Eclogite Rocks Research Articles

Immiscible melt droplets in garnet, as represented by ilmenite–magnetite–spinel spheroids in an eclogite-garnet peridotite association, Blanský les Granulite Massif, Czech Republic

![Figure][1] Interlayered eclogite and symplectitic garnet rock that is interpreted as former garnetite are found in the Gfohl Unit of the Bohemian Massif. They show unusual Fe–Ti-rich compositions, characterized by TiO2 contents up to 2.34 wt%, and Mg# of 59.8 and 51.6, respectively. Equilibration conditions of 1250 °C and 4.0 GPa are calculated for eclogite. The petrogenesis of this rock association can be best explained as high-temperature and ultrahigh-pressure magmatic cumulates. Highly decoupled Sr-Nd isotopic composition with nearly constant radiogenic 87Sr/86Sr values and a slightly negative e Nd value suggests interaction of aqueous fluid most likely derived from a subducting slab and/or from parental magmas. The garnetite contains large (up to 0.5 mm) Fe–Ti-rich spheroids of ilmenite–magnetite–spinel, interpreted as frozen droplets of a melt incorporated in the growing garnet. The interstices between these garnet crystals are filled by ilmenite–magnetite–spinel aggregates, with concave outer surfaces with trapped Fe–Ti-rich melt. These ilmenite–magnetite–spinel spheroids represent possibly the first record of such an oxidized assemblage in mantle rocks, and probably the first description of Fe–Ti-rich melt in eclogite-garnetite mantle rocks. A calculation based on mineral proportions in the spheroids and mineral composition indicates that the immiscible Fe–Ti-rich melt consisted of 28.7 TiO2, 3.7 Al2O3, 0.2 Cr2O3, 27.9 Fe2O3, 37.0 FeO, 0.8 MnO, and 1.7 MgO wt%. Petrology and geochemistry of the garnetite indicates an unusual composition for an upper mantle melt with a high oxygen fugacity and relatively high Fe content. [1]: pending:yes

Water incorporation in omphacite: concentrations and compositional relations in ultrahigh-pressure eclogites from Pohorje, Eastern Alps

Omphacite in ultra-high pressure (UH P ) eclogites from the Pohorje Mountains in Slovenia, south-eastern Alps, has been investigated by electron microprobe (EMP), infra-red (IR) and Mossbauer spectroscopy to determine OH concentrations and related incorporation mechanisms. Results from polarized IR measurements reveal high contents of structurally bound OH, varying from 530 to 870 ppm H 2 O. Characterization of omphacite composition by EMP analysis and Mossbauer spectroscopy shows that all samples contain vacancies at the M2 position, which can be expressed as a Ca-Eskola component (Ca 0.5 □ 0.5 AlSi 2 O 6 ). The amount of the Ca-Eskola component displays a positive correlation with the OH concentration, which confirms results from previous studies. The occurrence of precipitated quartz rods in some samples indicates that primary omphacite contained a larger Ca-Eskola component. Extrapolation of the observed trend-line for the Ca-Eskola and OH contents point to an original OH concentration around 1500 ppm H 2 O for these samples. The high water contents observed in omphacite are considered to be linked to the UH P origin of the eclogite rocks.

Eocene to Oligocene retrogression and recrystallization of the Stak eclogite in northwest Himalaya

Highly retrogressed eclogite is present in the Stak massif located on the northern edge of the Indian continental margin in northern Pakistan. Garnet in foliated samples contains omphacite inclusions (Xjd=0.33–0.40) and quartz inclusions and latter retain Raman spectroscopic evidence for high residual pressures up to 0.52GPa. These garnet grains do not show apparent compositional zoning. By contrast, one sample contains euhedral grains of garnet with quartz inclusions that show residual pressures as low as 0.25GPa. These garnet grains do not contain omphacite inclusions, and show different compositional zoning compared to the omphacite-bearing garnet. The metamorphic condition of this sample was estimated to be 1.0–1.4GPa/650–710°C using residual pressure values of quartz inclusions in garnets and the garnet–clinopyroxene geothermometer. The U–Pb ages of zircon grains range from 158 to 28Ma with a cluster between at ca. 32Ma, which is younger than that of the peak ultrahigh-pressure metamorphic ages of eclogitic massifs in the northwestern Himalaya, e.g. Kaghan and Tso Morari. We suggest that the retrogressed eclogitic rocks in the Stak massif were heated by nearby Nanga Parbat Haramosh massif at ca. 32Ma, subsequent to peak eclogite facies conditions. During this heating, part of the eclogite was largely recrystallized to form euhedral garnet grains. These results suggest that the Stak massif resided at a lower crustal depth while other ultrahigh-pressure massifs were exhumed in western Himalaya.

