rocks interaction at subduction-related tectonic settings have to be taken into account in considering the geochemical mass balance of seawater over geologic time.     

An unconformity in the early Miocene syn-rifting succession, northern Noto Peninsula, Japan: Evidence for short-term uplifting precedent to the rapid opening of the Japan Sea

Abstract   The present paper describes the newly discovered early Miocene unconformity in the northern Noto Peninsula, on the Japan Sea side, central Japan. The unconformity marks the boundary between an early Miocene non-marine to marine succession and a more extensive, late early to early middle Miocene marine succession, and contains a time gap of an order of 1 million years or less from 18 Ma or earlier to 17 Ma. The early Miocene succession likely represents an early phase of marine transgression and initial slow rifting. The overlying early to early middle Miocene succession records the climax of the opening of the Japan Sea at ca  16 Ma with widespread, rapid subsidence of the Japan Arc. The unconformity between the two transgressive successions may represent a global sealevel fall or, more likely, crustal uplifting because no upward-shallowing or regressive facies remains between the two successions. Early Miocene unconformities that are thought to be correlative with this unconformity in the northern Noto Peninsula occur in places along the Japan Sea coast of Sakhalin and Japan. They are likely to have been produced during rifting in response to upwelling of asthenospheric mantle, although more accurate age constraints are necessary to evaluate this idea.     

Early exhumation of the collisional orogen and concurrent infill of foredeep basins in the Miocene Eurasian–Okhotsk Plate boundary, central Hokkaido, Japan: Inferences from K–Ar dating of granitoid clasts

Abstract   Thick Middle (–Upper) Miocene turbiditic deposits filled very deep and narrow foredeep basins formed in the western margin of the Hidaka collision zone in central Hokkaido. Cobble- to boulder-sized clasts of eight monzogranites and a single granodiorite in the Kawabata Formation in the Yubari Mountains area yielded biotite K–Ar ages of 44.4 ± 1.0 to 45.4 ± 1.0 Ma and 42.8 ± 1.1 Ma, respectively. Major elemental compositions of the clasts all fall in the field of S-type granite on an NK/A (Na₂O + K₂O/Al₂O₃ in molecule) versus A/CNK (Al₂O₃/CaO + Na₂O + K₂O in molecule) diagram, verifying their peraluminous granite character (aluminium saturation index (ASI): 1.12–1.19). These geochronological and petrographical features indicate that the granitoid clasts in the Kawabata Formation correlate with Eocene granitic plutons in the northeastern Hidaka Belt, specifically the Uttsudake (43 Ma) and Monbetsu (42 Ma) plutons. Foredeep basins are flexural depressions developed at the frontal side of thickened thrust wedges. The results presented here suggest that deposition of the Middle Miocene turbidites was coeval with rapid westward up-thrusting and exhumation of the Hidaka Belt. This early mountain building may have occurred in response to thrusting in the Tertiary fold-and-thrust system of central Hokkaido.     

Possible origin of K-rich volcanic rocks from Virunga,East Africa,by metasomatism of continental crustal material: Pb,Nd and Sr isotopic evidence

