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Three types of amphibolites were found around a large serpentinite body in the basement of the Ohmachi Seamount in the Izu-Bonin-Mariana arc: epidote-albite amphibolite, epidote-garnet amphibolite, and garnet-zoisite amphibolite. The epidote-albite amphibolite shows a mid-ocean ridge basalt geochemical affinity based on whole-rock chemistry and Nd and Sr isotopes. The epidote-garnet amphibolite originated in an arc tectonic setting and contains relict glaucophane, showing blueschist-facies metamorphism (MD1) prior to amphibolite-facies recrystallization (MD2) that formed barroisite and paragonite. The Rb–Sr garnet-whole-rock isochron (289 ± 58 Ma) could represent the timing of blueschist-facies metamorphism. In contrast, the Rb-Sr barroisite-paragonite isochron of 67 ± 13 Ma is an errorchron or mixed age due to insufficient equilibrium with the whole-rock Sr isotope composition, with no geologic significance. The garnet-zoisite amphibolite formed in a mid-oceanic ridge setting and preserves relicts of eclogite-facies paragenesis (MR2a), including zoisite, kyanite, omphacite, and magnesiohornblende, followed by amphibolite-facies metamorphism (MR2b) represented by actinolite and then by a later MR3 event that resulted in the formation of edenite and pargasite. The Rb–Sr mineral isochron (244 ± 28 Ma) defined by zoisite + magnesiohornblende/actinolite + edenite/pargasite represents the time at which the rock recrystallized in the amphibolite facies (MR2b) or cooled (MR3) through ~ 500 °C (the closure temperature of amphibole), indicating that eclogite-facies metamorphism occurred before ca. 244 Ma. The two Permo-Triassic isochron ages (289–244 Ma) indicate that the high-pressure metamorphism of the basement in the Ohmachi Seamount occurred by ancient subduction rather than along the modern subduction zone. The different protoliths and P–T conditions of amphibolites exposed in the serpentine melange of the Ohmachi Seamount represent a mixture of rocks exhumed to the Earth’s surface from various depths in the subducted complex, which are interpreted as evidence for a fossil subduction channel preserved in a modern intra-oceanic arc.

The Mae Ping shear zone (MPSZ) is one of the major ductile strike-slip systems associated with the Cenozoic extrusion tectonics in Southeast Asia. However, its sinistral shear lacks a robust temporal constraint. This study attempts to acquire the deformation timing by applying 40Ar/39Ar thermochronology on a sheared pegmatitic leucogranite showing sinistral S–C fabrics with a thrust component. The contact of the leucogranite sub-paralleling to the major foliation in host gneiss indicates it could be a pre- to syn-shearing intrusion. Most minerals, including garnet, muscovite, K-feldspar, albite, and quartz, exhibit ductile to brittle deformation. Mineral microstructural analysis suggests a retrograde sinistral shear from > 600 to 250 °C. In situ 40Ar/39Ar dating on muscovite yield ages mainly within 42–38 Ma with calculated closure temperatures of 435–330 °C. Fine-grained muscovite aggregates are slightly older than fish, implying that grain size reduction may not always reset 40Ar/39Ar ages. The K-feldspar 40Ar/39Ar step heating age spectrum with two segments of contiguous steps at 24.5 and 35.4 Ma may reflect the coexistence of high-T porphyroclast and low-T K-rich fine-grain recrystallizing at pressure shadows. The reconstructed cooling path and inferred deformation temperatures constrain a shear duration of 42–30 Ma for the MPSZ. The activation of the MPSZ before 42 Ma could be linked to the Eocene metamorphism within a transpressional regime triggering crustal thickening that may further induce the leucogranitic melt. This study also shows leucogranite can be a nice thermal history recorder for a shear zone regarding its petrogenesis and suitable mineral assemblage for thermochronology.