The structural position and age of eclogite rocks in the area of Gridino village in the Belomorian mobile belt

Data on the localization of eclogitized mafic bodies in steeply and superposed gently dipping ductile shear zones in the area of Gridino village are considered. The process of eclogitization in steeply dipping ductile shear zones, which affect basites of rootless bodies (fragments of tectonic melange) and dikes, was determined by deformations. Such a relation between them is most evident in narrow ductile shear zones that cross Early Proterozoic (age of ca. 2.4 Ga) gabbronorite dikes. Gabbronorites involved in deformations of ductile shear zones experienced eclogitization, reflected in the total recrystallization of magmatic minerals with the formation of equilibrium high-pressure mineral parageneses, ordered metamorphic textures, and linear strain patterns. Synchronous metamorphic transformations in gabbronorites, which avoided plastic deformations and retained massive structures (in wall rocks of shear zones) resulted in the formation of reaction coronas at the boundaries of magmatic mineral grains. Younger Fe gabbro dikes, localized in steeply dipping ductile shear zones, which cross gabbronorite dikes, are totally eclogitized. This determines the temporal lag between the formation of gabbonorite dikes and eclogitization in steeply dipping ductile shear zones and indicates its Svecofennian age. As in steeply dipping ductile shear zones, eclogitized basites constituting rootless bodies in their gently dipping counterparts are characterized by ordered metamorphic textures indicating the deformation nature of eclogitization. Gently dipping ductile shear zones are never crossed by Early Proterozoic basite dikes, although they contain boudins of them, which implies the younger (Svecofennian) age of these zones. The absence of eclogitization features in flat ferruginous basite bodies, which take part in the structure of gently dipping ductile shear zones and demonstrate orientation of metamorphic structures similar to that of gneisses, indicates that bodies of eclogitized basites were involved into these zones after eclogitization. An Archean age of these seems unlikely, since rootless bodies of eclogitized basites are never crossed by Proterozoic basite dikes in the study area. The textural data imply the deformation nature and Svecofennian age of eclogitized basites of the area of Gridino village.

Metamorphic PT path and zircon U–Pb dating of Archean eclogite association in Gridino complex, Belomorian province, Russia

The eclogites from the Gridino eclogite-bearing mélange complex in the Belomorian province, Fennoscandian Shield may represent one of the oldest terrain in the world. The Belomorian province is a superposition of Archean and Palaeoproterozoic collisional orogens, which experienced repeated high-grade metamorphic reworking and intense polyphase tectonic deformation in the early Precambrian. Main purpose of this study is to determine the metamorphic age(s) and evolutionary PT path of eclogite as boudin-like body within the Gridino tectonic mélange. Representative rock materials of well-preserved, plagioclase-free eclogite and associated retrogressed eclogitic rock were sampled from Stolbikha Island in the Gridino area for our research. The eclogite sample mainly comprises omphacite, garnet, amphibole, rare quartz, and some accessory minerals (rutile, magnetite), whereas the retrogressed eclogite sample is made of diopside – Na-rich plagioclase symplectites, amphibole, quartz, and accessory minerals. Thermodynamic modeling points to a clockwise metamorphic PT path, with peak conditions of P=18.5kbar, T=710°C, and a “warm” subduction is inferred thus. LA-ICP-MS zircon U–Pb dating yields diverse metamorphic episodes from the Neoarchean to the Palaeoproterozoic. While metamorphic zircon formed at eclogite-facies condition is identified by studying the trace element distribution and partitioning in and between zircon and co-existing eclogite-facies minerals (omphacite, garnet); consequently, Neoarchean eclogite-facies metamorphic event of ca. 2.70Ga can be distinguished for the Gridino eclogite complex, and a regional-scale thermal event may have occurred in the period of 2.70–2.61Ga.

UHPM eclogite of the Maksyutov complex (Southern Urals)

This study reports the first find of a jadeite-bearing eclogite spatially associated with UHP ultramafic rocks of unit 1 of the Maksyutov complex (near the village of Karayanovo). Two phases of elcogitization were recognized based on mineralogical data, petrographic observations, and isotope geochronology. The first UHP metamorphic stage (533 ± 4.6 Ma, P > 4.4 GPa, T > 700°C) was defined by the assemblage jadeite + grossular-almandine + rutile ± phengite. This assemblage was later transformed into omphacite + grossular-almandine + phengite + albite + clinozoisite + titanite at a retrograde phase of stage I (392–485 ± 2–4 Ma, P > 3.1–3.4 GPa, T > 633–740°C) with decreasing pressure and temperature. The second prograde phase (360 ± 3 Ma, P > 1.1–2.2 GPa, T > 450–550°C) of HP metamorphism was marked by the development of a chlorite rim (almandine-grossular—pyrope-almandine-grossular, diopside, clinozoisite) around the eclogite body. A comparison shows that the studied eclogite and ultramafic rocks followed a common P-T-t path. Jadiet-bearing eclogites and meta-ultramafic rocks represent UHPM fragments of the mantle and crust, which were tectonically emplaced in quartzite schists of unit 1.

