Japan Sea Opening Research Articles

Geochronological and petrological investigations of Miocene felsic igneous rocks in the Amakusa Islands, southwest Japan: Possible extension of the Setouchi Volcanic Belt

AbstractThe opening of the Japan Sea led to the separation of southwest Japan from the Eurasian continent. Subsequent to this event, a diverse range of igneous activities occurred in southwest Japan. On the back‐arc side of the region, igneous activity commenced at approximately 22 Ma and persisted for an extended period. In the trench‐proximal region of southwest Japan, magmatism initiated around 15.6 Ma, immediately following the cessation of the Japan Sea opening, in correlation with the subduction of the Philippine Sea plate beneath southwest Japan. The Amakusa Islands in western Kyushu host felsic to intermediate igneous rocks with Miocene radiometric ages. There has been a debate regarding the attribution of the igneous rocks in Amakusa Island among the Miocene igneous rocks in southwest Japan. To address this issue, we conducted zircon U–Pb dating and analyzed the major‐ and trace‐element compositions of felsic igneous rocks in the Amakusa Islands to elucidate their characteristics. The obtained U–Pb ages range from 14.5 to 14.8 Ma, suggesting contemporaneity between magmatism in the Amakusa Islands and the Setouchi Volcanic Rocks in the trench‐proximal region of southwest Japan. The major and trace element compositions of the felsic igneous rocks exhibit similarities to the dacites of the Setouchi Volcanic Rocks. These findings support previous suggestions that the magmatism in the Amakusa Islands can be correlated with the Setouchi Volcanic Rocks, based on the discovery of a high‐Mg andesite dike and paleo‐stress analysis utilizing the direction of dikes and sills. Therefore, the Setouchi Volcanic Belt is proposed to extend further west than the previously identified Ohno volcanic rocks in eastern Kyushu. The subduction of the Shikoku Basin of the Philippine Sea plate toward western Kyushu supports the hypothesis that the Kyushu‐Palau Ridge was positioned west of Kyushu at ~15 Ma.

Thermal Evolution of the Lithosphere‐Asthenosphere Boundary Beneath Arc and Its Geodynamic Implications: Depth Variation of Thermal Histories of Mantle Xenoliths From Ichinomegata, Northeast Japan

AbstractQuantitative reconstruction of thermal history recorded in mantle xenoliths is imperative for understanding the temporal change of thermal state and dynamics of the lithosphere‐asthenosphere boundary (LAB). We challenged this problem in the arc settings by examining nine spinel peridotite xenoliths from Ichinomegata maar in the back‐arc side of Northeast Japan Arc. Extensive mineral chemical analyses combined with the derivation depths of the xenoliths revealed a depth‐dependent variation of chemical zoning patterns in olivine and pyroxenes. The depth variation of thermal histories of the Ichinomegata xenoliths was decoded by applying diffusion‐controlled reaction modeling to reproduce the zoning patterns. The decoded thermal events in the order of occurrence are (a) ∼14 million years of cooling causing lithosphere thickening up to ∼55 km depth, (b) subsequent ∼12 thousand years of heating from the underlying asthenosphere resulting in lithosphere thinning up to the depths of ∼40 km, and (c) 1–68 days of heating during xenolith transportation by the host magma. The duration of the lithosphere thickening is consistently explained by the period of the Japan Sea opening. On the other hand, the timescale of the lithosphere thinning is too short to be explained by heat conduction through the ∼15 km thick LAB and requires a more effective heat transportation mechanism such as direct magma injection into the LAB or significant viscosity reduction of the mantle peridotite aided by the pervasive permeable flow of silicate melt.

“Double Door” Opening of the Japan Sea Inferred by Pn Attenuation Tomography

AbstractThe extension of back‐arc basins and formation of marginal seas following the subduction of oceanic lithosphere are usually attributed to the rollback of subducting slabs and distorted mantle convection. However, for the Japan Sea, the largest marginal sea in the northwestern Pacific, its opening is unlikely only resulted from the subduction of the Pacific plate because of the coeval Philippine plate subduction and the arcuate arc volcanic zone. Therefore, the present‐day thermal structure in the uppermost mantle, which can be directly constrained by strong Pn‐wave attenuation, plays a vital role in understanding the Japan Sea opening. Here, we observe two belts of strong Pn attenuation beneath the Japan Sea; their strikes are generally consistent with local Pn anisotropy and the retreat directions of the Pacific and Philippine trenches. Hence, there seem to be two divergent mantle flows in the uppermost mantle, pushing a “double door” for the Japan Sea opening.