This study presents new whole-rock geochemical and Sr-Nd isotopic data, together with zircon U-Pb ages and Hf isotope ratios, of granitic rocks and associated dikes from the Okhotsk and Aniva plutons in South Sakhalin. The two plutons emplaced in the Mid-Eocene show markedly contrasting geochemical characteristics. The Okhotsk granites (44–42 Ma) are mildly peraluminous (A/CNK = 1.02–1.08), ferroan and alkali-calcic, and have transitional I- and A-type granite features. This granite suite contains rare inherited zircons. The Aniva granites (40 Ma) are peraluminous (A/CNK = 1.08–1.21), magnesian and calc-alkaline, typical of S-type features. This suite, essentially biotite-cordierite bearing, has abundant inherited zircons dated from 48 to 2446 Ma. Whole-rock Sr-Nd and zircon Hf isotopic data indicate that the genesis of the two plutons involved partial melting of mixed sources with dominant juvenile crustal components (newly accreted accretionary complexes or newly underplated basaltic materials) and magma mixing between underplated mantle-derived magmas and crustal partial melts. The Okhotsk granites have eNd (T) values of +3.1 to +3.7, initial 87Sr/86Sr ratios of 0.7047–0.7048, and zircon eHf (T) values of +11 to +16. The Aniva granites have eNd (T) values of +0.5 to +0.9 and initial 87Sr/86Sr ratios of 0.7052 to 0.7055, and zircon eHf (T) values of +4 to +12. According to Sr-Nd and Nd-Hf isotopic mixing calculations, the proportion of the juvenile component (=mantle-derived materials) is estimated to be >70%, and that of the upper crust component is less than 30% in the sources of the Okhotsk and Aniva plutons. We further argue that, based on Nd and Hf isotopic constraints, the Okhotsk and Aniva granites are correlated to the Mid-Eocene granites in the western and eastern parts, respectively, of the Hidaka terrane in Hokkaido. From the Sikhote-Alin area to the Sakhalin and Hokkaido Islands, the increase in juvenile sources and the decrease in recycled crustal materials are exhibited in the Nd isotopic signatures of the granitoids from early Cretaceous to Miocene. The granitic magmatism in South Sakhalin and Hokkaido may be the most prominent example of juvenile crustal growth among these areas.

Major and trace elements and whole rock Sr–Nd isotope data of Late Permian silicic plutonic and volcanic rocks from the Phan Si Pan–Tu Le region in NW Vietnam were collected in order to establish their petrogenetic relationship with the magmatic rocks of the Song Da zone (NW Vietnam) and the alkaline silicic rocks of the Panxi area of the Emeishan large igneous province (ELIP) in SW China. The granites and rhyolites have geochemical characteristics of anorogenic granites (e.g. high Fe# and high Ga/Al) and are further subdivided based on mineralogy. The Phu Sa Phin and Muong Hum granites contain sodic to sodic–calcic amphiboles and sodic pyroxene, while the Phan Si Pan granite does not. The Phu Sa Phin and Muong Hum granites occasionally show peralkaline to metaluminous compositions, while the Phan Si Pan granites and the Tu Le rhyolites have metaluminous to peraluminous compositions. The chondrite-normalized REE patterns [(La/Yb)N = 3.6–51.3, (Gd/Yb)N = 1.4–3.4] and the primitive mantle-normalized spidergrams (enrichment in high field strength elements) are similar to those of the Song Da silicic rocks and the Panzhihua and Taihe granites in the Panxi area. Rhyolite-MELTS modeling suggest that the silicic magmas were likely generated by fractionation of the Song Da high-Ti basalts. The eNd(t) values range from weakly negative to moderately positive values (−2.2 to +2.2), suggesting the silicic rocks may be assimilated by basement rocks during magma emplacement. Permian magmatic rocks in the Phan Si Pan–Tu Le region are petrologically, geochemically and geochronologically comparable to those of the inner zone of ELIP. Although Permian lithospheric mantle and lower crustal structure of the Phan Si Pan–Tu Le region may be destroyed by a lithospheric removal in response to Cenozoic India–Eurasia collision, our results along with recent geophysical analyses in NW Vietnam suggest that the upper to middle crust of Phan Si Pan–Tu Le region represent a remnant upper to middle crust of inner zone of ELIP.

High seismic velocity layers within the lower crust (i.e. ∼40 km) of the Yangtze Block are interpreted as mafic underplated rocks derived from the Late Permian Emeishan mantle plume. However, the region experienced a previous magmatic event during the Neoproterozoic (∼800 Ma) that produced the Kangdian basalts and associated mafic intrusions and therefore the lower crust seismic velocity layers may represent a mixture of two different magmatic episodes. The identification of inherited Neoproterozoic (i.e. ∼750 to ∼850 Ma) zircons within Emeishan magmatic rocks indicates either assimilation of older material during emplacement or that the rocks could be derived from a mafic Neoproterozoic precursor. Equilibrium partial melt modeling of Neoproterozoic Kangdian basalts can produce compositions similar to Emeishan basalt at a pressure of 1.2 GPa (i.e. ≈40 km depth). It is suggested that the injection of high temperature picritic magmas into the lower crust of the Yangtze Block could be sufficient to induce high degrees of melting of the lower crust and produce magmas that are similar in composition to some Emeishan mafic rocks and/or produce melts that could contaminate ‘pristine’ Emieshan magmas.

