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The Zedang ophiolite, containing pods of massive chromitite, crops out in the eastern segment of the Yarlung-Zangbo suture zone, Southern Tibet. Chromite grains in the Zedang chromitite bodies have high Cr# [Cr/(Cr + Al)] (0.73–0.82) and Mg# [Mg/(Mg + Fe)] (0.73–0.79), and low TiO2 content (0.13–0.25 wt%); they contain diopside (Di) exsolution lamellae and abundant inclusions of diopside, enstatite, hydrous silicates (e.q., Cr-bearing amphibole), and serpentine. The inclusions occur as isolated single-phase or multiphase (clinopyroxene + amphibole ± platinum group mineral) grains. Diopside lamellae-bearing chromite was probably a precursor CaFe2O4-phase (CF phase) forming at >12.5 GPa. Individual octahedral serpentine inclusions are likely pseudomorphs after isometric ringwoodite. Both the CF phase and ringwoodite crystallized within the mantle transition zone (MTZ). Equilibrium pressure-temperature (P-T) conditions of coexisting clinopyroxene-orthopyroxene (Cpx-Opx) inclusions are 8.5–21.5 ± 1.5 kbar, and 996–1097 ± 30 °C. These P-T conditions suggest that the inferred parental melts of the Zedang chromitites were boninitic. The boninitic liquid in turn was generated from depleted, metasomatized mantle in a supra-subduction zone (SSZ) environment. Linear alignment of hydrous silicate inclusions in chromitites imply fluid infiltration along fracture zones at shallow depths during consolidation of the ophiolite. Based on our new data, we propose a new three-stage model to explain the formation and evolution of the Zedang chromitites. It experienced a long journey from (i) the MTZ forming high-P chromite (containing ultrahigh-pressure minerals), (ii) to the upper mantle attending to the formation of the major part of the chromite, whereas it's host chromitites formed at melt-rock reaction process, and (iii) where high-P chromite was incorporated in low-P neoblastic chromite during subducting slab rollback-induced channel return flow to shallow depths (involving fluid input and alteration).

This memoir highlights the scientific and academic contributions of Bor-ming Jahn who passed away in last December. He is best known for his ability to draw novel insights about the Earth’s tectonics, evolution, and geological processes from in-depth field and laboratory studies of isotopic and elemental geochemistry. He is recognized as a global leader in the applications of geochemical and isotope tracers (Sr-Nd-Hf-Pb-O) to regional tectonic evolution, and made outstanding contributions to the formation of dominant juvenile crust (>70%) for the Phanerozoic Central and Northeast Asian Orogenic Belts. He created the term “TTG” for tonalite-trondhjemite-granodiorite suites formed in ancient and present-day magmatic arc setting and is the first to date the oldest Chinese rocks and to constrain Triassic continental subduction for the Dabie-Sulu UHP terrane. He received numerous honors, and was the first Chinese Earth scientist to be the Editor-in-Chief of the celebrated Journal of Asian Earth Sciences , Elsevier. We are sad for his sudden passing; yet his scientific devotion and contributions will be remembered for decades to come.

Clinoenstatite is an uncommon mineral, and has been documented in Sulu ultrahigh-pressure (UHP) garnet pyroxenite. However, clinoenstatite (CLEN) was rarely reported in the previous studies of ophiolitic mantle peridotites. The studied ophiolitic peridotites are recovered from a drilling hole of the Luobusa ophiolitic massif, Tibet that is unique in occurrences of super reducing UHP minerals in both chromitite and peridotite. Our petrographic and TEM studies reveal abundant CLEN as lamellae associated with diopside exsolution in orthoenstatite (OREN) from the Luobusa mantle peridotite. In addition, termination of CLEN lamellae and Guinier-Preston zones (nanometer-scale precipitates with compositions between enstatite and diopside) are also observed in the OREN. Both CLEN lamellae and host OREN have nearly identical (Mg0.9Fe0.1)2Si2O6 compositions. The CLEN lamellae in host OREN from two core samples have even numbers of 9 A (1 0 0) fringes without twins, and are oriented parallel to (1 0 0) of the host OREN. The crystallographic data of the CLEN are d100 = 9.06 A (R1), d020 = 4.34 A (R2), d120 = 4.03 A (R3), R1 ∠ R2 = 90°, R1 ∠ R3 = 64° and R2 ∠ R3 = 26°, whereas the host OREN have d100 = 18.1 A (R1), d020 = 4.48 A (R2), d120 = 4.34 A (R3), R1 ∠ R2 = 90°, R1 ∠ R3 = 77° and R2 ∠ R3 = 13°. No antiphase domain boundaries are observed in TEM images; it probably indicates that the clinoenstatite is the low-P polymorph of clinoenstatite (LCLEN) with P21/c symmetry. The exsolution of diopside and the presence of Guinier-Preston zones are attributed by cooling of primary high-T Ca-bearing (very minor) OREN. The transformation of LCLEN from OREN may have occurred mainly by cooling along the P-T boundaries between the OREN and LCLEN stability fields; shear stress only enhances the transition. The occurrence of CLEN lamellae in OREN and the existence of super reducing UHP minerals in peridotite provide an additional constraint for a very deep mantle origin of the Luobusa peridotite.