Temporal and spatial changes of magmatism related to the Japan Sea opening in the Hokuriku region, central Japan:

Temporal and spatial changes of magmatism related to the Japan Sea opening in the Hokuriku region, central Japan:

Genetic diversification of the Kanehira bitterling Acheilognathus rhombeus inferred from mitochondrial DNA, with comments on the phylogenetic relationship with its sister species Acheilognathus barbatulus.

The Kanehira bitterling, Acheilognathus rhombeus, is a freshwater fish, discontinuously distributed in western Japan and the Korean Peninsula. Unusually among bitterling it is an autumn-spawning species and shows developmental diapause. Consequently, the characterization of its evolutionary history is significant not only in the context of the fish assemblage of East Asia, but also for understanding life-history evolution. This study aimed to investigate the phylogeography of A. rhombeus and its sister species Acheilognathus barbatulus, distributed in China, using a mitochondrial analysis of the ND1 gene from 311 samples collected from 50 localities in Japan and continental Asia. Phylogenetic analysis revealed that A. barbatulus is included in A. rhombeus and genetically closer to Japanese A. rhombeus than to Korean A. rhombeus. Divergence of Korean A. rhombeus and A. barbatulus from Japanese A. rhombeus was estimated to be from the late Pliocene (3.44 Mya) and the early Pleistocene (1.98 Mya), respectively. Each event closely coincided with the time of the Japan Sea opening. Japanese A. rhombeus comprised seven lineages: three in Honshu and four in Kyushu. One lineage in central Kyushu was genetically closer to the Honshu lineages than to other lineages in northern Kyushu. Divergence of Japanese lineages was estimated to be from the early to middle Pleistocene (0.55-0.93 Mya), during a period of geological and paleoclimatic change, including volcanic activity. Population expansion in the late Pleistocene (<0.10Ma) was suggested in many of the lineages, which accords with other freshwater fishes. Biogeographically the ancestral A. rhombeus/A. barbatulus was likely to have repeatedly colonized Japan from the continent through land bridges in the late Pliocene and the early Pleistocene. However, the close genetic relationship between Japanese A. rhombeus and A. barbatulus suggests another possibility, with the second colonization occurring in reverse, from Japan to China. The small genetic distance between them indicates that the colonization occurred later than colonization events of other freshwater fishes, including other bitterling species.

Styles of mesoscale brittle deformations associated with the Miocene folding of the Taishu Group, Tsushima, between the Japan Sea and East China Sea backarc basins

AbstractThe Taishu Group is a folded, Eocene–Lower Miocene, thick sedimentary package exposed widely on Tsushima Island between the Japan Sea and East China Sea. This location makes the strata important to understand tectonics and paleo‐environments in the Far East, but the timing of the folding is controversial. We studied the styles of brittle deformations of the strata. It was found that flextural‐slip folds were dominant. Mesoscale faults were classified into two groups: NE–SW trending reverse faults and NW–SE trending strike‐slip faults. Members of both the groups showed movements largely perpendicular to the fold axes. The latter group consisted of sinistral and dextral faults. Accordingly, we interpreted that they were transfer faults activated during the folding. Consequently, mesoscale faults and flexural‐slip faults evidence the map‐scale plane strain of the Taishu Group in the plane perpendicular to the NE‐trending fold axes. There were few transpressional deformations in the group. This is inconsistent with the transpression hypothesis for explaining the simultaneous folding and Japan Sea opening. Another hypothesis in which the folds in Tsushima are regarded as an onshore part of the Taiwan‐Shinji fold belt is inconsistent with the timing of folding suggested by mining geologists to be consistent with and contemporaneous with this deformation. On the other hand, we found that dolerite dikes and sills were involved in the folding. Therefore, we conclude that the folding began during the late Early Miocene time and climaxed during the ore mineralization at around 15 Ma. We suggest that the folding in Tsushima was the easternmost manifestation of the compressional regime around the Yellow Sea and East China Sea in the Early to early Middle Miocene, and that the compression was brought about by the arrival of the Philippine Sea plate to initiate buoyant subduction under Kyushu.