In-situ zircon U–Pb and Hf isotope analyses were carried out for alkaline silicic magmatic rocks from the Phan Si Pan-Tu Le region in northern Vietnam to constrain their possible sources and to determine their petrogenetic relationship with the Emeishan Large Igneous Province (ELIP), SW China. Nine granites and nine rhyolites yield zircon 206Pb/238U ages from 262 Ma to 249 Ma, coinciding with the timing of silicic magmatism in the Panxi area of the ELIP. The zircon eHf(t) values (+14 to +3) of these granites and rhyolites suggest a moderately depleted mantle source and overlap with those of peralkaline and metaluminous granites in the Panxi area [i.e. eHf(t) = +14 to +4]. The zircon Hf isotope ratios show that the zircons probably record the original source characteristics whereas whole-rock Nd isotope data indicate an evidence for crustal contamination that may have occurred at lower temperatures during magma emplacement. The synchroneity, coupled with petrological and geochemical similarities, indicate that silicic rocks from the Phan Si Pan-Tu Le region are cogenetic with the Panxi silicic plutonic rocks and that they are likely derived by similar petrogenetic processes (i.e. fractionation of mafic magmas or partial melting of mafic rocks). Therefore, we propose that the Phan Si Pan-Tu Le region represents a displaced portion of the ELIP inner zone.

Nephrite in the Fengtien area of the eastern part of the Central Mountain Range, Taiwan, is associated with antigorite-serpentinite within the Yuli belt, a late Cenozoic subduction–accretionary complex related to the eastward subduction of the South China Sea plate forming the Luzon arc. Diopsidite and clinozoisite rock are two other metasomatic components accompanying nephrite between serpentinites and the greenschist-facies country rock (carbonaceous material-)quartz-mica schist. Detrital zircons were separated from one clinozoisite rock sample, formed through metasomatic replacement after mica-quartz schist at temperatures of 320–420°C or slightly lower, which is lower than the metamorphic temperature conditions of the Yuli belt. Most of the detrital zircons have thin zircon rims less than 15–20 μm wide. These zircon rims, considered as newly formed during metasomatism leading to nephrite/diopsidite/clinozoisite rock formation, were dated by a high lateral resolution secondary ion mass spectrometer (CA...

The island of Hokkaido is a young accretionary terrane, basically built with a Jurassic accretionary complex and Cretaceous arc in the west (= NE Japan arc terrane), a Cretaceous-Paleogene forearc basin and accretionary complex with the Hidaka metamorphic belt in the center, and a Cenozoic island arc with Cretaceous basement in the east (= Chishima or Kuril arc terrane). Though volumetrically small, Paleogene and Neogene granitoids are widespread in central Hokkaido (Hidaka Belt). Granitoids are the most representative component of the continental crust, so in this work we aimed to study the mode of generation and source characteristics of these granitoids in order to assess the crustal composition of Hokkaido and examine the general problem of continental growth. New zircon geochronology on nine granitic and one gabbroic rocks from the Hidaka Belt reveals three distinct magmatic episodes, two in the Eocene at 45–46 Ma (3 granites), and 37.0 ± 0.5 Ma (1 granite), and one in the Miocene at 18 to 19 Ma (5 granites and 1 gabbro). The Miocene episode represents the most important granitic emplacement in Hokkaido. The early Eocene zircon ages of 45 to 46 Ma are identified for the first time for granitoids that occur in the northern part of the Hidaka Belt. The zircon age of 37 Ma for a granite from Shirataki is rather rare in Hokkaido, but similar ages had been reported for a tonalite and a granite from the Hidaka metamorphic belt. Geochemically, all granites are slightly peraluminous but not S-type, and they possess volcanic arc granitoid characteristics. Their REE distribution patterns are typically “granitic,” showing fractionated patterns with LREE enrichment and distinct negative Eu anomaly. The whole-rock isotopic signatures [ISr = 0.7044 to 0.7061; eNd(t) = +1.0 to +4.7; TDM-1 = 400-1000 Ma] reveal their largely juvenile characteristics. This is corroborated by the zircon Hf isotopic compositions [eHf(t) = +8 to +19]. The Eocene granites were most probably generated by melting of subducted accretionary complex in a prolonged period from 46 to 37 Ma in supra-subduction zone; whereas the Miocene granites were also generated by melting of accretionary complex in a back-arc rifting setting. In both cases, the involved accretionary complex was probably dominated by the mantle-derived lithological component with little Paleozoic or older crustal material. Hokkaido provides an excellent example of juvenile crust addition to the continental crust.