The isotopic compositions of Sr, Nd and Pb together with the abundances of Rb, Sr, U and Pb have been determined for mafic and felsic potassic alkaline rocks from the young Virunga volcanic field in the western branch of the East African rift system.⁸⁷Sr/⁸⁶Sr varies from 0.7055 to 0.7082 in the mafic rocks and from 0.7073 to 0.7103 in the felsic rocks. The latter all come from one volcano, Sabinyo. Sabinyo rocks have negative ε_Ndvalues ofε_Nd = ?10. Nd and Sr isotopic variations in the basic potassic rocks are correlated and plot between Sabinyo and previously reported [1] compositions (ε_Nd = +2.5;⁸⁷Sr/⁸⁶Sr≈ 0.7047) for Nyiragongo nephelinites. The Pb isotopic compositions for Sabinyo rocks are nearly uniform and average²⁰⁶Pb/²⁰⁴Pb≈ 19.4,²⁰⁷Pb/²⁰⁴Pb= 15.79–15.84,²⁰⁸Pb/²⁰⁴Pb≈ 41.2. The basic potassic rocks have similar²⁰⁶Pb/²⁰⁴Pb values but range in²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb from the Sabinyo values to less radiogenic compositions.Excellent correlations of⁸⁷Sr/⁸⁶Sr with Rb/Sr, 1/Sr and²⁰⁷Pb/²⁰⁶Pb for Sabinyo rocks suggest these to be members of a hybrid magma series. However, the nearly uniform Pb compositions for this series points to radiogenic growth of⁸⁷Sr in the magma source region following an event which homogenized the isotopic compositions but not Rb/Sr. The Rb-Sr age derived from the erupted Sabinyo isochron-mixing line is consistent with the ～500 Myr Pb-Pb age from Nyiragongo [1], which suggests that this event affected all Virunga magma sources. The event can again be traced in the Pb-Pb, Pb-Sr and Nd-Sr isotopic correlations for all Virunga rocks, including Nyiragongo, when allowances are made for radiogenic growth subsequent to this mixing or incomplete homogenization event. Inferred parent/daughter element fractionations point to a metasomatic event during which a mantle fluid invaded two lithospheric reservoirs: a +ε_Nd reservoir sampled by the Nyiragongo nephelinites and suggested to be the subcontinental mantle and a ?ε_Nd reservoir sampled by the mafic and felsic potasssic volcanism. Whether this ?ε_Nd reservoir is the crust, continental crustal material in the mantle or anomalous mantle cannot be decided from the data. The simplest answer, that this reservoir is the continental crust, seems to be at variance with experimental evidence suggesting a subcrustal origin for basic potassic magmas. Partial melting of the ancient metasomatised lithospheric domains and ensuing volcanism seems to be entirely a response to decompression and rising geotherms during rifting and thinning of the lithosphere.     

Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: implications for their origin from suboceanic-type mantle reservoirs

Major- and rare-earth-element (REE) concentrations and UThPb, SmNd, and RbSr isotope systematics are reported for Cenozoic volcanic rocks from northeastern and eastern China. These volcanic rocks, characteristically lacking the calc-alkaline suite of orogenic belts, were emplaced in a rift system which formed in response to the subduction of the western Pacific plate beneath the eastern Asiatic continental margin. The rocks sampled range from basanite and alkali olivine basalt, through olivine tholeiite and quartz tholeiite, to potassic basalts, alkali trachytes, pantellerite, and limburgite. These rock suites represent the volcanic centers of Datong, Hanobar, Kuandian, Changbaishan and Wudalianchi in northeastern China, and Mingxi in the Fujian Province of eastern China.The major-element and REE geochemistry is characteristic of each volcanic suite broadly evolving through cogenetic magmatic processes. Some of the outstanding features of the isotopic correlation arrays are as follows: (1) NdSr shows an anticorrelation within the field of ocean island basalts, extending from the MORB end-member to an enriched, time-averaged high Rb/Sr and Nd/Sr end-member (EM1), (2) SrPb also shows an anticorrelation, similar to that of Hawaiian and walvis Ridge basalts, (3) NdPb shows a positive correlation, and (4) the ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb plot shows linear arrays parallel to the general trend (NHRL) for MORB on both sides of the geochron, although in the ²⁰⁸Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb plot the linear array is significantly displaced above the NHRL in a pattern similar to that of the oceanic island basalts that show the Dupal signatures. In all isotope correlation patterns, the data arrays define two different mantle components—a MORB-like component and an enriched mantle component. The isotopic data presented here clearly demonstrate the existence of Dupal compositions in the sources of the continental volcanic rocks of eastern China. We suggest that the subcontinental mantle beneath eastern China served as the reservoir for the EMI component, and that the MORB component was either introduced by subduction of the Kula-Pacific Ridge beneath the Asiatic plate in the Late Cretaceous, as proposed by Uyeda and Miyashiro, or by upwellings in the subcontinental asthenosphere due to subduction.     

