Metamorphic zoning and thermal structure of the Dabie ultrahigh- pressure–high-pressure terrane, central China

The ultrahigh- and high-pressure (UHP–HP) metamorphic belt of the Dabie Mountains, central China, formed by the Triassic continental subduction and collision, is divided into four metamorphic zones; from south to north, the greenschist facies zone, epidote amphibolite to amphibolite facies zone, quartz eclogite zone, and coesite eclogite zone, based on metabasite mineral assemblages. Most of the coesite-bearing eclogites consist mainly of garnet and omphacite with homogeneous compositions and have partially undergone hydration reactions to form clinopyroxene + plagioclase + calcic amphibole symplectites during amphibolite facies overprinting. However, the least altered eclogites sometimes contain garnet and omphacite that preserve compositional zoning patterns which may have originated during their growth at peak temperature conditions of ∼ 750 °C, suggesting a short duration of UHP metamorphic conditions and/or consequent rapid cooling during exhumation. Systematic investigation on peak metamorphic temperatures of coesite eclogite have revealed that, contrary to the general trend of metamorphic grade in the southern Dabie unit, the coesite eclogite zone shows rather flat thermal structure (T = 600 ± 50 °C) with the highest temperature reaching up to 850 °C and no northward increase in metamorphic temperature, which is opposed to the previous interpretations. This feature, along with the preservation of compositional zonation, implies complicated differential movement of each eclogite mass during UHP metamorphism and the return from the deeper subduction zone at mantle depths to the surface.     

Petrology of the diamond-grade eclogite in the Kokchetav Massif, northern Kazakhstan

Abstract The Kokchetav Massif of northern Kazakhstan is unique because of the abundant occurrence of microdiamond inclusions in garnet, zircon and clinopyroxene of metasediments. In order to determine precise pressure–temperature (P–T) conditions, we have systematically investigated mineral inclusions and the compositions of major silicates in Ti–clinohumite–garnet peridotite and diamond-grade eclogite from Kumdy–Kol. It was found that garnet peridotites from Kumdy–Kol contain assemblages of garnet, olivine, Ti–clinohumite and ilmenite. The garnet contains inclusions that are indicative of both ultrahigh pressure (UHP) and retrograde conditions. Inclusions of hydrous phases such as chlorite, amphibole and zoisite were formed at the post-UHP stage. The study also found that eclogite from Kumdy–Kol contains albite–augite symplectites after omphacitic pyroxene. The core of pyroxene (sodic augite) contains high K₂O (up to 1wt%; average 0.24wt%). Phengite is included in the core. Applying the K₂O-in-augite geobarometry, which is based on recent experiments, and the garnet–clinopyroxene (Grt–Cpx) geothermometer for peak metamorphism, the eclogites yield P–T estimates of > 6 GPa and > 1000 °C, and the diamond-grade eclogites yield lower temperature estimates at 900–1000 °C and 5 GPa.     

Diamond-bearing and diamond-free metacarbonate rocks from Kumdy–Kol in the Kokchetav Massif, northern Kazakhstan

Abstract Dolomite marble from the Kumdy–Kol area of the Kokchetav Massif contains abundant microdiamond, mainly in garnet and a few in diopside. The mineral assemblage at peak metamorphic condition consists of dolomite + diopside + garnet (+ aragonite) ± diamond. Inclusions of very low MgCO₃ calcite and almost pure calcite occur in diopside and are interpreted as aragonite and/or aragonite + dolomite. Single-phase Mg–calcite in diopside with a very high MgCO₃ component (up to 21.7 mol%) was also found in diamond-free dolomitic marble, and is interpreted as a retrograde product from aragonite + dolomite to Mg–calcite. The dolomite stability constrains the maximum pressure (P) at < 7 GPa using previous experimental data, whereas the occurrence of diamond yields the minimum peak pressure–temperature (P–T) condition at 4.2 GPa and 980 °C at X co ₂ = 0.1. The highest MgCO₃ in Mg–calcite constrains the minimum P–T condition higher than 2.5 GPa and 800 °C for the exhumation stage. As these marbles were subjected to nearly identical P–T metamorphic conditions, the appearance of diamond in some carbonate rocks was explained by high X co ₂. A low X co ₂ condition refers to high oxidized conditions and diamond (and/or graphite) becomes unstable. Difference in X co ₂ for marble from the same area suggests local heterogeneity of fluid compositions during ultrahigh-pressure metamorphism.     