The oxygen fugacity at which graphite or diamond forms from carbonate-bearing melts in eclogitic rocks

The oxygen fugacity (fO2) at which carbonate-bearing melts are reduced to either graphite or diamond in synthetic eclogite compositions has been measured in multi-anvil experiments performed at pressures between 3 and 7 GPa and temperatures between 800 and 1,300 °C using iron–iridium and iron–platinum alloys as sliding redox sensors. The determined oxygen fugacities buffered by the coexistence of elemental carbon and carbonate-bearing melt are approximately 1 log unit below thermodynamic calculations for a similar redox buffering equilibrium involving only solid phases. The measured oxygen fugacities normalized to the fayalite–magnetite–quartz oxygen buffer decrease with temperature from ~−0.8 to ~−1.7 log units at 3 GPa, most likely as a result of increasing dilution of the carbonate liquid with silicate. The normalized fO2 values also decrease with pressure and show a similar decrease with temperature at 6 GPa from ~−1.5 log units at 1,100 °C to ~−2.4 log units at 1,300 °C. In contrast to previous arguments, the stability field of the carbonate-bearing melt extends to lower oxygen fugacity in eclogite rocks than in peridotite rocks, which implies a wider range of conditions over which carbon remains mobile in natural eclogites. The raised prevalence of diamonds in eclogites compared to peridotites may, therefore, reflect more effective scavenging of carbon by melts in these rocks. The ferric iron contents of monomineralic layers of clinopyroxene and garnet contained in the same experiments were also measured using Mossbauer spectroscopy. A preliminary model was derived for determining the fO2 of eclogitic rocks from the compositions of garnet and clinopyroxene, including the Fe3+/ΣFe ratio of garnet, using the equilibrium, $$\mathop { 5 {\text{CaFeSi}}_{2} {\text{O}}_{6} }\limits_{\text{cpx}} + \mathop {1/3{\text{Ca}}_{3} {\text{Al}}_{2} {\text{Si}}_{3} {\text{O}}_{12} }\limits_{\text{garnet}} + {\text{O}}_{2} = \mathop { 2 {\text{Ca}}_{3} {\text{Fe}}_{2} {\text{Si}}_{3} {\text{O}}_{12} }\limits_{\text{garnet}} + \mathop {1/3{\text{Fe}}_{3} {\text{Al}}_{2} {\text{Si}}_{3} {\text{O}}_{12} }\limits_{\text{garnet}} + \mathop {4{\text{SiO}}_{2} }\limits_{\text{coesite}}$$ The model, which reproduces the independently determined fO2 of the experimental data to within 0.5 log units, can be used to estimate the fO2 of ultrahigh-pressure metamorphic eclogites and cratonic eclogitic xenoliths. Although there are very few analyses of garnet Fe3+/ΣFe ratios from eclogite samples, the range in fO2 recorded by available eclogitic xenoliths is similar to that reported for peridotitic xenoliths and generally within the graphite/diamond stability field. Estimates for the average bulk Fe3+/ΣFe ratio of modern basaltic oceanic crust, however, are higher than the values for most of these xenoliths, and upon subduction, crustal carbon is likely to remain in the carbonate stability field to depths of at least 250 km. If eclogite xenoliths originated from subducted oceanic crust, then their generally lower fO2 most likely reflects either lower initial basaltic Fe3+/ΣFe ratios, loss of Fe2O3 through partial melting or the initial presence of organic carbon.