Petrogenetic implications and geochronology of middle Miocene Tannayama igneous rocks, Goto Islands, Japan Sea southern margin, northwestern Kyushu, Japan

AbstractThe subduction of “hot” Shikoku Basin and the mantle upwelling related to the Japan Sea opening have induced extensive magmatism during the middle Miocene on both the back‐arc and island‐arc sides of southwest Japan. The Goto Islands are located on the back‐arc side of northwestern Kyushu, and middle Miocene granitic rocks and associated volcanic, hypabyssal, and gabbroic rocks are exposed. The igneous rocks at Tannayama on Nakadori‐jima in the Goto Islands consist of gabbronorite, granite, granite porphyry, diorite porphyry, andesite, and rhyolite. We performed detailed geological mapping at a 1:10 000 scale, as well as petrographical and geochemical analyses. We also determined the zircon U–Pb age dating of the igneous rocks from Tannayama together with a granitic rock in Yagatamesaki. The zircon U–Pb ages of the Tannayama igneous rocks show the crystallization ages of 14.7 Ma ± 0.3 Ma (gabbronorite), 15.9 Ma ± 0.5 Ma (granite), 15.4 Ma ± 0.9 Ma (granite porphyry), and 15.1 Ma ± 2.1 Ma (rhyolite). Zircons from the Yagatamesaki granitic rock yield 14.5 Ma ± 0.7 Ma. Considering field relationships, new zircon data indicate that the Tannayama granite formed at ~16–15 Ma, and the gabbronorite, granite porphyry, diorite porphyry, andesite, and subsequently rhyolite formed at 15–14 Ma, which overlaps a plutonic activity of the Yagatamesaki. The geochemical characteristics of the Tannayama igneous rocks are similar to those of the tholeiitic basalts and dacites of Hirado, and the granitic rocks of Tsushima in northwestern Kyushu. This suggests that the Tannayama igneous rocks can be correlated petrogenetically with the igneous rocks in those areas, with all of them generated by the upwelling of hot mantle diapirs during crustal thinning in an extensional environment during the middle Miocene.

Cenozoic lithospheric deformation in Northeast Asia and the rapidly-aging Pacific Plate

Northeast Asia underwent widespread rifting and magmatic events during the Cenozoic. The geodynamic origins of these tectonic events are often linked to Pacific plate subduction beneath Northeast Asia. However, the Japan Sea did not open until the late Oligocene, tens of millions of years after Pacific Plate subduction initiation in the Paleocene. Moreover, it is still not clear why the Baikal Rift Zone extension rate increased significantly after the late Miocene, while the Japan Sea opening ceased at the same time. Geodynamic models suggest these enigmatic events are related to the rapidly-aging Pacific Plate at the trench after Izanagi–Pacific spreading ridge subduction. Subduction of the young Pacific Plate delayed the Japan Sea opening during the Eocene while advection of the old Pacific Plate towards the trench increases seafloor age rapidly, allowing the Japan Sea to open after the early Miocene. The Japan Sea opening promotes fast trench retreat and slab stagnation, with subduction-induced wedge zone convection gradually increasing its extent during this process. The active rifting center associated with wedge zone convection upwelling also shifts inland-ward during slab stagnation, preventing further Japan Sea spreading while promoting the Baikal Rift Zone extension. Our geodynamic model provides a good explanation for the temporal-spatial patterns of the Cenozoic tectonic and magmatic events in Northeast Asia.

Spatial distribution of the apatite fission‐track ages in the Toki granite, central Japan: Exhumation rate of a Cretaceous pluton emplaced in the East Asian continental margin

AbstractThe Cretaceous Toki granitic pluton of the Tono district, central Japan was emplaced in the East Asian continental margin at about 70 Ma. The Toki granite has apatite fission‐track (AFT) ages ranging from 52.1 ±2.8 Ma to 37.1 ±3.6 Ma (number of measurements, n = 33); this indicates the three‐dimensional thermal evolution during the pluton's low‐temperature history (temperature in the AFT partial annealing zone: 60–120 °C). The majority of the Toki granite has a spatial distribution of older ages in the shallower parts and younger ages in the deeper parts, representing that the shallower regions arrived (were exhumed) at the AFT closure depth earlier than the deeper regions. Such a cooling pattern was predominantly constrained by the exhumation of the Toki granitic pluton and was related to the regional denudation of the Tono district. The age–elevation relationships (AERs) of the Toki granite indicate a fast exhumation rate of about 0.16 ±0.04 mm/year between 50 Ma and 40 Ma. The AFT inverse calculation using HeFTy program gives time‐temperature paths (t–T paths), suggesting that the pluton experienced continuous slow cooling without massive reheating since about 40 Ma until the present day. A combination of the AERs and AFT inverse calculations represents the following exhumation history of the Toki granite: (i) the fast exhumation at a rate of 0.16 ±0.04 mm/year between 50 Ma and 40 Ma; (ii) slow exhumation at less than 0.16 ±0.04 mm/year after 40 Ma; and (iii) exposure at the surface prior to 30–20 Ma. The Tono district, which contains the Toki granite, underwent slow denudation at a rate of less than 0.16 ±0.04 mm/year within the East Asian continental margin before the Japan Sea opening at 25–15 Ma and then within the Southwest Japan Arc after the Japan Sea opening, which is in good agreement with representative denudation rates obtained in low‐relief hill and plain fields.