We conducted major, trace element, Sr-Nd-Hf isotopic, and U-Pb geochronologic analyses of early Cretaceous Kitakami granitic plutons, northeast Japan. We suggest that these plutons include rocks of adakitic affinity, which indicate partial melting of an eclogitic slab. The Kitakami adakites were mostly derived from juvenile oceanic crustal sources, but include inherited zircons with Archean to Neoproterozoic Hf model ages. Rather than being directly derived from crustal rocks of these ages, zircons may have been detrital in the voluminous trench fill sandstone within the Jurassic northern Kitakami accretionary prism. The exhumation of older basement that served as the detrital zircon source may have been triggered by ridge subduction. Although the plutons show a general southward younging from northern Kitakami (120–130 Ma), southern Kitakami (115–125 Ma), and Abukuma (100–115 Ma), restoration of post-crystallization left-slip faults inverts this spatial pattern and results in a northward-younging pattern. This suggests northward migration of a trench-ridge-transform triple junction that included the Izanagi–Farallon ridge. The Sambagawa eclogite may be genetically related to the Kitakami adakite in time and space if the transform duplexing is restored.

Cordilleran-type batholiths are useful in understanding the duration, cyclicity and tectonic evolution of continental margins. The Dalat zone of southern Vietnam preserves evidence of Late Mesozoic convergent zone magmatism superimposed on Precambrian rocks of the Indochina Block. The Dinhquan, Deoca and Ankroet plutons and their enclaves indicate that the Dalat zone transitioned from an active continental margin producing Cordilleran-type batholiths to highly extended crust producing within-plate plutons. The Deoca and Dinhquan plutons are compositionally similar to Cordilleran I-type granitic rocks and yield mean zircon U/Pb ages between 118 ± 1.4 Ma and 115 ± 1.2 Ma. Their Sr–Nd whole rock isotopes (ISr = 0.7044 to 0.7062; eNd(T) = − 2.4 to + 0.2) and zircon Hf isotopes (eHf(T) = + 8.2 ± 1.2 and + 6.4 ± 0.9) indicate that they were derived by mixing between a mantle component and an enriched component (i.e. GLOSS). The Ankroet pluton is chemically similar to post-orogenic/within-plate granitic rocks and has a zircon U/Pb age of 87 ± 1.6 Ma. Geobarometric calculations indicate that amphibole within the Ankroet pluton crystallized at a depth of ~ 6 kbar which is consistent with the somewhat more depleted Sr–Nd isotope (ISr = 0.7017 to 0.7111; eNd(T) = − 2.8 to + 0.6) and variable eHf(T) compositions suggesting a stronger influence of crustal material in the parental magma. The compositional change of the Dalat zone granitic rocks during the middle to late Cretaceous indicates that the tectonic regime evolved from a continental arc environment to one of post-orogenic extension. The appearance of sporadic post-90 Ma magmatism in the Dalat zone and along the eastern margin of Eurasian indicates that there was no subsequent orogenic event and the region was likely one of highly extended crust that facilitated the opening of the South China Sea during the latter half of the Cenozoic.

To constrain the paleoposition of Indochina within Gondwana during the Early Paleozoic, we performed in-situ U–Pb and Hf isotope analyses on detrital zircons from three river sediment samples in the Truong Son Belt of the Indochina block. The age distributions yield dominant Neoarchean (~ 2.5 Ga), Mesoproterozoic (1.7–1.4 Ga), Grenvillian (~ 0.95 Ga), and Pan-African (0.65–0.5 Ga) age groups and minor Paleo- to Meso-archean zircons. Hf isotope compositions of zircons for each age group exhibit large ranges of eHf (T) , suggesting that the zircon host rocks have diverse sources. The oldest Hf model ages for zircons of Neoarchean, Grenvillian, and Pan-African age group yield ~ 3.7 Ga or older, while those of Mesoproterozoic age group show ~ 3.3 Ga. The remarkable similarity of age distribution and Hf isotope compositions among detrital zircons of Indochina and those of Tethyan Himalaya, western Cathaysia, and Qiangtang suggests that Indochina was located outboard of Qiangtang and south of South China in the Indian margin of Gondwana during the Early Paleozoic. Our results are consistent with the paleontological correlations of east Gondwana margin during the Early Paleozoic.