Polycrystalline microdiamonds are rare in ultrahigh-pressure (UHP) rocks worldwide. Among samples collected at Erzgebirge, Germany, we found abundant polycrystalline microdiamonds as inclusions in zircons from a quartzofeldspathic rock. To illuminate their origin and forming age, we investigated morphologies and Raman spectra of 52 microdiamond inclusions, and dated the zircon host. The zircons have low Th/U values (0.03–0.07) and a concordia U/Pb age of 335.8 ± 1.9 Ma. Polycrystalline diamond (10–40 µm) consists of many fine-grained crystals (1.5–3 µm) with different orientations; discrete single diamonds (2–20 µm) are rare. All measured Raman spectra show an intense diamond band at 1332–1328 cm−1 and have a negative correlation with full width at half maximum (FWHM) of 5.8–11.3 cm−1. These data combined with previously reported diamond band data (1331–1337 cm−1) are compatible with those of diamond inclusions in various host minerals from other UHP terranes, but are different from those of ureil...

Field and petrologic characteristics of two new eclogite localities within the Guatemala Suture Complex (GSC) north of the Motagua Fault are presented. The Tuncaj Hill locality exposes a coherent body of retrogressed eclogite hundreds of metres long that is associated with serpentinite of the North Motagua Unit. The Tanilar River locality exposes numerous bands and lenses of eclogite hosted in sialic gneisses of the Chuacus Complex. The Tuncaj eclogite has a two-stage prograde evolution containing the peak assemblage Grt + Omp + Ttn + Czo + Zo ± Am, formed at temperatures

The Kokchetav ultrahigh-pressure ( UHP ) metamorphic belt, located in the western part of the Central Asia Orogenic Belt (CAOB), consists of a well-studied diamond-bearing western domain and a less well-known coesite-bearing domain. To better understand the time of subduction and exhumation of the eastern domain and the tectonic evolution of the entire Kokchetav microcontinent, we conducted SIMS U-Pb dating of zircon from eclogite and rutile from whiteschist of the eastern domain. We identified two types of zircon from the Kulet eclogite: type-I zircon grains possess a large, low-cathodoluminescence (CL) magmatic core with high Th/U ratios of 1.0–1.5, yielding a Proterozoic protolith age (1437 ± 29 Ma), and a thin rim of intermediate or high-CL. Type-II zircon crystals are rounded to ovoid, and have very low Th/U ratios ( UHP rocks from both the eastern and western domains represent Proterozoic protoliths, and that they were subjected to nearly coeval UHP metamorphism at 527–537 Ma, even though the two domains formed at different peak P-T conditions. Exhumation took place during 526–498 Ma for the eastern domain, comparable to the western domain (526–507 Ma). The UHP slabs were exhumed from depths of 100–200 km to a mid-crustal level over 20–25 My. The final suturing of the uplifted Kokchetav microcontinent against the Stepnyak paleo-island arc occurred at ≤500 Ma. Our new SIMS U-Pb age data and previous petrological and structural analyses provide new constraints on the metamorphism and tectonic evolution of the Kokchetav microcontinent, and on the continental history in the western part of the CAOB.