日本海拡大変動と大陸リソスフェアの流動( ユーラシア東縁における白亜紀-中新世テクトニクス-リフティングはいかに準備されたか-)

日本海拡大変動と大陸リソスフェアの流動( ユーラシア東縁における白亜紀-中新世テクトニクス-リフティングはいかに準備されたか-)

A15 日本海拡大時のマグマ活動の広域変化 : 前期〜中期中新世の北海道中央部における玄武岩質火山岩の地球化学的特徴(火山の物質科学(1),日本火山学会2008年秋季大会)

A15 日本海拡大時のマグマ活動の広域変化 : 前期〜中期中新世の北海道中央部における玄武岩質火山岩の地球化学的特徴(火山の物質科学(1),日本火山学会2008年秋季大会)

Peridotite xenoliths from the Shiribeshi Seamount, Japan Sea: insights into mantle processes in a back-arc basin

Orthopyroxene-rich and orthopyroxene-poor peridotite xenoliths were sampled from quaternary basaltic to andesitic lava flows of the Shiribeshi seamount, Japan Sea. These xenoliths were affected by reactions with the host magma during transportation to the surface, which caused partial orthopyroxene dissolution and intergrowth with vermicular spinel. Chromian spinel and clinopyroxene major element compositions in the Shiribeshi peridotite are similar to those in abyssal peridotites. REE modeling indicates that the Opx-rich peridotite experienced decompression partial melting from the garnet to the spinel peridotite stability field. Rare earth element (REE) patterns of clinopyroxene in the Opx-rich peridotite show various degrees of enrichment in light REE, which resulted from melt percolation through the reaction with host magma. Comparison with peridotite xenoliths from two other localities (Seifu and Oshima-Ōshima) in the Japan Sea suggests that the Oshima-Ōshima peridotite record higher degree of partial melting than the Shiribeshi and Seifu peridotites. Oxygen fugacities calculated from chromian spinel in the Japan Sea peridotites are comparable to those of arc peridotites. The high degree of partial melting of the Oshima-Ōshima peridotite was possibly caused by the infiltration of a H2O-bearing flux released from the subducted slab. The Shiribeshi peridotite is interpreted as the residue formed after the extraction of depleted back-arc basalts during a later stage of the Japan Sea opening in the Middle Miocene, whereas the Oshima-Ōshima peridotite is residual after the extraction of enriched basalts during an earlier stage of the opening of the Japan Sea.

Do Miocene igneous dikes in the Tajima area, San'in region, evidence the transition of stress field associated with the cessation of the Japan Sea opening?

Do Miocene igneous dikes in the Tajima area, San'in region, evidence the transition of stress field associated with the cessation of the Japan Sea opening?

Subduction of bending Slab and Japan Sea Opening

Subduction of bending Slab and Japan Sea Opening

Miocene–Pleistocene magmas in the Monbetsu area, Northeast Hokkaido, tap N-MORB-like sources contaminated by slab-derived fluids