Zircon separates from one jadeitite sample (JJ) from Tone, Japan and one from Guatemala (GJ) were studied for mineral inclusions, age dating, trace-element determination and Hf isotope analysis. These zircons can be categorized into two types. Type I (igneous) zircons are characterized by the presence of mineral inclusions, among others K-feldspar, which is not present in jadeitite matrix. They also show higher Th/U ratios, larger Ce anomalies and higher 176 Lu/ 177 Hf ratios. Type II (metasomatic/solution-precipitate) zircons contain omphacite/jadeite inclusions and exhibit lower Th/U ratios, smaller Ce anomalies and lower 176 Lu/ 177 Hf ratios. Both types of zircons display high eHf( t ) values, slightly lower than the depleted mantle evolution line. The JJ sample contains both type I and II zircons. SHRIMP and geochemical data indicate that this jadeitite sample was formed through the mechanism of whole-sale metasomatic replacement at ~80 Ma from an igneous protolith of juvenile origin with an age of 136 ± 2 Ma. The GJ sample contains only type II zircons and may have formed through a mechanism of, or close to, vein precipitation at 98 ± 2 Ma. The two samples therefore testify that both mechanisms may have been in operation during jadeitite formation. Based on Hf isotope composition of type I zircons and the back-calculated REE pattern of the presumed protolith, the geochemical characteristics of the protolith of the Tone jadeitite were shown to be similar to those of oceanic plagiogranites derived from partial melting of cumulate gabbros or subduction-zone adakitic granites originated from partial melting of subducted oceanic crust. The latter, however, is a more probable candidate because the former is known to be poor in K 2 O, which, in contrast, is a notable chemical component in Tone jadeitite. On the basis of the available data, it is also suggested that the protolith, the physicochemical conditions and the extent of jadeitization may all play a role in dictating the chemical variations of jadeitites.

Zircons from one jadeitite sample, collected from serpentinite melange north of the Motagua fault, Guatemala, were separated for SHRIMP-RG U–Pb dating and trace element analyses. The sub- to anhedral crystal form, lack of typical magmatic oscillatory zoning, the presence of fluid and albite/quartz/jadeite inclusions, and the low Th/U ratios ( Both the whole-rock jadeitite and its zircons have very low REE contents, ∼ 1 ppm and 0.5–42 ppm, respectively. The whole-rock jadeitite shows a flat and slightly concave REE pattern and a positive Eu anomaly (Eu/Eu* = 1.24). Zircons are enriched in HREE and their REE patterns can be divided into two groups: one with negative Eu anomaly and one with positive Eu anomaly. The latter tends to have smaller positive Ce anomalies. The fluid from which jadeitite formed probably evolved over time, becoming more reducing and more dominated by plagioclase decomposition reactions. Alternatively, trace element compositions of zircons simply demonstrate complicated variations of fluid chemistry during jadeitite formation. A reducing fluid with high pH values capable of mobilizing Al, Na, Zr and Hf is inferred to be the media during jadeite/zircon formation. If jadeitite formation was related to serpentinization fluid, the ultramafics must contain olivine as the major primary phase subjected to serpentinization, yielding reducing fluids with high pH values. Two additional requirements are (1) the presence of feldspar/mica-bearing protolithic blocks survived from rodingitization in peridotites to provide Al and Na, and (2) the presence of fluid channels as well as pulses of in/out-flux fluid to trigger metasomatic reactions. These preconditions in subduction zones may contribute to the rare occurrences of jadeitite in global high-P metamorphic belts.

To constrain the timing of collisional event in the Indochina block, SHRIMP U-Pb dating and REE analyses of zircon were carried out for two paragneiss samples of the Kham Duc Complex, central Vietnam. Both samples contain kyanite, staurolite, and zoisite as relics from an early metamorphic stage (M1), and biotite and sillimanite as major minerals constituting the foliation formed during the late metamorphic stage (M2). The change in mineral assemblages indicates a clockwise P-T path composed of a high- or medium-P + low-T stage (M1) and a subsequent low- P + high-T stage (M2). The U-Pb concordia ages of zircon rims are 447 ± 6 Ma and 452 ± 6 Ma for the two samples, respectively. These results are distinctly different from the available Ar-Ar mineral ages of 254–225 Ma. Following the clockwise P-T path and phase equilibrium analyses of the Complex, we suggest that the zircon rims were formed near peak temperatures during the decompression. The ∼450 Ma zircon rim thus gives the minimum age constraint for a possible crustal thickening event during Early Paleozoic, whereas the reported Permo-Triassic Ar-Ar ages would result from an Indosinian overprint. This Early Paleozoic event is most likely related to a collisional orogeny between the Indochina and South China blocks. Late Neoproterozoic to Neoarchean ages are recorded from detrital zircon cores of the Kham Duc Complex, the Kontum Massif and Truong Son Belt, suggesting that their protoliths might have derived from sediments at the Gondwana margin.