Newly recognized occurrences of ultrahigh-pressure (UH P ) minerals including diamonds in ultrahigh-temperature (UH T ) felsic granulites of orogenic belts, in chromitites associated with ophiolitic complexes, and in mantle xenoliths suggest the recycling of crustal materials through deep subduction, mantle upwelling, and return to the Earth’s surface. This circulation process is supported by crust-derived mineral inclusions in deep-seated zircons, chromites, and diamonds from collision-type orogens, from eclogitic xenoliths in kimberlites, and from chromitities of several Alpine–Himalayan and Polar Ural ophiolites; some of these minerals contain low-atomic number elements typified by crustal isotopic signatures. Ophiolite-type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone. In addition to ringwoodite and inferred stishovite, a number of nanometric minerals have been identified as inclusions employing state-of-the-art analytical tools. Reconstitution of now-exsolved precursor UH P phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier UH P conditions. For example, Tibetan chromites containing exsolution lamellae of coesite + diopside suggest that the original chromitites formed at P > 9–10 GPa at depths of >250–300 km. The precursor phase most likely had a Ca-ferrite or a Ca-titanite structure; both are polymorphs of chromite and (at 2000 °C) would have formed at minimum pressures of P > 12.5 or 20 GPa respectively. Some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin, including zircon, feldspars, garnet, kyanite, andalusite, quartz, and rutile; the zircons possess much older U–Pb ages than the time of ophiolite formation. These UH P mineral-bearing chromitite hosts evidently had a deep-seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes. These new findings together with stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere, recycling of surface ‘organic’ carbon into the lower mantle, and ascent to the Earth’s surface through mantle upwelling. Intensified study of UH P granulite-facies lower crustal basement and ophiolitic chromitites should allow a better understanding of the geodynamics of subduction and crustal cycling.

The Gangdese magmatic arc, southeastern Tibet, was built by mantle-derived magma accretion and juvenile crustal growth during the Mesozoic to Early Cenozoic northward subduction of the Neo-Tethyan oceanic slab beneath the Eurasian continent. The petrological and geochronological data reveal that the lower crust of the southeastern Gangdese arc experienced Oligocene reworking by metamorphism, anatexis and magmatism after the India and Asia collision. The post-collisional metamorphic and migmatitic rocks formed at 34–26 Ma and 28–26 Ma respectively. Meta-granitoids have protolith ages of 65–38 Ma. Inherited detrital zircon from metasedimentary rocks has highly variable ages ranging from 2708 to 37 Ma. These rocks underwent post-collisional amphibolite facies metamorphism and coeval anatexis under P–T conditions of ~710–760 °C and ~12 kbar with geothermal gradients of 18–20 °C km−1, indicating a distinct crustal thickening process. Crustal shortening, thickening and possible subduction erosion due to the continental collision and ongoing convergence resulted in high-P metamorphic and anatectic reworking of the magmatic and sedimentary rocks of the deep Gangdese arc. This study provides a typical example of the reworking of juvenile and ancient continental crust during active collisional orogeny.

The precise constraints on the timing of metamorphism of the Changhai metamorphic complex is of great importance considering the prolonged controversial issue of the north margin and the extension of the Sulu–Dabie HP–UHP Belt. While the monazite U–Th–Pb and muscovite 40 Ar/ 39 Ar techniques are widely accepted as two of the most powerful dating tools for revealing the thermal histories of medium–low grade metamorphic rocks and precisely constraining the timing of metamorphism. The Changhai metamorphic complex at the SE Jiao–Liao–Ji Belt, North China Craton consists of a variety of pelitic schist and Grt–Ky-bearing paragneiss, and minor quartzite and marble. Analyses of mineral inclusions and back-scattered electric (BSE) images of monazites, combined with LA–ICP–MS U–Th–Pb ages for monazites and 40 Ar/ 39 Ar ages for muscovites, provide evidence of the origin and metamorphic age of the Changhai metamorphic complex. Monazites separates from various Grt–Mus schists and Grt–Ky–St–Mus paragneisses exhibit homogeneous BSE images from cores to rims, and contain inclusion assemblages of Grt + Mus + Qtz ± Ctd ± Ky in schist, and Grt + Ky + St + Mus + Pl + Kfs + Qtz inclusions in paragneiss. These inclusion assemblages are very similar to matrix minerals of host rocks, indicating they are metamorphic rather than inherited or detrital in origin. LA–ICP–MS U–Th–Pb dating reveals that monazites of schist and paragneiss have consistent 206 Pb/ 238 U ages ranging from 228.1 ± 3.8 to 218.2 ± 3.7 Ma. In contrast, muscovites from various schists show slightly older 40 Ar/ 39 Ar plateau ages of 236.1 ± 1.5 to 230.2 ± 1.2 Ma. These geochronological and petrological data conclude that the pelitic sediments have experienced a metamorphic event at the Mid–Late Triassic (236.1–218.2 Ma) rather than the Paleoproterozoic (1950–1850 Ma), commonly regarded as the Precambrian basement for the Jiao–Liao–Ji Belt. Hence, the Changhai metamorphic complex should be considered as a discrete lithotectonic group. This newly recognized Mid–Late Triassic metamorphic event (236.1–218.2 Ma) for the Changhai metamorphic complex is coeval with the HP–UHP metamorphic event (235–220 Ma) for Sulu–Dabie rocks. This leads us to speculate that the metamorphism of the Changhai complex belt along the SE margin of the North China Craton was genetically related to the Mid–Late Triassic collision of the North China and South China cratons. By the same token, the Sulu–Dabie HP–UHP Belt may have extended through Yantai, and the southern Yellow Sea, and to the southern side of the Changhai metamorphic complex.