Petrology of the Hujialin garnet clinopyroxenite in the Su-Lu ultrahigh-pressure province, eastern China

Abstract Garnet clinopyroxenite containing porphyroclastic clinopyroxene with garnet lamellae from the Su-Lu ultrahigh-pressure (UHP) province, eastern China, records a three-stage evolutionary history. Stage A: precursor aluminous clinopyroxene associated with Mg-rich spinel was stable. Stage B: Mg-rich spinel and aluminous clinopyroxene recrystallized to form porphyroblasts of garnet and diopside with garnet lamellae, respectively. Thereafter, these porphyroblasts were granulated and recrystallized to form equigranular neoblasts in the matrix, being driven by subsolidus deformation. Stage C: the assemblage developed retrograde hornblende, epidote, spinel, chlorite, calcite, and dolomite. The estimated composition of the precursor aluminous clinopyroxene indicates that equilibrium conditions of the stage A were 900-1000°C, and 1.0-1.5 GPa. The neoblast garnet-clinopyroxene pairs, not conjectural, give 740 ± 50°C and higher than 2.5 GPa for the climax conditions of stage B. This implies that the Hujialin garnet clinopyroxenite was transported from a low dP/dT setting to a high dP/dT setting, probably related to subduction during stage B. The Hujialin garnet clinopyroxenite as well as adjacent UHP eclogite which records 700°C and 3.0 GPa as the maximum physical conditions, experienced amphibolite facies retrogression, suggesting that they shared a common P-T history during exhumation.     

Ultrahigh-pressure metamorphism and decompressional P-T paths of eclogites and country rocks from Weihai, eastern China

Abstract Altered quartz-rich and nearly quartz-free eclogitic rocks and completely retrograde quartz-rich garnet amphibolites occur as blocks or lenses in gneisses at Weihai, northeastern tip of the Sulu ultrahigh-P belt. Eclogitic rocks with assemblage garnet ± clinopyroxene ± coesite + rutile have experienced three-stage metamorphic events including ultrahigh-pressure eclogite, granulite and amphibolite facies. Granulite metamorphic event is characterized by formation of the hypersthene + salite + plagioclase ± hornblende corona between garnet and quartz + clinopyroxene. P-T conditions for the three-stage recrystallization sequence are 840 ± 50°C, >28 kbar, about 760±50°C, 9 kbar, and ~650°C, <8 kbar respectively. Most country rock gneisses contain dominant amphibolite-facies assemblages; some garnet-bearing clinopyroxene gneisses recrystallized under granulite-facies conditions at about 740±50°C and 8.5 kbar; similar to granulite-facies retrograde metamorphism of the enclosed eclogitic blocks. Minor cale-silicate lenses within gneisses containing an assemblage grossular + salite + titanite + quartz with secondary zoisite and plagioclase may have formed within a large pressure range of 14-35 kbar. Eclogitic boudins and quartzo-feldspathic country rocks may have experienced coeval in situ UHP and subsequent retrograde metamorphism. The established nearly isothermal decompression P-T path suggests that this area may represent the interior portion of a relatively large subducted sialic block. The recognized UHP terrane may extend eastward across the Yellow Sea to the Korean Peninsula.     