In-situ U-Pb geochronology and Hf isotopic studies of zircons from the Pre-Tertiary basement complex and Eocene rocks of the Central Range of Taiwan were carried out to elucidate the history of detrital zircons older than 2.3 Ga. Zircons from the eastern Backbone Range and Eastern Central Range show Paleoproterozoic to late Neoarchean ages (2.3−≥2.5 Ga), whereas those from the Hsuehshan Range yield Neoarchean to Mesoarchean ages (2.6–3.2Ga). The ɛHf(T) for the Paleoproterozoic to late Neoarchean zircons varies from −7.4 to +5.1, implying both juvenile crustal growth and reworking of old crusts. This dual origin could extend back to Mesoarchean-Paleoarchean (3.0–3.4 Ga) based on Hf isotopic model ages. Consistent but narrower ɛHf(T) values and Hf crustal model ages for zircons from Taiwan compared to Cathaysia suggest a possible common origin for these zircons, although Taiwan has a generally younger crustal evolution history than SE China.

To examine the tectonic history of the Taiwan segment of the eastern margin of South China, six rock samples from the Tailuko belt, the metamorphic basement of Taiwan, were selected for zircon SHRIMP dating. The aim was to identify evidence shedding light on the timing of the change from passive to active tectonics for this part of the continental margin since South China separated from the supercontinent of Rodinia. The results lead to two age groups, 190–200 and 88–90 Ma. These age groups, augmented by the previously published age data, suggest that they could have resulted from two Mesozoic accretion/subduction events. In addition, this mid‐late Mesozoic Tailuko belt might have also been reactivated and structurally complicated by the late Cenozoic collision/accretion of the Luzon arc with the Eurasian continent. Records of older tectonic events, such as those derived from the Japanese Islands, are absent in this metamorphic basement. An important finding of this study is the existence of the 191±10 Ma...

Spongy minerals, especially clinopyroxenes, and fine-grained, often glass-bearing mineral assemblages , commonly referred to as melt pockets, occur in many mantle xenolith suites worldwide, but their origins remain far from being clearly understood. We describe a suite of spongy clinopyroxene- and melt pockets-bearing peridotite xenoliths from the Plio-Quaternary volcanic field in the Al Ghab Depression, a pull-apart basin of the Dead Sea Fault System in northwestern Syria. The melt pockets comprise fine-grained olivines, clinopyroxenes, spinels and feldspars ± amphiboles ± glasses. Petrography and major and trace element mineral chemistry suggest that the xenoliths have experienced at least two stages of metasomatism with the formation of the melt pockets being associated with the latest event. The first metasomatic episode featuring LREE, Na, Th, U enrichment and relative Ti and Zr depletion in the cores of primary clinopyroxenes involved metasomatism by a low-silica, CO2-rich agent. The second metasomatic episode was associated with the development of melt pockets which evolved from decompressional and perhaps metasomatism-induced incongruent melting of clinopyroxene ± spinel. The spongy clinopyroxenes that occur as coronas around clear, primary clinopyroxenes represent a transitional stage of the partially melted crystals. The incongruent melting produced a liquid that evolved within the melt pockets and eventually migrated out to form amphiboles and micas elsewhere in the lithosphere. Albeit with some uncertainty, geothermobarometric estimations reveal significant, systematic differences in the equilibration pressures between the primary minerals (0.8–1.4 GPa), and spongy and melt-pocket minerals (0.7–0.9 GPa), lending good support for the decompressional origins of the spongy clinopyroxenes and melt pockets. It is interpreted that decompression resulted from local transtension associated with the development of the Al Ghab pull-apart basin, a step-over zone of the Dead Sea Fault System, in Plio-Quaternary time.