Cenozoic intraplate volcanism in northern and northeastern Hokkaido appears to have been closely associated with the Late Oligocene to Mid-Miocene opening of the Japan and Okhotsk Seas (c. 23–12Ma). Between 12 and 2Ma, volcanism appeared in the Monbetsu area of northeastern Hokkaido, whose magmatic products are characterized by relative depletions in Nb and Ta and enrichments in LILE and Pb, suggesting a subduction-related source. Likewise, their N-MORB-like isotopic affinity resembles that of active arcs in eastern Hokkaido. These geochemical features are distinct from those of intraplate Cenozoic magmas in Sikhote-Alin, Sakhalin and within the Japan Sea itself, which appear to show a secular change from high-87Sr/86Sr, -206Pb/204Pb and low-143Nd/144Nd (EM2-rich) mantle sources to relatively low-87Sr/86Sr and -206Pb/204Pb (EM1-rich) types, resembling the progression of magmatic activity during early stages of Japan Sea opening. The EM2 is interpreted to derive from lithospheric mantle while EM1-like components may have been introduced by material delaminated from the Sino-Korea craton, following the Indo-Eurasia collision. In contrast, the Cenozoic Monbetsu volcanics in northern and northeastern Hokkaido appear to lack any such temporal pattern, implying that their respective source regions have not changed since at least c. 14Ma. We suggest, accordingly, that upper mantle beneath Hokkaido may have been isolated after opening of the Japan and Okhotsk Seas, to the west and northwest, the Kurile Basin to the northeast, and continuing west-vergent subduction of the Pacific plate. Given the fact that Cenozoic volcanism in north and northeastern Hokkaido shows apparent subduction-related character, and that the relationship between this activity and opening of the Kurile Basin remains unclear, we tentatively conclude that intraplate mantle melting may reflect a combination of effects, that of slab-derived hydrous fluid superimposed on upwelling asthenosphere.

島根県隠岐諸島島後の中新統久見層および都万層の珪藻化石層序

A biostratigraphic study of fossil diatoms revealed that the most of the Kumi Formation in Dogo, Oki Islands, is correlative with the Crucidenticula kanayae Zone to Thalassiosira yabei Zone, and that the Tsuma Formation with the Rouxia californica Zone to lower Neodenticula kamtschatica Zone, respectively. On the other hand, the lowermost of the Kumi Formation yielded a Lower Miocene freshwater planktonic diatom assemblage. The assemblage mainly consists of various forms of the genus Auracoseira associated with lacustrine Actinocyclus species. It is overlain by the marine Kumi Formation correlated to, or lower than, the Lower Miocene Crucidenticula kanayae Zone (16.6-17.0 Ma) which evidences that the geologic section recorded the first marine water invasion related to the Japan Sea opening in the area. This abrupt change of the diatom assemblage from freshwater to marine planktons suggests the rapid transgression in the area where wide continental shelf did not develop. In addition, this study shows the first discovery of the biostratigraphic index diatom fossil N. kamtschatica from the Tsuma Formation which is the westernmost occurrence from the Japan Sea side.

Repeated magmatism at 34Ma and 23-20Ma producing high magnesian adakitic andesites and transitional basalts on southern Okushiri Island, NE Japan arc

Abstract The southern part of Okushiri Island in the present-day back-arc margin of the NE Japan arc is one of the rare convergent plate boundaries where similar magma types (high-magnesian adakitic andesite (HMAA) and high-TiO 2 basalt (HTB)) have been erupted concurrently at more than one time. Oligocene HMAA can be divided into two types: HMAA-I is characterized by high Sr/Y and low Y, and HMAA-II by relatively low Sr/Y and high Y. HMAA-I is primitive in terms of MgO (8.5 wt.%), Mg# (67), Ni (232 ppm) and Cr (613 ppm) contents, and the most Mg-rich olivine phenocrysts plot within the mantle olivine array in terms of Fo and NiO. The similar Cr versus Ni relations of types I and II HMAA indicate some interaction of slab-derived adakitic melts with mantle peridotite, whereas Ni contents are higher than those of most boninites derived by partial melting of mantle peridotite at a given Cr content. Types I and II HMAA have more enriched Sr and Nd isotopic compositions than N-MORB. The petrography and geochemistry of these rocks, combined with published results on the genesis of high-magnesian andesite (HMA) indicate that types I and II HMAA could be produced by interaction of slab (N-MORB and sediment)-derived adakitic melts with mantle peridotite. The comagmatism of HMAA and HTB is ascribed to the following model. A cool, less hydrous, adakite magma (spherical diapir) would rise from the subducting slab (Pacific Plate) and become more hydrous as a result of its interaction with overlying hydrous peridotite. This hydrated adakitic diapir further ascends and is heated on entering the overlying mantle wedge. Subsequently, the temperature and H 2 O gradients in the ascending adakitic diapir and surrounding mantle peridotite would have been established. The HTB magma segregated from the surrounding mantle peridotite region (high temperature and low H 2 O content) at a depth of 60 km or more, whereas the adakitic diapir (low temperature and high H 2 O content) continued to rise, with its chemical composition modified due to interaction with the surrounding mantle peridotite. Type I HMAA then segregated at about 50 km. The most attractive tectono-magmatic model to account for production of adakitic magma at two different periods in the same cool subduction zone region involves upwelling of depleted hot asthenosphere into the subcontinental lithosphere beneath the back-arc margin of the NE Japan arc, coincident with back-arc rifting which took place at the initiation of the Japan Sea opening. The unusually high temperature conditions established in the mantle wedge due to upwelling of depleted hot asthenosphere caused partial melting of a limited part of the cool oceanic crust subducting beneath the NE Japan arc, resulting in the generation of adakitic magma.