A petrogenetic grid for eclogite and related facies under high-pressure metamorphism

The petrogenetic grid between the eclogite and other high-pressure/temperature (P/T) metamorphic facies in a basaltic system is constructed by considering barroisite as one of the important phases in high-P/T metamorphism and by using previous petrological data combined with Schreinemakers' analysis and slope calculation. In the constructed petrogenetic grid, the eclogite facies is bounded by the blueschist and epidote–amphibolite facies with negative-slope reactions at lower temperatures (450–550 °C) and by the epidote–amphibolite, amphibolite and granulite facies with positive-slope reactions at higher temperatures (> 550–600 °C). The eclogite facies does not contact the greenschist facies, and the lowest P condition for the eclogite facies exists at the boundary between the eclogite and epidote–amphibolite facies. The temperature range of the epidote–amphibolite facies increases with increasing pressure until 8–11 kbar and then decreases up to 13–15 kbar. Compared to boundaries of other facies, boundaries of the eclogite facies may have wider P–T ranges. The boundary between the blueschist and eclogite facies occurs over a large temperature range from 450 to 620 ± 30 °C, and the transitions between the eclogite and amphibolite or high-pressure granulite facies occur over a pressure range in excess of 6–10 kbar.     

Petrogenetic grid for ultrahigh-pressure metamorphism in the model system CaO-MgO-SiO2-CO2-H2O

Abstract Petrogenetic grids for ultrahigh-pressure (UHP) metamorphism were calculated at different X_co2 conditions in the model system CaO-MgO-SiO₂-CO₂-H₂O involving coesite (Co), diopside (Di), dolomite (Do), enstatite (En), forsterite (Fo), magnesite (Ms), quartz (Qz), talc (Tc), tremolite (Tr) using a published internally consistent thermodynamic data set. Two P-T grids at X_co2= 0.01 and 0.5 are described. In the calculated P-T grid at X_co2= 0.01, four out of 10 stable invariant points, Co-En-Ms-Tc, Co-Di-En-Tc-Tr, Co-Di-Ms-Tc-Tr and Di-En-Ms-Tc-Tr lie within the stability field of coesite. If the fluid phase has X_co2= 0.5, no invariant point is stable under UHP conditions. Some magnesite-bearing assemblages are stabilized by the following three reactions: Di + Ms = Do + Fo + CO₂, Ms + Tr = Do + Fo + CO₂+ H₂O and Ms + Tc = Fo+ CO₂+ H₂O at X_co2= 0.01 and by reaction Ms + Tc = Fo + CO₂+ H₂O together with these three at X_co2= 0.5. Ten possible UHP assemblages for mafic and ultramafic compositions at very low X_co2 conditions include the following: Co-Do-Ms, Co-Di-Ms, Co-Di-Tc, Di-Ms-Tc, Di-En-Tc-, Di-En-Ms, Co-Di-En, Di-En-Fo, Di-Fo-Ms, Di-Do-Fo. Among them, talc-bearing assemblages are restricted to X_co2 < 0.02 and their high-P limit is 31.7 kb (749°C) at X_co2= 0.01. Dolomite-magnesite-silica assemblages have large P-T stability fields even if X_co2 is as low as 0.1, and could occur in cold subduction zones with very low geothermal gradients. Reported UHP coesite-dolomite assemblage is restricted only to a calc-silicate rock interlayered with marble where X_co2 is relatively higher; no such assemblage appears for mafic and ultramafic rocks with low X_co2 evidenced by the occurrence of diopside (or omphacite) at the expense of dolomite + coesite. The effect of X_co2 on the stability of coesite-dolomite-magnesite, diopside-enstatite-magnesite, diopside-talc assemblages is examined and the occurrence of coesite-dolomite, magnesite-bearing and talc-bearing assemblages in the Dabie UHP rocks are interpreted by employing the calculated P-T grids.     