In order to define the timing of granulite facies metamorphism, sensitive high- resolution ion microprobe (SHRIMP) U-Pb analyses were performed on zircons of three pelitic granulites from the lower metamorphic sequence of the Hidaka Metamorphic Belt, southern central Hokkaido, Japan. Both rounded and prismatic zircons were found in the granulite samples. The rounded zircons had thin (10-20 µm) concentric overgrowth rims on detrital cores, while the prismatic zircons did not have detrital cores. Both the overgrowth rims on the rounded zircons and the entire prismatic zircons were formed under granulite facies metamorphism and consistently yield Latest Oligocene-Early Miocene ages (23.7 ± 0.4 Ma to 17.2 ± 0.5 Ma; 206 Pb/ 238 U ages (n = 31) with low Th/U ratios, mostly

Marked variations of biotite Fe-Mg compositional profiles on approach to interfaces with garnet as a function of biotite crystallographic orientation are found in a natural high-temperature metamorphic rock. For biotite with (001) planes nearly parallel to the interface, Mg increases and Fe decreases steeply toward the interface. If oblique, the profile is less steep, and if nearly perpendicular, the profile is flat. These variations are evidences for anisotropic Fe-Mg intradiffusion within biotite during cooling. For precise application of geothermobarometry and geospeedometry, the crystallographic orientation and compositional gradient in biotite must be considered.

北海道中央部, 穂別地域に分布する中部中新統川端層について堆積相解析を行った.その結果, 川端層はタービダイトと関連粗粒重力流堆積物からなり, 構成岩相の累重様式・水平分布は, 堆積盆東縁に発達した小規模あるいは短命のチャネルやスロープエプロンなどによる斜面基部～堆積盆底環境での形成を示唆する.また, その堆積の後期には, 側方の陸域からの供給が大きく寄与する側方流卓越型のシステムが成立していた可能性があり, 川端層の堆積に際する軸流と側方流の寄与は地域あるいは層準によって大きく異なっていたと推定される.また川端層砂岩には蛇紋岩片等の不安定な岩片や, 片麻岩起源のざくろ石などが含まれる.そのため, 川端層の供給源としては東に隣接する枝幸-日高帯南部地域があげられ, この時期にはすでに枝幸-日高帯南部が陸化していたことが示唆される.このことは従来の古地理の復元や日高帯の上昇時期について再考を促すものである.

The central part of the Hidaka Metamorphic Belt (HMB) is divided into six metamorphic zones : lower-biotite, upper-biotite, garnet, sillimanite-muscovite, garnet-cordierite and orthopyroxene zones. A significant temperature gap with lithological contrast is recognized between the sillimanite-muscovite and garnet-cordierite zones. In the garnet-cordierite and orthopyroxene zones, marked prograde zoning profiles occur in garnets in amphibolites. Geothermometry and diffusion modeling for the zoning profiles of these garnets suggest that the rims were formed under upper amphibolite-facies and granulite-facies conditions, with a brief and rapid increase in temperature at peak metamorphism. The latest pulse of SHRIMP zircon dating of granulite (Early Miocene) in the Hidakahorobetsu River area may explain the granulite-facies metamorphism recorded the garnet rims, which occurred just before the uplift episode indicated by K-Ar ages.

The Ohmachi Seamount in the Izu-Bonin frontal arc is one of the very rare localities where rocks from a deep subduction zone are exposed on the modern sea floor. Submersible and dredge results revealed that the basement serpentinite body is accompanied by small amounts of amphibole schist (six float stones less than 20 cm in diameter were collected) with relics of the blueschist to eclogite facies minerals, and is covered by volcanic and sedimentary sequences of Eocene to Miocene ages. In contrast to the occurrences of well-known serpentine mud volcanoes in the Mariana forearc, the Ohmachi Seamount serpentinite body is a coherent mass composed dominantly of ­massive serpentinite in upper horizons and of schistose serpentinite with amphibole schist in lower horizons. Both types of serpentinites consist mainly of antigorite ± ­olivine, and suffered greenschist to amphibolite grade metamorphism. Geologic structures are truncated by the base of the Paleogene, and the serpentinite body is interpreted as a basement complex representing a set of the hanging-wall wedge mantle (massive serpentinite) and the subduction channel (schistose serpentinite), which trapped pieces from the foot-wall subducted slab. The complex was exhumed probably along with one of the back-arc spreading in the Philippine Sea plate.

球晶形態が異なる3タイプのポリフェニレンスルフィド (PPS) フィルムを室温で延伸し, 応力-歪挙動とモルホロジー変化を調べた. 試料は, 成形条件を変えて以下の3タイプを調製した. タイプI: 球晶サイズが大きく球晶数が少ない. タイプII: 球晶サイズが小さく球晶数が多い. タイプIII: 球晶が未発達で完全な球晶構造を取らない. 球晶が大きく数が少ないほど, 延伸応力は比較的高い結晶化度 (20%付近) まで増大せず, 高い破断伸びを示した. 一方, 球晶構造が不完全な試料では, 結晶化度の増加に比例して延伸応力が増大し, 破断伸びは急激に低下した. 延伸による変形は主に球晶間の非晶領域で進行し, 非晶分子鎖の配向度の増加が見られた. また, 動的粘弾性測定において非晶分子鎖の運動性が各タイプにより異なることが分かった. 以上の結果から, PPSの室温での延伸挙動は結晶化度, 球晶サイズ, 及び非晶鎖の拘束度により著しい影響を受けることを明らかにした.