Fission track and U–Pb zircon ages of psammitic rocks from the Harushinai unit,Kamuikotan metamorphic rocks,central Hokkaido,Japan: constraints on metamorphic histories

Accurate pressure–temperature–time (P–T–t) paths of rocks from sedimentation through maximum burial to exhumation are needed to determine the processes and mechanisms that form high‐pressure and low‐temperature type metamorphic rocks. Here, we present a new method combining laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb with fission track (FT) dates for detrital zircons from two psammitic rock samples collected from the Harushinai unit of the Kamuikotan metamorphic rocks. The concordant zircon U–Pb ages for these samples vary markedly, from 1980 to 95 Ma, with the youngest age clusters in both samples yielding Albian‐Cenomanian weighted mean ages of 100.8 ± 1.1 and 99.3 ± 1.0 Ma (2σ uncertainties). The zircon U–Pb ages were not reset by high‐P/T type metamorphism, because there is no indication of overgrowth within the zircons with igneous oscillatory zoning. Therefore, these weighted mean ages are indicative of the maximum age of deposition of protolithic material. By comparison, the zircon FT data yield a pooled age of ca. 90 Ma, which is almost the same as the weighted mean age of the youngest U–Pb age cluster. This indicates that the zircon FT ages were reset at ca. 90 Ma while still at their source, but have not been reset since. This conclusion is supported by recorded temperature conditions of less than about 300 °C (the closure temperature of zircon FTs), as estimated from microstructures in the deformed detrital quartz grains in psammitic rocks, and no shortening of fission track lengths in the zircon. Combining these new data with previously reported white mica K–Ar ages indicates that the Harushinai unit was deposited after ca. 100 Ma, and underwent burial to its maximum depth before being subjected to a localized thermal overprint during exhumation at ca. 58 Ma.     

Low-degree melting of a metasomatized lithospheric mantle for the origin of Cenozoic Yulong monzogranite-porphyry,east Tibet: Geochemical and Sr–Nd–Pb–Hf isotopic constraints

SHRIMP zircon U–Pb dating, mineral chemical, element geochemical and Sr–Nd–Pb–Hf isotopic data have been determined for the Yulong monzogranite-porphyry in the eastern Tibet, China. The Yulong porphyry was emplaced into Triassic strata at about 39 Ma. The rocks are weakly peraluminous and show shoshonitic affinity, i.e., alkalis-rich, high K₂O contents with high K₂O / Na₂O ratios, enrichment in LREE and LILE. They also show some affinities with the adakite, e.g., high SiO₂ and Al₂O₃, and low MgO contents, depleted in Y and Yb, and enrichment in Sr with high Sr / Y and La / Yb ratios, and no Eu anomalies. The Yulong porphyry has radiogenic ⁸⁷Sr / ⁸⁶Sr (0.7063–0.7070) and unradiogenic ¹⁴³Nd / ¹⁴⁴Nd (ε_Nd = − 2.0 to − 3.0) ratios. The Pb isotopic compositions of feldspar phenocrysts separated from the Yulong porphyry show a narrow range of ²⁰⁶Pb / ²⁰⁴Pb ratios (18.71–18.82) and unusually radiogenic ²⁰⁷Pb / ²⁰⁴Pb (15.65–15.67) and ²⁰⁸Pb / ²⁰⁴Pb (38.87–39.00) ratios. In situ Hf isotopic composition of zircons that have been SHRIMP U–Pb dated is characterized by clearly positive initial ε_Hf values, ranging from + 3.1 to + 5.9, most between + 4 and + 5. Phenocryst clinopyroxene geothermometry of the Yulong porphyry indicates that the primary magmas had anomalously high temperature (> 1200 °C). The source depth for the Yulong porphyry is at least 100 km inferred by the metasomatic volatile phase (phlogopite–carbonate) relations. Detailed geochemical and Sr–Nd–Pb–Hf isotopic compositions not only rule out fractional crystallization or assimilation-fractional crystallization processes, but also deny the possibility of partial melting of subducted oceanic crust or basaltic lower crust. Instead, low degree (1–5%) partial melting of a metasomatized lithosphere (phlogopite–garnet clinopyroxenite) is compatible with the data. This example gives a case study that granite can be derived directly by partial melting of an enriched lithospheric mantle, which is important to understand the source and origin of diverse granites.     