A jadeite–quartz–glaucophane rock from Karangsambung, central Java, Indonesia

High-pressure metamorphic rocks are exposed in Karangsambung area of central Java, Indonesia. They form part of a Cretaceous subduction complex (Luk–Ulo Complex) with fault-bounded slices of shale, sandstone, chert, basalt, limestone, conglomerate and ultrabasic rocks. The most abundant metamorphic rock type are pelitic schists, which have yielded late Early Cretaceous K–Ar ages. Small amounts of eclogite, glaucophane rock, garnet–amphibolite and jadeite–quartz–glaucophane rock occur as tectonic blocks in sheared serpentinite. Using the jadeite–garnet–glaucophane–phengite–quartz equilibrium, peak pressure and temperature of the jadeite–quartz–glaucophane rock are P  = 22 ± 2 kbar and T  = 530 ± 40 °C. The estimated P–T conditions indicate that the rock was subducted to ca 80 km depth, and that the overall geothermal gradient was ∼ 7.0 °C/km. This rock type is interpreted to have been generated by the metamorphism of cold oceanic lithosphere subducted to upper mantle depths. The exhumation from the upper mantle to lower or middle crustal depths can be explained by buoyancy forces. The tectonic block is interpreted to be combined with the quartz–mica schists at lower or middle crustal depths.     

Metamorphic petrology of the Barchi–Kol metabasites, western Kokchetav ultrahigh-pressure–high-pressure massif, northern Kazakhstan

Abstract In the Barchi–Kol area, located at the westernmost part of the Kokchetav ultrahigh pressure (UHP) to high-pressure (HP) massif, northern Kazakhstan, metabasites from the epidote amphibolite (EA) facies to the coesite eclogite (CEC) facies are exposed. Based on the equilibrium mineral assemblages, the Barchi–Kol area is divided into four zones: A, B, C and D. Zone A is characterized by the assemblage: epidote + hornblende + plagioclase + quartz, with minor garnet. Zone B is characterized by the assemblage: garnet + hornblende + plagioclase + quartz + zoisite. Zone C is defined by the appearance of sodic–augite, with typical assemblage: garnet + sodic–augite + tschermakite–pargasite + quartz ± plagioclase ± epidote/clinozoisite. Zone D is characterized by the typical eclogite assemblage: garnet + omphacite + quartz + rutile, with minor phengite and zoisite. Inclusions of quartz pseudomorph after coesite were identified in several samples of zone D. Chemical compositions of rock-forming minerals of each zone were analyzed and reactions between each zone were estimated. Metamorphic P-T conditions of each zone were estimated using several geothermobarometers as 8.6 ± 0.5 kbar, 500 ± 30 °C for zone A; 11.7 ± 0.5 kbar, 700 ± 30 °C for zone B; 12–14 kbar, 700–815 °C for zone C; and 27–40 kbar, 700–825 °C for zone D.     

Dating of prograde metamorphic events deciphered from episodic zircon growth in rocks of the Dabie–Sulu UHP complex, China

The timing of ultra-high pressure (UHP) metamorphism has been difficult to determine because of a lack of age constraints on crucial events, especially those occurring on the prograde path. New Sensitive High-Resolution Ion Microprobe (SHRIMP) U–Pb age and rare-earth element (REE) data of zircon are presented for UHP metamorphic rocks (eclogite, garnet peridotite, garnet pyroxenite, jadeite quartzite and garnet gneiss) along the Dabie–Sulu UHP complex of China. With multiphase metamorphic textures and index mineral inclusions within zircon, the Dabie data define three episodes of eclogite-facies metamorphism, best estimated at 242.1 ± 0.4 Ma, 227.2 ± 0.8 Ma and 219.8 ± 0.8 Ma. Eclogite-facies zircons of the Sulu UHP complex grew during two major episodes at 242.7 ± 1.2 and 227.5 ± 1.3 Ma, which are indistinguishable from corresponding events in the Dabie UHP complex. A pre-eclogite metamorphic phase at 244.0 ± 2.6 Ma was obtained from two Sulu zircon samples which contain low pressure–temperature (plagioclase, stable below the quartz/Ab transformation) and hydrous (e.g., amphibole, stable below  2.5 Gpa) mineral inclusions. In terms of Fe–Mg exchange of trapped garnet–clinopyroxene pairs within zircon domains, we are able to determine the Pressure–Temperature (PT) conditions for a specific episode of metamorphic zircon growth. We suggest that mineral phase transformations and associated dehydration led to episodic eclogite-facies zircon growth during UHP metamorphism ( 2.7 Gpa) began at 242.2 ± 0.4 Ma (n = 74, pooling the Dabie–Sulu data), followed by peak UHP metamorphism (>  4 Gpa) at 227.3 ± 0.7 Ma (n = 72), before exhumation (<  220 Ma) to quartz stability (~ 1.8 Gpa). The Dabie–Sulu UHP metamorphism lasted for about 15 Ma, equivalent to a minimum subduction rate of 6 mm/year for the descending continental crust.     