White mica K–Ar geochronology of Sanbagawa eclogites from Southwest Japan: implications for deformation-controlled K–Ar closure temperature

White mica (phengite and paragonite) K–Ar ages of eclogite-facies Sanbagawa metamorphic rocks (15 eclogitic rocks and eight associated pelitic schists) from four different localities yielded ages of 84–89 Ma (Seba, central Shikoku), 78–80 Ma (Nishi-Iratsu, central Shikoku), 123 and 136 Ma (Gongen, central Shikoku), and 82–88 Ma (Kotsu/Bizan, eastern Shikoku). With the exception of a quartz-rich kyanite-bearing eclogite from Gongen, white mica ages overlap with the previously known range of phengite K–Ar ages of pelitic schists of the Sanbagawa metamorphic belt and can be distinguished from those of the Shimanto metamorphic belt. The similarity of K–Ar ages between the eclogites and surrounding pelitic schists supports a geological setting wherein the eclogites experienced intense ductile deformation with pelitic schists during exhumation. In contrast, phengite extracted from the Gongen eclogite, which is less overprinted by a ductile shear deformation during exhumation, yielded significantly older ages. Given that the Gongen eclogite is enclosed by the Higashi-Akaishi meta-peridotite body, these K–Ar ages are attributed to excess 40Ar gained during an interaction between the eclogite and host meta-peridotite with mantle-derived noble gas (very high 40Ar/36Ar ratio) at eclogite-facies depth. Fluid exchange between deep-subducted sediments and mantle material might have enhanced the gain of mantle-derived extreme 40Ar in the meta-sediment. Although dynamic recrystallization of white mica can reset the Ar isotope system, limited-argon-depletion due to lesser degrees of ductile shear deformation of the Gongen eclogite might have prevented complete release of the trapped excess argon from phengites. This observation supports a model of deformation-controlled K–Ar closure temperature.

Geochemistry and petrogenesis of the Cretaceous A‐type granites in the Laoshan granitic complex, eastern China

AbstractMajor element and trace element compositions, and Sr, Nd and Pb isotopic compositions for postcollisional granites from the Laoshan granitic complex, in the eastern side of the Triassic suture between the South China and North China tectonic blocks were determined. The granites are alkaline, A‐type and can be further classified as A1 granites. The trace element composition of these granites is transitional between those of oceanic island basalt and enriched mid‐oceanic ridge basalt, with depletions in Ba, Sr, P, and Ti that can be ascribed to mineral fractionation and enrichments in Cs, Rb, Th and U possibly resulted from the involvement of slab fluids. The isotopic signature of Laoshan granites represent a mixture between an enriched mantle type 1 (EMI)‐like end‐member and lower continental crust (LCC). We propose that the magmas that formed the Laoshan A1 granites are a mixture between those derived from the EMI‐like delaminated eclogitic rocks (subsequently enriched by fluids released from Mesozoic Pacific subducted slab) and those derived from the LCC, which consists of granulites or metamorphic residues from the prior generation of I‐type granites in the region. The mixed magmas then experienced a strongly alkali feldspar‐dominated fractionation prior to their emplacements as A‐type granites in the Laoshan granitic complex.

Single-crystal elasticity of diopside to 14 GPa by Brillouin scattering

The single-crystal elastic moduli (Cij) of diopside have been measured up to 14GPa using Brillouin spectroscopy, from which the aggregate compressional and shear velocities, adiabatic bulk modulus, shear modulus, and their pressure derivatives were obtained. A least-squares fit of the velocity–pressure data to third-order finite strain equation yields KS′=4.8(2), G′=1.7(1) with ρ0=3.264(6)g/cm3, Ks=114.6(7)GPa and G=72.7(4)GPa. The current study provides the first high-pressure experimental values for the individual Cij’s of diopside and extends the range of direct measurements on the shear modulus to higher pressure. From the single-crystal moduli, the acoustic anisotropy of diopside is calculated and found to be higher than that of other major mantle minerals such as olivine, orthopyroxene, or garnet. The high anisotropy of diopsidic pyroxene is solely responsible for the observed acoustic anisotropy of eclogitic rocks. In mantle rocks containing both olivine and clinopyroxene, such as lherzolites, the diopside component could either enhance or diminish the bulk acoustic anisotropy, depending on the relationship of preferred orientation of these phases.

Resistivity logs of the Chinese Continental Scientific Drilling main hole: Implication for the crustal electrical structure of Dabie-Sulu Terrane, Central-Eastern China

To understand the electrical structure of the Sulu (苏鲁) terrane, we analyzed the resistivity logs of the Chinese Continental Scientific Drilling main hole (CCSD-MH) and obtained the statistical characteristics of the resistivity log of ultrahigh-pressure (UHP) metamorphic rocks. To check the logs’ quality, we compared the resistivity log data with the core test data. The two datasets show a good coherence. On the whole, the resistivity of the UHP metamorphic rocks from the CCSD-MH is very high (mostly higher than 1 000 Ω·m). From orthogneiss to paragneiss, amphibolite, retrograded eclogite, rutile eclogite, phengite eclogite, and ultramafic rock, the resistivity decreases gradually. The fracture zone, enrichment of metal minerals, and serpentinization of peridotite can probably cause the decrease of resistivity. Comparing the electrical structure of the crust of Dabie (大别) terrane and Sulu terrane, we can conclude that (1) the electrical structure of the crust of Dabie terrane and Sulu terrane shows a very big difference, (2) the high-resistivity zone of the north Sulu orogen extended to a very great depth (60 km), but the high-resistivity zone of Dabie orogen is over the depth of 25 km.