Vitrinite reflectance and consolidation characteristics of the post‐middle Miocene Forearc Basin in central and eastern Boso Peninsula,central Japan: Implications for basin subsidence

To understand the sedimentary development of the Boso Forearc Basin, central Japan, since ~ 3 Ma, we investigated paleothermal structure and consolidation trends in the central and eastern parts of the forearc basin through vitrinite reflectance measurements and consolidation tests. Vitrinite reflectance (R_m) was in the range 0.33 % to 0.61 % for the Miura Group in the central part of the forearc basin and 0.34 % to 0.41 % for the Miura and Kazusa Groups in the eastern. These values suggest a roughly uniform vitrinite reflectance for the Miura Group from the central to eastern parts. No significant vitrinite reflectance difference is observed across the ~ 3 Ma Kurotaki Unconformity in the eastern part of the basin. The consolidation yield stress (p_c) was calculated as 27.5 MPa and 32.2 MPa for the Kiyosumi and Amatsu Formations of the Miura Group in the eastern part, respectively. Both the p_c values are consistent quantitatively with represent the trend of the maximum overburden pressure estimated from the thickness and density of overlying sediments, and the difference in p_c is expected by the maximum burial depths of the strata at the sampling localities. Values of p_c in the eastern part of the basin increase with thickness of overlying sediment, showing no break across the Kurotaki Unconformity. Considering the eroded thickness of the Miura Group, the continuous trends in vitrinite reflectance and consolidation between the Miura and Kazusa Groups in the eastern part reflect the greater deposition of the eastern part of the Boso Forearc Basin since ~ 2.3 Ma.     

Rb–Sr and Sm–Nd isotopic studies of mafic igneous rocks from the Ryoke plutono-metamorphic belt in the Setouchi area, Southwest Japan: implications for the genesis and thermal history

Abstract Rb–Sr and Sm–Nd isochron ages were determined for whole rocks and mineral separates of hornblende‐gabbros and related metadiabases and quartz‐diorite from Shodoshima, Awashima and Kajishima islands in the Ryoke plutono‐metamorphic belt of the Setouchi area, Southwest Japan. The Rb–Sr and Sm–Nd whole‐rock‐mineral isochron ages for six samples range from 75 to 110 Ma and 200–220 Ma, respectively. The former ages are comparable with the Rb–Sr whole‐rock isochron ages reported from neighboring Ryoke granitic rocks and are thus due to thermal metamorphism caused by the granitic intrusions. On the contrary, the older ages suggest the time of formation of the gabbroic and related rocks. The initial ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of the gabbroic rocks (0.7070–0.7078 and 0.51217–0.51231 at 210 Ma, respectively) are comparable with those of neighboring late Cretaceous granites and lower crustal granulite xenoliths from Cenozoic andesites in this region. Because the gabbroic rocks are considered to be fragments of the lower crustal materials interlayered in the granulitic lower crust, their isotopic signature has been inherited from an enriched mantle source or, less likely, acquired through interaction with the lower crustal materials. The Sr and Nd isotopic and petrologic evidence leads to a plausible conclusion that the gabbroic rocks have formed as cumulates from hydrous mafic magmas of light rare earth element‐rich (Sm/Nd < 0.233) and enriched isotopic (?_Sr > 0 and ?_Nd < 0) signature, which possibly generated around 220–200 Ma by partial melting of an upper mantle. We further conclude that they are fragments of refractory material from the lower crust caught up as xenoblocks by granitic magmas, the latter having been generated by partial melting of granulitic lower crustal material around 100 Ma.     