Aragonite and omphacite-bearing metapelite from Besshi region, Sambagawa belt in central Shikoku, Japan and its implication

Aragonite and omphacite-bearing metapelite occurs in the albite–biotite zone of the Togu (Tohgu) area, Besshi region, Sambagawa metamorphic belt, central Shikoku, Japan. This metapelite consists of alternating graphite-rich and graphite-poor layers that contain garnet, phengite, chlorite, epidote, titanite, calcite, albite, and quartz. A graphite-poor layer contains a 1.5-cm ivory-colored lens that mainly consists of phengite, calcite, albite, and garnet. Aragonite, omphacite, and paragonite occur as inclusions in the garnet of the ivory lens. The aragonite has a composition that is close to the CaCO₃ end-member: the FeCO₃ and MnCO₃ components are both less than 0.3 mol% and the SrCO₃ component is about 1 mol%. The aragonite + omphacite + quartz assemblage in garnet indicates equilibrium conditions of P  > 1.1–1.3 GPa and T  = 430–550°C. Quartz grains sealed in garnet of the aragonite and omphacite-bearing sample and other metapelites in the Togu area preserve a high residual pressure that is equivalent to the Sambagawa eclogite samples. These facts suggest that: (i) the Togu area experienced eclogite facies metamorphism; and (ii) thus, eclogite facies metamorphism covered the Sambagawa belt more extensively than previously recognized.     

Eclogites from the Chinese continental scientific drilling borehole, their petrology and different P-T evolutions

Abstract Four phengite‐bearing eclogites, taken from different depths of the Chinese continental scientific drilling (CCSD) borehole in the Sulu ultrahigh pressure terrane, eastern China, were studied with the electron microprobe. The compositional zonations of garnet and omphacite are moderate, whereas phengite compositions generally vary significantly in a single sample from core to rim by decrease of the Si content. Various geothermobarometric methods were applied to constrain the P‐T conditions of these eclogites on the basis of the compositional variability of the above minerals. The constrained P‐T path for sample B218 is characterized by pressure decrease from ca 3.0 GPa (ca 600°C) to 1.3 GPa (ca 550°C). Eclogite B310 yielded P‐T conditions of 3.0 GPa and 750°C. The path for eclogite B1008 starts at about 650°C and 3.6–3.9 GPa (stage I) followed by a pressure decrease to 2.8–3.0 GPa and a significant temperature rise (stages II and IIIa, 750–810°C). Afterwards, this rock cooled down to 620–660°C at still high pressures (2.5–2.7 GPa, stage IIIb). Retrograde conditions were about 670°C and 1.3 GPa (stage IV). Eclogite B1039 yielded a P‐T path starting at ca 600°C and 3.3–3.9 GPa (stage I). A pressure decrease to about 3.0 GPa (stage II, 590–610°C) and then a moderate isobaric temperature increase to ca 630°C (stage III) followed. Stage IV is characterized by temperatures of 650°C at pressures close to 1.3 GPa. During and after this stage (hydrous) fluids partially rich in potassium penetrated the rocks causing minor changes. Relatively high oxygen fugacities led to andradite and magnetite among the newly formed minerals. We think that the above findings can be best explained by mass flow in a subduction channel. Thus, we conclude that the assembly of UHP rocks of the CCSD site, eclogites, quartzofeldspathic rocks, and peridotites, cannot represent a crustal section that was already coherent at UHP conditions as it is the common belief currently. The coherency was attained after significant exhumation of these UHP rocks.     