Mid-Eocene high Sr/Y granites in the Northern Himalayan Gneiss Domes: Melting thickened lower continental crust

Within the Himalayan collisional belt, granotoids occur along two sub-parallel belts, the Northern Himalayan Gneiss Domes (NHGD) and the High Himalayan Crystalline Series (HHCS). In the Yardoi area of NHGD, two-mica granite, a new type granite occurs in the core of the Yardoi gneiss dome (YGD), Dala and Quedang from north to south, and extends at least 50km long. These granites have similar mineral composition, elemental and radiogenic isotope geochemistry, and age of formation. SHRIMP zircon U/Pb dating indicates that the Yardoi and the Quedang two-mica granites formed at 42.6±1.1Ma and 42.8±0.6Ma, respectively, similar to the Dala pluton. These two-mica granites have (1) high SiO2 (>68wt.%), Al2O3 (>15wt.%), and A/CNK(>1.0); (2) relatively high Sr and LREE, but low Y(<10ppm) and Yb (<1ppm); (3) high Sr/Y (>40 and up to 250) and La/Yb (>30); (4) very weak or no Eu anomalies; and (5) as compared with those in the Himalayan leucogranites, low initial Sr (87Sr/86Sr(i)<0.7120) and similarly unradiogenic Nd (εNd(i)=−8.9–−15.0) isotopic compositions. These granites have initial Sr and Nd isotope compositions similar to those in the amphibolites but significantly different from those in the metapelite and granitic gneiss. Two-mica granites from the Yardoi area are of peraluminous granite with relatively high Na/K and Sr/Y ratios. Such features are distinct from those in the younger leucogranites along the HHCS as well as in the NHGD, and require melting of source consisting dominantly of amphibolite at thickened crustal conditions. This is also supported by the presence of amphibolites with similar Sr and Nd isotope compositions, and similar ages of metamorphism. Two-mica granites of similar age also occur in the other NHGD gneiss domes and along the HHCS belt, implying that Mid-Eocene melting of thickened crustal materials was widespread and might be a primary factor that led to the formation of high density materials (e.g. eclogitic rocks) beneath the Tethyan Himalaya.

Elasticity of glaucophane, seismic velocities and anisotropy of the subducted oceanic crust

Upon subduction, the oceanic crust transforms to blueschists and eclogites, with seismic properties that gradually become similar to those of the surrounding mantle. In order to evaluate the anisotropy of blueschists and glaucophane-bearing eclogites, the elastic constants of glaucophane single-crystal plates from the Sesia–Lanzo Zone (Aosta Valley, Western Alps) were measured using Brillouin spectroscopy at ambient conditions. The mean P- and S-wave velocities are 7.8 and 4.6 km s −1 respectively, and the anisotropy is high (38.1% (AV P) and 27.3% (AV S)). Glaucophane develops strong LPO, characterized by the [001]-axes concentrated sub-parallel to the lineation, and the {110} poles concentrated sub-perpendicular to the foliation in both blueschist and eclogite rocks. The measured LPO is in good agreement with viscoplastic self-consistent numerical models. Seismic properties of glaucophane-bearing blueschist and eclogite are calculated by combining measured LPO and the single-crystal elastic moduli of glaucophane with the other main mineral constituents of the rock: mostly epidote for blueschist, and omphacite and garnet for eclogite. Blueschists present stronger anisotropies (AV P = 16.1% and AV S = 10.3%) than eclogites (AV P = 2.9% and AV S = 1.7%). The shear-wave splitting and resulting delay times for a 7-km thick layer of eclogite or blueschist are low for the eclogite (< 0.03 s), but significant for blueschist (0.16 s). Application to the subducted oceanic crust yields a decrease of velocity contrast with the surrounding mantle and of anisotropy at depth depending on the temperature of the slab. The details of the velocity variations in subducted oceanic crust are difficult to reconcile with the blueschist–eclogite transition as probed by exhumed metamorphic rocks and may require additional phases such as lawsonite or chemical variations such as oxidation state.