Geochemical and Sr–Nd–Pb isotopic investigation of Triassic granitoids and basement rocks in the northern Gyeongsang Basin, Korea: Implications for the young basement in the East Asian continental margin

Abstract We present chemical and Sr–Nd–Pb isotopic compositions of three Triassic (226–241 Ma) calc‐alkaline granitoids (the Yeongdeok granite, Yeonghae diorite and Cheongsong granodiorite) and basement rocks in the northern Gyeongsang basin, south‐eastern Korea. These plutons exhibit typical geochemical characteristics of I‐type granitoids generated in a continental magmatic arc. The Yeongdeok and Yeonghae plutons have similar initial Sr, Nd and Pb isotope ratios (⁸⁷Sr/⁸⁶Sr_initial = 0.7041 ~ 0.7050, ?_Nd(t) = 2.3 ~ 4.0, ²⁰⁶Pb/²⁰⁴Pb_feldspar = 18.22 ~ 18.34), but distinct rare earth element patterns, suggesting that the two plutons formed from partial melting of a similar source material at different depths. The Cheongsong pluton has slightly more enriched Sr–Nd–Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr_initial = 0.7047 ~ 0.7065, ?_Nd(t) = 3.9 ~ 2.8, ²⁰⁶Pb/²⁰⁴Pb_feldspar = 18.24 ~ 18.37) than the other two plutons. The Nd model ages of the basement rocks (1.1 ~ 1.4 Ga) are slightly older than those of the plutons (0.6 ~ 1.0 Ga). The initial Sr and Nd isotopic ratios of the plutons can be modeled by the mixing between the mid‐oceanic ridge basalt‐like depleted mantle component and the crustal component represented by basement rocks, which is also supported by Pb isotope data. The Sr and Nd isotope data from granitoids and basement rocks suggest that the Gyeongsang basin, the Hida belt and the inner zone of south‐western Japan share relatively young basement histories (middle Proterozoic), compared with those (early Proterozoic to Archean) of the Gyeonggi and Yeongnam massifs and the Okcheon belt. The Nd isotope data of basement rocks suggest that the Hida belt might be better correlated with the basement of the Gyeongsang basin than the Gyeonggi massif, the Okcheon belt or the Yeongnam massif, although it may represent an older continental margin of East Asia than the Gyeongsang basin considering its slightly older Nd model ages.     

Shimajiri Group equivalent sedimentary rocks dredged from sea knolls off Kume Island,central Ryukyus: Implications for timing and mode of rifting of the middle Okinawa Trough back-arc basin

The late Cenozoic geohistory of the Ryukyu arc is closely related to the rifting history of the Okinawa Trough. The submarine geology and stratigraphy of areas around Kume Island, which is situated near the eastern rifted margin of the middle Okinawa Trough, provide key constraints to understand the timing and mode of Okinawa Trough rifting. Here we report the lithology of sedimentary rocks dredged along slopes of ~1000-m-deep sea knolls located north and northwest off Kume Island, and their depositional ages determined by calcareous nannofossils and strontium (Sr) isotope analyses. Various types of sedimentary rocks, such as siltstone, very fine-grained sandstone, medium-grained sandstone, fossiliferous coarse-grained sandstone, and tuffaceous sandstone, were recovered at two dredge sites. These sedimentary rocks are lithologically similar to those in the Aka Formation and a part of the Maja Formation of the Shimajiri Group in nearby Kume Island. Calcareous nannofossils and Sr isotope analyses indicate their depositional ages from the early Pliocene to the early Pleistocene, which are generally consistent with those of the Aka Formation. The finding of the dredged rocks similar in lithology and ages to the Aka Formation indicates that marine deltaic area continued toward north and northwest from Kume Island during these periods. The presence of the Shimajiri Group equivalent sedimentary rocks at the dredge sites are likely related to the main rifting of the Okinawa Trough after ca. 2 Ma in the central Ryukyus.     