Prograde pressure-temperature path of jadeite-bearing eclogites and associated high-pressure/low-temperature rocks from western Tianshan, northwest China

Abstract Jadeite‐bearing eclogites and associated blueschists locally crop out in a greenschist facies area at Kuldkourla, near the Akeyazhi River in the western Chinese Tianshan region, northwestern China. Garnet in these metamorphic rocks shows prograde zoning with increasing Mg and decreasing Mn from the crystal center towards the rim, and is divided into Ca‐poor/Fe‐rich core and Ca‐rich/Fe‐poor mantle parts. The garnet cores include the assemblages of (i) jadeite/omphacite (X_jd = 0.34–0.96) + barroisite/taramite; and (ii) omphacite + barroisite/pargasite, with paragonite, epidote, rutile and quartz as major phases with rare albite. The garnet mantles rarely contain inclusions of omphacite, glaucophane, epidote, rutile and quartz. Major matrix phases of the pre‐exhumation stage are omphacite, glaucophane, paragonite, rutile and quartz. These mineral parageneses give pressure (P)‐temperature (T) conditions of 0.9 GPa/390°C?1.4 GPa/560°C for the stage of the garnet core formation, 1.8 GPa/520°C for the stage of the garnet mantle formation, and 2.2 GPa/495°C‐2.4 GPa/535°C for the peak eclogite facies assemblage in the matrix. The estimated P‐T conditions and continuous changes of mineral parageneses imply a counterclockwise P‐T path which is a combination of (i) an early prograde stage of high‐pressure/low‐temperature (HP/LT) blueschist facies and/or LP/LT eclogite facies; (ii) a later prograde stage involving compression with minimal heating; and (iii) a climax‐of‐subduction stage characterized by a slight decrease of temperature with increasing pressure. The negative dP/dT of the latest subduction stage is possibly a record of the following events after a continuous subduction and ridge approach: (i) material migration within the upper part of the subducting slab, which has an inverse thermal gradient caused by ductile flow and/or slab break during subduction; and/or (ii) temporary cooling of the wedge mantle–slab interface by continuous subduction of a relatively cold slab following subduction of a hotter ridge.     

Petrology and retrograde P-T path for eclogites of the Maksyutov Complex, Southern Ural Mountains, Russia

Abstract The Maksyutov Complex, situated in the southern Ural Mountains of Russia, is the first location where quartz aggregates within garnets exhibiting radial fractures were identified as coesite pseudomorphs (Chesnokov & Popov 1965). The complex consists of two tectonic units: a structurally lower eclogite-bearing schist unit and an overlying meta-ophiolite unit. Both units show evidence for multiple stages of metamorphism and deformation. The high-pressure metamorphism of the eclogite-bearing schist unit, discussed in this report, is suspected to be related to a collision between the Russian platform and a fragment of the Siberian continent during the early Cambrian. At least three stages of metamorphism (M_1-3) and two stages of deformation (S₁ and S₂) were observed in thin sections: M₁) garnet (Alm_55-60, Prp_22-28, Grs_16-20) + omphacite (Jd_46-56) + phengite (Si ≅ 3.5) + rutile; M₂) garnet + glaucophane ± lawsonite + white mica; and M₃) epidote + chlorite ± albite ± actinolite + white mica. Observed mineral parageneses define a retrograde P-T path for the eclogite. Mineral assemblages within the most representative eclogite from the lower unit of the Maksyutov Complex indicate minimum peak pressures of 15 kbar at temperatures of approximately 600°C. If the presence of coesite pseudomorph is confirmed, the peak ultrahigh-pressure metamorphism may be as high as 27 kbar at 615°C.     

Significant cooling during exhumation of UHP eclogite from the Taohang area in the Sulu region of eastern China and its tectonic significance