Petrology and thermobarometry of eclogite rocks in the Krasnaya Guba dike field, belomorian mobile belt

A sequence of mineral associations was examined in eclogitized basites of the Krasnaya Guba dike field in the Belomorian Mobile Belt. Two morphological types of eclogite and eclogite-like rocks were recognized: (1) eclogite rocks that developed after ferrogabbro dikes and completely replaced these dikes from contact to contact and (2) eclogite-like rocks that developed after gabbronorites in zones of ductile deformations and shearing. According to data mineral geothermobaromety, both rock types were formed within temperature and pressure ranges corresponding to high-pressure and high-temperature amphibolite facies at T = 700 ± 40°C and P = 10.0 ± 0.5 kbar. The peak metamorphic parameters of the host gneisses are analogous. The decompressional stage, which is unambiguously identified by reaction textures, occurred at 630–660°C and 7.9–8.2 kbar. As the temperature and, first of all, pressure decreased, the SiO2 activity in the fluid systematically varied. The eclogitization of the basites took place locally in relation to fluid fluxes, which were restricted to zones of intense deformations, at variable SiO2 activity. The rocks show evidence of two stages of post-eclogite amphibolization. Older amphibolization 1 was coeval with the late prograde metamorphic stage (T = 650°C, P = 10–11 kbar). Younger amphibolization 2 affected eclogitized basite dikes and unaltered gabbronorites (together with their host gneisses) over large areas. This process coincided with decompression (T = 580°C, P = 7–8 kbar) and was likely accompanied by the exhumation of deep zones of BMB to upper-middle crustal levels.

Intrusion of UHP metamorphic rocks into the upper crust of Kyrgyzian Tien-Shan: P-T path and metamorphic age of the Makbal Complex

Coesite occurs in garnets from quartz schists and pelitic schists in the Makbal Complex of Kyrgyzian Tien-Shan. The thick quartz schists interbedded with pelitic schists are the transitional facies after coesite schists. Quartz-pseudomorph after coesite also appears widely in pelitic schists and quartz schists, but is rare in eclogitic rocks. The growth zoning of garnets and Na-Ca amphiboles in eclogites and the garnet-omphacite geothermometry indicate that the metamorphic P-T paths of eclogites are different from each. Ultra-high (UHP) pressure metamorphic rocks of the Makbal Complex are mainly composed of pelitic and quartz schists. All eclogite lenses are included in the host UHPM rocks, although most of them do not contain UHP metamorphic (UHPM) evidences. The K-Ar ages of phengite in a host pelitic schist and of paragonite in an eclogite are approximately 500 Ma, while those of biotite and phengite in a biotite-bearing mica schist and of winchite in a winchite schist are approximately 769-717 Ma and 881 Ma, respectively. The evidence indicates that this complex is a tectonic melange. These geological relationships suggest that the ascending substances must be pelitic and quartz schists. The UHPM rocks in the ascending material capture many exotic blocks as xenoliths during intrusion into the upper crust.

プロト伊豆‐マリアナ島弧の衝突付加テクトニクス―レビュー―

This paper reviews the geology, petrology, and tectonics of the Proto-Izu-Mariana arc against the Honshu arc, Japan, since 15 Ma, to formulate current topics related to the Pacific-type orogeny. Since the first pioneering work by Sugimura (1972), which placed the plate boundary on-land Japan north of the Izu peninsula collision-accretion tectonics of the Proto-Izu-Mariana arc against the Honshu arc started to be extensively investigated through multi-disciplinary methods. Aoike (1999) proposed a comprehensive scenario of five successive accretions of Proto-Izu to the present, with all bounded by top- and bottom-faults since 15 Ma. Oroclinal bending of the Honshu arc began simultaneously with the collision and indentation of the Proto-Izu arc, as is well documented by the Neogene paleomagnetic declination of sedimentary rocks in the Kanto district (Niitsuma et al., see summary, 1989). The entire Izu-Mariana arc is subducting without any accretion now under Honshu, as is well documented by seismic tomography imaging by Hasegawa et al. (see Hasegawa et al., 2010). Tanzawa Tonalite–Trondhjemite–Granodiolite (TTG) pluton intruded into the already-accreted Tanzawa Group at 4 Ma (Tani et al., 2010), resulting in a contact metamorphic aureole ranging from amphibolite through greenschist to zeolite facies at a depth of ca. 10 km, and tectonically denudated along the Kan-nawa fault to be exposed on the surface by 1.0 Ma. The Proto-Izu-Mariana arc was built on the Pre-Paleogene, presumably Cretaceous MORB crust on which boninitic and fractionated silicic lava flows accumulated since 48 Ma, which is older than the bending of the Hawaii–Emperor seamount chain at 43 Ma. Hence, we need another plate off Proto-Ogasawara arc, called North New Guinea plate (Seno, 1985), and ridge subduction underneath Ogasawara arc to initiate subduction magmatism. Both calk-alkali rock series (CA) and coeval blueschist–eclogite rocks are exposed along the inner wall of the Izu–Mariana trench. This strongly suggests that extensive tectonic erosion occurred, resulting in the coexistence of two rock units that were formed 100-150 km from each other horizontally, i.e., rocks on the volcanic front and rocks on the descending oceanic slab. Petrogenesis of juvenile crust and TTG rocks are being debate with two models: basaltic arc magma fractionation (Kawate and Arima, 1998) and delaminated mafic lower crust (Nakajima and Arima, 1998). In the latter model, a difficulty is derived from density at the Moho depth and small size of delaminated residue. Instead, extensive tectonic erosion that transports not only the upper crust but also the lower mafic crust together with parts of the descending slab could be more plausible than these two models in order to present a thinner mafic lower crust.