Sr–Nd isotopic systematics and geochemistry of intermediate plutonic rocks from Ikoma Mountains, Southwest Japan: evidence for a sequence of Mesozoic magmatic activity in the Ryoke Belt

Abstract The Ryoke Belt in the Ikoma Mountains, Nara Prefecture, Japan, is composed mainly of various granitic, intermediate and gabbroic rocks. Igneous activity in this area is divided into two periods, early–middle Jurassic and late Cretaceous, based on isotopic dating. The intermediate plutonic rocks in the Fukihata area are composed of two rock types: Kyuanji quartz diorite and Fukihata tonalite. Rb–Sr whole-rock isochron ages have been determined for both plutonic rocks. Their ages and initial ⁸⁷Sr/⁸⁶Sr ratios are as follows: the Kyuanji quartz diorite has an age of 161.0 ± 17.9 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70727 ± 0.00007, while the Fukihata tonalite has an age of 121.4 ± 24.6 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70753 ± 0.00020. Our chronological results indicate that the Kyuanji quartz diorite belongs to the Jurassic mafic rocks, such as the Ikoma gabbroic mass, while the Fukihata tonalite belongs to the early Cretaceous granitic rocks. Both these intermediate plutonic rocks have different chemical characteristics and were derived from different magmas.     

Geochemistry and Ar/Ar geochronology of Miocene volcanic rocks from the Karaburun Peninsula: Implications for amphibole-bearing lithospheric mantle source,Western Anatolia

New geochemical and ⁴⁰Ar/³⁹Ar age data are presented from the Neogene volcanic units of the Karaburun Peninsula, the westernmost part of Western Anatolia. The volcanic rocks in the region are associated with Neogene lacustrine deposition and are characterized by (1) olivine-bearing basaltic-andesites to shoshonites (Karaburun volcanics), high-K calc-alkaline andesites, dacites and latites (Yaylaköy, Arma?anda? and Kocada? volcanics) of ~ 16–18 Ma, and (2) mildly-alkaline basalts (Ovac?k basalt) and rhyolites (Urla volcanics) of ~ 11–12 Ma. The first group of rocks is enriched in LILE and LREE with respect to the HREE and HFSE on N-MORB-normalised REE and multi-element spider diagrams. They are comparable geochemically with volcanic rocks in the surrounding regions such as Chios Island and other localities in Western Anatolia. The Ovac?k basalt is geochemically similar to the first stage early–middle Miocene volcanic rocks but differs from NW Anatolian late Miocene alkali basalts.     

Geochronological characterization and petrogenesis of granitoids in the Ryoke belt, Southwest Japan Arc: constraints from K–Ar, Rb–Sr and Sm-Nd systematics

Abstract Rb–Sr and K–Ar chronological studies were carried out on granitic and metamorphic rocks in the Ina, Awaji Island and eastern Sanuki districts, Southwest Japan to investigate the timing of intrusion of the granitoids in the Ryoke belt. Intrusions of 'younger' Ryoke granitic magmas took place in the Ina district between 120 Ma and 70 Ma, and cooling began immediately after the emplacement of the youngest granitic bodies. Igneous activity in Awaji Island was initiated at 100 Ma and continued to 75 Ma. Along-arc variations of Rb–Sr whole-rock isochron ages suggest that magmatism began everywhere in the Ryoke and San-yo belts at almost the same time ( ca 120 Ma). The last magmatism took place in the eastern part of both belts. Rb–Sr and K–Ar mineral ages for the granitoids young eastwards. The age data suggest that the Ryoke belt was uplifted just after the termination of igneous activity. Initial Sr and Nd isotopic ratios for the Ryoke granitoids indicate that most were derived from magmas produced in the lower crust and/or upper mantle with uniform Sr and Nd isotopic compositions. Several granitoids, however, exhibit evidence of assimilation of Ryoke metamorphic rocks or older Precambrian crustal rocks beneath the Ryoke belt.     