The decompressional pressure–temperature (P–T) path was estimated for ultrahigh‐pressure (UHP) eclogite from the Sulu region of eastern China by applying geo‐thermobarometers to well‐preserved equilibrium mineral pairs. The sample studied is a kyanite‐bearing eclogite that was collected from the Taohang area of the Sulu region. Garnet is relatively homogeneous in chemical composition, but omphacite has a clear chemical zoning with decreasing jadeite content from core to rim. Assuming that peak‐P equilibrium compositions are preserved in the cores of garnet (Grt) and omphacite (Omp), P–T conditions were calculated to be about 700°C and 3.4 GPa. On the other hand, the jadeite content of omphacite rims varies from 0.35 to 0.46 mol.%. Nevertheless, the variation in Fe/Mg ratios of omphacite rims is very small. Temperatures of 566 ± 54°C were obtained at 1.5 GPa for garnet rim and omphacite rim pairs. These petrological considerations indicate that temperatures should have significantly declined during the early decompression stage of this eclogite. In other areas of the Sulu region, isothermal decompression paths were proposed, and it was concluded that the UHP rocks were exhumed as a large mass tens of kilometers in thickness to avoid thermal effects from the surrounding materials. However, the newly identified decompression path accompanying the significant cooling may indicate that the Taohang outcrop was located at the margin of the Sulu UHP terrane. Thus, the decompressional P–T path is not unique in the Sulu region and varies depending on the location.     

An introduction to ultrahigh-pressure metamorphism

Abstract Ultrahigh-pressure (UHP) metamorphism refers to mineralogical and structural readjustment of supracrustal protoliths and associated mafic-ultramafic rocks at mantle pressures greater than ∼ 25 kbar (80-90 km). Typical products include metapelite, quartzite, marble, granulite, eclogite, paragneiss and orthogneiss; minor mafic and ultramafic rocks occur as eclogitic-ultramafic layers or blocks of various dimensions within the supracrustal rocks. For appropriate bulk compositions, metamorphism at great depths produces coesite, microdiamond and other characteristic UHP minerals with unusual compositions. Thus far, at least seven coesite-bearing eclogitic terranes and three diamond-bearing UHP regions have been documented. All lie within major continental collision belts in Eurasia, have similar supracrustal protoliths and metamorphic assemblages, occur in long, discontinuous belts that may extend several hundred kilometers or more, and typically are associated with contemporaneous high-P blueschist belts. This paper defines the P-T regimes of UHP metamorphism and describes mineralogical, petrological and tectonic characteristics for a few representative UHP terranes including the western gneiss region of Norway, the Dora Maira massif of the western Alps, the Dabie Mountains and the Su-Lu region of east-central China, and the Kokchetav massif of the former USSR. Prograde P-T paths for coesite-bearing eclogites require abnormally low geothermal gradients (approximately 7°C/km) that can be accomplished only by subduction of cold, oceanic crust-capped lithosphere ± pelagic sediments or an old, cold continent. The preservation of coesite inclusions in garnet, zircon, omphacite, kyanite and epidote, and microdiamond inclusions in garnet and zircon during exhumation of an UHP terrane requires either an extraordinarily fast rate of denudation (up to 10 cm/year) or continuous refrigeration in an extensional regime (retreating subduction zone).     

Eclogitic metapelites in the western segment of the north Qaidam Mountains: Evidence on "in situ" relationship between eclogite and its country rock

The presence of relics of high-pressure and ultra-high pressure metamorphic assemblages in metasedi-ments and granitoid gneisses provides important evi-dence for deep subduction of continental crust (litho-sphere), and also an important criteria on "in situmetamorphism" and "tectonic emplacement" relation-ship between gneisses and enclosed eclogites. In re-cent years, eclogite and garnet peridotite lenses en-closed within quartz-feldspathic gneisses or peliticgneisses were discovered separately…     

Ultrahigh-pressure metamorphic records hidden in zircons from amphibolites in Sulu Terrane, eastern China