Late Cenozoic magmatic transitions in the central Great Xing'an Range, Northeast China: Geochemical and isotopic constraints on petrogenesis

The Wuchagou and Halaha volcanic fields are located in the central Great Xing'an Range, Northeast China, cover an area of ~1400km2 in China, and continue westward into Mongolia. These regions provide an important opportunity to study temporal changes in magma source regions accompanying the evolution of geodynamic-processes. Late Miocene (10.5–8.0Ma) volcanic rocks from the Wuchagou volcanic field are composed of basaltic andesite and andesite. The characteristics of these rocks include a high concentration of Si (SiO2=55.7–59.2wt.%), a pronounced negative Nb―Ta anomaly, and a slightly LREE enriched pattern with (La/Yb)N ratios varying from 2.1 to 6.9. They have Sr and Nd isotopic compositions indicative of long-term incompatible element enrichment (87Sr/86Sr=0.7053–0.7067, 143Nd/144Nd=0.5118 to 0.5122), with a trend toward the EM1 component. Their isotopic compositions can also be described as Dupal-like, but with less-radiogenic Pb isotopic ratios (206Pb/204Pb=16.99–17.83, 207Pb/204Pb=15.46–15.52, 208Pb/204Pb=37.42–37.91). Quaternary (2.0Ma to Holocene) lavas from the Halaha volcanic field consist of alkali olivine basalt and olivine tholeiite, which show alkalic affinities and are characterised by ocean island basalt-like REE and trace element patterns, typical of an intraplate sodic basalt composition. The low 87Sr/86Sr (0.7034–0.7041), high 143Nd/144Nd (0.5128 to 0.5130), and moderately radiogenic 206Pb/204Pb (18.39–18.55), 207Pb/204Pb (15.54–5.55), and 208Pb/204Pb (38.32–38.47) ratios are similar to those of most late Cenozoic basalts of South China and the Abaga region, Inner Mongolia. The different melt compositions between the volcanic rocks of the two volcanic fields are most likely due to changes in geodynamic processes and magma sources. A possible scenario for the genesis of these volcanic rocks is that Wuchagou andesitic magma formed by high degrees of partial melting of the subcontinental lithospheric mantle with some lower-crust contamination as a result of the thermal reactivation of the continental lithosphere by shallow-level rifting following the cessation of the Japan Sea opening. During the Quaternary, extensive volcanism resumed, with eruptions along deep rifting systems in the Halaha volcanic field, which might have been related to the geodynamic forcing of the continuous piling up and upward thickening of the stagnant subducted Pacific slab beneath Northeast China. Basaltic magmas of this stage were primarily produced by melting of convecting asthenosphere. Using the batch melting of a hypothetical LREE-enriched mantle source in the garnet stability field proposed by Xu et al. (2005), we estimated that the Halaha basalts could be generated from 9 to 14% of partial melting of garnet peridotites.

Reconstruction of back-arc volcanism related to Japan Sea opening in Dewa Mountains in Sakata, Yamagata Prefecture, Northeast Japan

Reconstruction of back-arc volcanism related to Japan Sea opening in Dewa Mountains in Sakata, Yamagata Prefecture, Northeast Japan

Trace element and Sr-Nd isotopic features of ultramafic xenoliths from Kurose, Southwest Japan

Ultramafic xenoliths from Kurose, Southwest Japan, are fragments of the lithospheric mantle beneath the back arc of the active Southwest Japan volcano-tectonic arc. Trace element and Sr-Nd isotopic compositions of the clinopyroxenes in the ultramafic xenoliths suggest that several enrichment processes are responsible for the geochemical variation. These xenoliths would have witnessed the following events: (1) open-system partial melting of the continental lithosphere with an influx of an asthenospheric melt with MORB-like isotopic composition during the formation of the back-arc basin that resulted from the Japan Sea opening, (2) open-system melting under an influx of a fluid/melt derived from a subducted slab, and (3) metasomatism after these partial melting events by an island arc tholeiitic basalt melt in the Neogene period. Moreover, variations in the depletion levels of Zr and Ti in the clinopyroxenes suggest that source depletion preceded the melting and metasomatic events.