Distribution,age, and origin of a submarine landslide deposit in the Pleistocene Kiwada Formation,forearc basin fill on the Boso Peninsula,east‐central Japan: Constraints from tephro‐ and biostratigraphy

A mass‐transport deposit named MTD1 (up to 100 m in thickness) is intercalated in the upper Kiwada Formation, a Pleistocene forearc basin fill on the Boso Peninsula, east‐central Japan. The present study aims to examine the origin, age, and distribution of MTD1. MTD1 consists mainly of mudstone blocks containing thin very fine‐ to medium‐grained sandstones, and ranges from tens of centimeters to more than tens of meters in length and thickness. Correlation of marker tuff beds and application of the biostratigraphy of calcareous nannofossils suggest that the blocks in MTD1 were derived from the underlying strata. The total thickness of the stratified blocks from the different stratigraphic horizons exceeds 60 m, implying that MTD1 originated from deeply‐excavated slope failure. The slope failure occurred in a short time interval at ca 1.3 Ma. MTD1 provides an estimate of the height of the escarpment on the basis of the stratigraphic origin of the blocks.     

U–Pb zircon ages of the Nakanogawa Group in the Hidaka Belt,northern Japan: Implications for its provenance and the protolith of the Hidaka metamorphic rocks

Zircon U–Pb ages of two acidic tuff and two turbidite sandstone samples from the Nakanogawa Group, Hidaka Belt, were measured to estimate its depositional age and the development of the Hokkaido Central Belt, northeast Japan. In the northern unit, homogeneous zircons from pelagic acidic tuff from a basal horizon dated to 58–57 Ma, zircons from sandstone from the upper part of the unit dated to 56–54 Ma, and zircons from acidic tuff from the uppermost part dated to 60–56 Ma and 69–63 Ma. Both of the tuff U–Pb ages are significantly older than the youngest radiolarian fossil age (66–48 Ma). Therefore, the maximum depositional age of the turbidite facies in the northern unit is 58 Ma and the younger age limit, estimated from the fossil age, is 48 Ma. In the southern unit, homogeneous zircons from turbidite sandstone dated to 58–57 Ma. Thus the depositional age of this turbidite facies was interpreted to be 66–56 Ma from the fossil age, probably close to 57 Ma. Most of the zircon U–Pb ages from the Nakanogawa Group are younger than 80 Ma, with a major peak at 60 Ma. This result implies that around Hokkaido volcanic activity occurred mainly after 80 Ma. Older zircon ages (120–80 Ma, 180–140 Ma, 340–220 Ma, 1.9 Ga, 2.2 Ga, and 2.7 Ga) give information about the provenance of other rocks in the Hidaka Belt. It is inferred that the Nakanogawa Group comprises protoliths of the upper sequence of the Hidaka Metamorphic Zone, which therefore has the same depositional age as the Nakanogawa Group (66–48 Ma). The depositional ages of the lower sequence of the Hidaka Metamorphic Zone and the Nakanogawa Group are probably the same.     

SHRIMP U–Pb zircon ages of the Hida metamorphic and plutonic rocks,Japan: Implications for late Paleozoic to Mesozoic tectonics around the Korean Peninsula

A new U–Pb zircon geochronological study for the Hida metamorphic and plutonic rocks from the Tateyama area in the Hida Mountains of north central Japan is presented. The U–Pb ages of metamorphic zircon grains with inherited/detrital cores in paragneisses suggest that a metamorphic event took place at around 235–250 Ma; the cores yield ages around 275 Ma, 300 Ma, 330 Ma, 1 850 Ma, and 2 650 Ma. New age data, together with geochronological and geological context of the Hida Belt, indicate that a sedimentary protolith of the paragneisses is younger than 275 Ma and was crystallized at around 235–250 Ma. Detrital ages support a model that the Hida Belt was located in the eastern margin of the North China Craton, which provided zircon grains from Paleoproterozoic to Paleozoic rocks and also from Archean and rare Neoproterozoic rocks. Triassic regional metamorphism possibly reflects collision between the North and South China Cratons.