Mantle xenoliths entrained in the Neogene basalts at central and southern Vietnam provide important constraints on the nature of the lithospheric mantle beneath the eastern Indochina block. The basalts contain mantle xenoliths (garnet lherzolite, spinel lherzolite, harzburgite, and eclogite), cumulate xenoliths (wehrlite, websterite, and pyroxenite) and megacrysts (olivine, Al-rich clinopyroxene, orthopyroxene, anorthoclase, Ti-amphibole, phlogopite, sapphire, and zircon). The mineral compositions of spinel lherzolite xenoliths studied are typical of mantle phases: Fo-rich olivine, En-rich orthopyroxene, Di-rich clinopyroxene and Cr-rich spinel. Concentrations of trace elements for clinopyroxene were determined in situ by laser ablation ICP-MS. Clinopyroxenes exhibit large compositional variations ranging from LREE-depleted (LaN/YbN = 0.5–0.7) to cryptically metasomatized LREE-enriched (LaN/YbN = 6.7–20.3). Slightly LREE-depleted REE patterns with YbN of 10 indicate these peridotites were originated from very low degree of partial melting. However, most REE patterns present much lower HREE concentrations with YbN of 2–6 suggesting these peridotites were products of either high degree of partial melting or the residual from the removal of garnet. Spidergrams of most clinopyroxenes show Ba, Nb–Ta, Hf–Zr and Ti depletion. Therefore, the hydrous silicate melts/fluids of subduction zone origin are likely candidates for the required metasomatic agent. The subcontinental lithospheric mantle beneath central and southern Vietnam was complicated and heterogeneous.

Electron-microscope and electron-microprobe observations of amphibolites from the Omachi Seamount in the Izu-Bonin frontal arc show that they contain micrometer-sized inclusions of omphacite ± kyanite, indicating eclogite facies metamorphism. The garnet-zoisite amphibolite was subjected to metamorphic conditions of 600–700 °C and ∼2 GPa in the early stage of metamorphism. These metabasites were subducted to a depth of ∼70 km beneath the Izu-Bonin arc and were exhumed together with serpentinite schist. Their intraoceanic unroofing was probably induced by backarc extension of the upper plate, whose axes were intersected and cooled by the newly formed Izu-Bonin subduction system. This intraoceanic exhumation associated with suprasubduction extension can be a modern analogue for ancient on-land eclogite terranes accompanied by ophiolites.

In the design and strength analysis of a spot welded structure, the mechanical property of spot weld, and the number, spacing or arrangement, etc. of spot welds in a joint are considered as the primary factors. However, elastic deformation of weld, that is displacement coefficient, which is one of the mechanical properties of spot weld is not sufficiently applied because the characteristics have not been made clear.This report deals with the relationship between the displacement coefficient value and the quality of spot weld, the influence of joint type, etc. in a mild steel spot welded joint. As the result of the investigation, it is found that the displacement coefficient value of mild steel spot weld is smaller than those of aluminum alloy and riveted joint of same size, and there is hyperbolic relationship between the displacement coefficient value and the nugget diameter of spot weld and so on.Moreover, the application of displacement coefficient to spot welded structure design is studied. And, it is shown with some examples that the displacement coefficient is applicable to analysis of torsional rigidity and calculation of load share of spot weld in a spot welded structure.

This paper contains the following two experimental results obtained by “Micro-Tension Tester” recently provided in our labolatory. The size of the specimen was shown in Fig. 3.For the 1st experiment, notch toughness in the vicinity of welded part was investigated. In this study, a simplified method to judge the notch-sensitivity of material, which was suggested by one of the authors previously, was applied. The specimens were cut as shown in Fig. 6. From the test result shown in Fig. 8, it can be said, for both rimmed and killed steels, that (a) the notch toughness become worse as the part is far from the weld metal in the heat-affected zone and (b) far from this part the notch toughness recovers.For the 2nd experiment, the tensile terts of the eleven kinds of killed steels were performed, in each of three directions, i. e. rolling, transverse and thickness directions. Fig. 9 shows the schematic load-elongation curve of each direction. The experimental results were shown in Fig. 11 for all materials. From this figure, we know that tensile properties in the thickness direction are inferior to other two directions, especially in ductility.