A new model for the evolution of diamond-forming fluids: Evidence from microinclusion-bearing diamonds from Kankan, Guinea

We report major, trace and volatile element compositions for seven coated diamonds from Kankan, Guinea. Four diamonds trapped microinclusions with high-Mg carbonatitic high-density fluids (HDFs) and three carry silicic to low-Mg carbonatitic HDFs. Mineral inclusions and nitrogen aggregation in the cores indicate growth at ∼ 5 GPa, 1100–1200 °C. Significant differences in the nitrogen aggregation state (A + B, A and A + C centres in cores, inner coats and rims of three diamonds) indicate growth during multiple events, well separated in time. All diamonds show radial chemical evolution. The HDFs in high-Mg carbonatitic diamonds grow more carbonatitic towards the rim, the silicic one grows more silicic, and the zoned diamonds show contrasting trends in the different parts of the coats. The HDFs are highly enriched in Ba, Th, U and the light REE; enrichment in K, Rb and Cs is somewhat lower. Normalized Sr, Ti and Y concentrations are low relative to the middle and heavy REE. Trace elements in the low-Mg carbonatitic to silicic suite vary systematically with increasing silicic character of the HDFs. The high-Mg HDFs commonly deviate from such arrays. A new model is proposed for the evolution of diamond-forming fluids, suggesting that high-Mg carbonatitic HDFs are formed through interaction of saline HDF with peridotitic rock, while low-Mg silicic to carbonatitic HDFs are the result of penetration of K-rich hydrous fluid into eclogitic rock. Another possible source for the high potassium content of the HDFs is K-bearing phases in the mineral assemblage of the rock. Sinusoidal REE patterns that are common in garnet inclusions in monocrystalline diamonds may form when HDFs with steep REE patterns are introduced into a LREE-depleted harzburgite. Unless garnet and diamond were formed in two separate events, this similarity suggests growth of monocrystalline diamonds from fluids similar to the HDFs.

Natural and experimental constraints on formation of the continental crust based on niobium–tantalum fractionation

Fractionation between Nb and Ta, elements generally regarded as geochemical ‘identical twins’, is a key to deciphering the formation of the continental crust (CC). Here we show that Nb/Ta of rutile grains in eclogitic rocks from the Chinese Continental Scientific Drilling (CCSD) project are remarkably heterogeneous but overall subchondritic at core depths of 100–700 m, and are less variable and mainly suprachondritic at core depths of 700–3025 m, indicating clear Nb/Ta fractionation across a subducted slab. To understand the potential mechanism of Nb/Ta fractionation within the subducted plate, we analysed by laser ablation ICPMS a thermal migration experiment in which a wet andesite was placed in a large thermal gradient (300°C/cm with ends ranging from 950–350°C) at 0.5Gpa. Results show that Nb, Ta and Ti, driven by the thermal gradient, preferentially migrate by diffusion through supercritical fluids into the cooler end of the experiment (at ∼650–350°C). Due to contrasting Nb and Ta thermal migration patterns, dramatic fractionation between Nb, Ta, and Ti took place in the cooler end. Experimental results are consistent with the measured Nb, Ta in rutile from CCSD drillhole samples. We consider that major fractionation between Nb, Ta must occur before rutile appears, most likely during the prograde blueschist to amphibole–eclogite transformation, when Ti is also mobile. Before rutile appears, partitioning between Ti‐rich dominant minerals such as amphiboles and fluids in the hotter region where dehydration preferentially occurs, produces Nb–Ta–Ti‐rich fluids with subchondritic Nb/Ta, and dehydration residues with suprachondritic Nb/Ta. Meanwhile, owing to evolution of the thermal gradient within the subducting slab, thermal migration of Nb, Ta, and Ti in aqueous fluids result in Nb, Ta, and Ti enrichment in the cooler region and depletion in the hotter region. As a result of high‐pressure metamorphism, hydrous rutile‐rich eclogites with overall subchondritic Nb/Ta form in the cooler region, whereas relatively anhydrous rutile‐poor eclogites with suprachondritic Nb/Ta form in the hotter region. Subsequently, partial melting of hydrous rutile‐rich eclogites with initial subchondritic Nb/Ta at deeper levels transfers overall subchondritic Nb/Ta coupled with Nb, Ta, and Ti depletion characteristics to the CC, leaving dry rutile‐poor eclogites with suprachondritic Nb/Ta and rutile‐rich residual eclogites with overall, heterogeneous subchondritic Nb/Ta as a complementary reservoir to the CC.