Abstract The amphibolites occur sporadically as thin layers and blocks throughout the Sulu Terrane, eastern China. All analyzed amphibolite from outcrop and drill cores from prepilot drill hole CCSD‐PP1 and CCSD‐PP2, Chinese Continental Scientific Drilling Project in the Sulu Terrane, are retrograded eclogites overprinted by amphibolite‐facies retrograde metamorphism, with characteristic mineral assemblages of amphibole + plagioclase + epidote ± quartz ± biotite ± ilmenite ± titanite. However, coesite and coesite‐bearing ultrahigh‐pressure (UHP) mineral assemblages are identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from these amphibolites. In general, coesite and other UHP mineral inclusions are preserved in the cores and mantles of zircons, whereas quartz inclusions occur in the rims of the same zircons. The UHP mineral assemblages consist mainly of coesite + garnet + omphacite + rutile, coesite + garnet + omphacite, coesite + garnet + omphacite + phengite + rutile + apatite, coesite + omphacite + rutile and coesite + magnesite. Compositions of analyzed mineral inclusions are very similar to those of matrix minerals from Sulu eclogites. These UHP mineral inclusion assemblages yield temperatures of 631–780°C and pressures of ≥2.8 × 10³ MPa, representing the P–T conditions of peak metamorphism of these rocks, which are consistent with those (T = 642–726°C; P ≥ 2.8 × 10³ MPa) deduced from adjacent eclogites. These data indicate that the amphibolites are the retrogressive products of UHP eclogites.     

Permo–Triassic Barrovian-type metamorphism in the ultrahigh-temperature Kontum Massif, central Vietnam: Constraints on continental collision tectonics in Southeast Asia

The Kontum Massif in central Vietnam is composed of various metamorphic complexes including a high-temperature southern part (Kannak and Ngoc Linh complexes) and a low- to medium-temperature northern part (Kham Duc complex). The Kham Duc complex exhibits Barrovian-type medium-pressure metamorphism evidenced by kyanite- and/or staurolite-bearing metapelites. The garnet–gedrite–kyanite gneiss, which is the focus of the present study, preserves several mineral parageneses formed during a prograde and retrograde metamorphic history: staurolite + quartz in gedrite, garnet + gedrite + kyanite in the matrix, and spinel + cordierite symplectite between gedrite and sillimanite. The calculated semiquantitative petrogenetic grid reveals peak pressure conditions of 620–650°C at 1.1–1.2 GPa and peak temperature conditions of 730–750°C at 0.7–0.8 GPa. The monazite U–Th–Pb electron microprobe ages of the garnet–gedrite–kyanite gneiss and associated gneisses yield 246 ± 3 Ma for the Kham Duc complex, which is similar to the age of the high- to ultrahigh-temperature metamorphism in the adjacent Kannak and Ngoc Linh complexes of the southern Kontum Massif. The present results indicate that both the Barrovian-type and ultrahigh-temperature metamorphism occurred simultaneously in the Kontum Massif during an event strongly related to Permo–Triassic microcontinental collision tectonics in Asia.     

Oxygen and carbon isotope composition from the UHP Shuanghe marbles, Dabie Mountains, China

Investigations on the oxygen and carbon isotope compositions from the ultrahigh-pressure (UHP)-metamorphosed Shuanghe marbles, that occur as a member of a UHP slab, show that the δ¹⁸ O values range from +11.1% to+20.5% SMOW, and δ¹³ C from+1.0% to+5.7% PDB, respectively. The variations in isotope compositions show a centimeter scale of homogeneity and a heterogeneity of regional scale larger than 1 meter. In contrast to the eclogite marbles from Norway, the Shuanghe marbles have inherited the carbon isotope compositions from their sedimentary precursor. The δ¹³C shows positive correlation to the content of dolomite. The depletion in¹⁸O, compared with the pmtolithic carbonate strata, might result from three possible geological processes: 1) exchanging oxygen isotope with meteoric water before the UHP metamorphism, 2) decarbonation during the UHP metamorphism, and 3) exchanging oxygen isotope with country gneiss at local scale during retrograde metamorphism. It seems that the advection of fluid in the orogenic belt was very limited during subduction and exhumation of UHP rocks. Project supported by a U. S. -China cooperative project led by Prof. Cong Bolin of the Institute of Geology. Chinese Acade-my of Sciences, and Prof. J. G. Liou of the Department of Geological and Environmental Sciences, Stanford University and by the National Natural Science Foundation of china (Grant No. 49794042). Chinese Academy of Sciences (Grant No. KZ951-A1-401r, and National Science Foundation (Grant No. EAR-95-26700).