Ultrahigh-pressure minerals and metamorphic terranes – The view from China

Ultrahigh-pressure (UHP) metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths ± associated mafic-ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere, but which subsequently have experienced P– T conditions within or above the coesite stability field (>∼2.7 GPa, ∼700 °C). Typical products include eclogite, garnet peridotite, and UHP varieties of metapelite, quartzite, marble, paragneiss, and orthogneiss. UHP metamorphic assemblages require relatively cold lithospheric subduction to mantle depths; some recrystallization even occurs under “forbidden” P– T conditions, characterized by a geotherm of <5 °C/km. In appropriate bulk compositions, UHP metamorphism produces coesite, microdiamond and other indicator phases such as majoritic garnet, TiO 2 with α-PbO 2 structure, supersilicic clinopyroxene, high- P clinoenstatite, K-cymrite and stishovite. Globally, at least 20 coesite-bearing eclogitic, eight diamond-bearing, and five majoritic garnet-bearing UHP regions have been documented thus far; they are mostly of Phanerozoic ages. The presence of majoritic garnet, and even apparent stishovite pseudomorph in supracrustal rocks suggests continental subduction to mantle depths exceeding 300 km; such UHP metamorphic terranes should be distinguished from deep-seated mantle xenoliths that contain UHP minerals. Cold subduction zones may be sites of major recycling of H 2O back into the mantle; high- P experiments on mafic-ultramafic bulk compositions reveal that many important hydrous and formally anhydrous phases are stable under such UHP conditions. The current explosion of research on continental UHP terranes reflects their significance for mantle dynamics and the tectonics of continental subduction, collision, exhumation, mantle–slab interactions, and geochemical recycling. A further characterization of UHP phases and positive identification of UHP minerals requires new experimental studies coupled with state-of-the-art analyses. For example, the very rare occurrence of microdiamond inclusions in zircons from Dabie–Sulu UHP rocks may reflect high f O 2 attending recrystallization inasmuch as epidote is rather common. Rutile needles within garnets from Sulu UHP eclogitic rocks may not be the result of exsolution, so in such cases the apparent UHP pressure may have been over estimated. Hadean igneous microdiamond inclusions in Jack Hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital Phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from New South Wales might have been derived from unexposed UHP metamorphic terranes. Micro-mineral intergrowth and nano-size minerals may hold important key to deciphering the actual P– T paths of subduction and mantle return flow. Although most exhumed terranes have returned surfaceward relatively rapidly after short time of UHP condition, the long duration of storage at great depth and slow exhumation for the largest UHP terranes remain as major problems.

Coesite inclusions in garnet from eclogitic rocks in western Tianshan, northwest China: Convincing proof of UHP metamorphism

Coesite inclusions in garnet have been recognized in eclogitic rocks from western Tianshan, northwest China. The coesite grains exhibit distinct radial cracks in host porphyroblastic garnet; some coesite relics are well preserved, whereas others are partially replaced by quartz. Coesite has been identified optically and then confirmed by in situ Raman spectroscopy, showing the characteristic band at 522 cm-1 and subsidiary bands at 428, 326, 271, 178, 151, and 121 cm-1. The eclogitic rocks contain garnet, omphacite, and Na-Ca-amphibole, and they are rich in white mica (>30%) and graphite. Peak conditions of 570-630 °C and 2.7-3.3 GPa are constrained by garnet-clinopyroxene geothermometry and the occurrence of coesite. The presence of coesite and widespread quartz inclusions in garnet with radial cracks indicative of former coesite in these unique graphitic rocks confirms the previous suggestion of the UHP terrane for the western Tianshan, China.