The Gangdese batholith of the southern Tibet is regarded as a principal component of the pre-Himalayan collision, Andean-type continental margin. However, the Gangdese arc root remains poorly understood because most of previous studies concentrated on granitoids from the middle and upper crust of the arc. In this paper, we report new petrochemical and geochronological characteristics of metagabbros occurred as a part of the Late Cretaceous Lilong batholith in the eastern Gangdese arc. The gabbros with protolith ages of ∼82–95 Ma, underwent slightly late (68–77 Ma) granulite-facies metamorphism and nearly coeval anatexis, typical of lower arc crusts. The metagabbros and diorites of the Lilong batholith show geochemical features of juvenile crust in continental magmatic arcs, and an original magmatic differentiation trend, with gabbros formed as the cumulates, whereas diorites as the crystalline products of evolved magmas. The Lilong batholith forms an arc crustal section with a possible thickness of 20–30 km, and may have an overall andesitic composition, providing a support for the “andesite model” of continental crust growth.

Metatexite and diatexite migmatites are widely distributed within the upper amphibolite and granulite facies zones of the Higo low-P/high-T metamorphic terrane. Here, we report data from an outcrop in the highest grade part of the granulite facies zone, in which diatexite occurs as a 3 m thick layer between 2 m thick layers of stromatic-structured metatexite within pelitic gneiss. The migmatites and gneiss contain the same peak mineral assemblage of biotite + plagioclase + quartz + garnet + K-feldspar with retrograde chlorite ± muscovite and some accessory minerals of ilmenite ± rutile ± titanite + apatite + zircon + monazite ± pyrite ± zinc sulphide ± calcite. Calculated metamorphic P–T conditions are 800–900 °C and 9–12 kbar. Zircon in the diatexite forms elongate euhedral crystals with oscillatory zoning, but no core–rim structure. Zircon from the gneiss and metatexite forms euhedral–subhedral grains comprising inherited cores overgrown by thin rims. The overgrowth rims in the metatexite have lower Th/U ratios than zircon in the diatexite and yield a 206Pb/238U age of 116.0 ± 1.6 Ma, which is older than the 110.1 ± 0.6 Ma 206Pb/238U age derived from zircon in the diatexite. Zircon from the diatexite has variable REE contents with convex upward patterns and flat normalized HREE, whereas the overgrowth rims in the metatexite and gneiss have steep HREE-enriched patterns; however, both types have similar positive Ce and negative Eu anomalies. 176Hf/177Hf ratios in the overgrowth rims from the metatexite are more variable and generally lower than values from zircon in the diatexite. Based on U–Pb ages, trace element and Hf isotope data, the zircon rims in the metatexite are interpreted to have crystallized from a locally derived melt, following partial dissolution of inherited protolith zircon during anatexis, whereas the zircon in the diatexite is interpreted to have crystallized from a melt that included an externally derived component. By integrating zircon and petrographic data for the migmatites and pelitic gneiss, the metatexite migmatite is interpreted to have formed by in situ partial melting in which the melt did not migrate from the source, whereas the diatexite migmatite included an externally derived juvenile component. The Cretaceous high-temperature metamorphism of the Higo metamorphic terrane is interpreted to reflect emplacement of mantle-derived basalts under a volcanic arc along the eastern margin of the Eurasian continent and advection of heat via hybrid silicic melts from the lower crust. Post-peak crystallization of anatectic melts in a high-T region at mid-crustal depths occurred in the interval c. 116–110 Ma, as indicated by the difference in zircon ages from the metatexite and diatexite migmatites.

Two microdiamond-bearing samples, a dolomite marble and a garnet-clinopyroxene rock, from the Kokchetav ultrahigh-pressure metamorphic terrane were selected in the present study to explore the possible origin of KAlSi3O8 rod inclusions oriented along the c-axis of clinopyroxene host. The KAlSi3O8 rod inclusions at clinopyroxene cores, where K2O content is high in the range of 0.5–1.0 wt.%, are mostly fine-grained with a rod width less than 1 μm. AEM studies showed that the KAlSi3O8 phase in most rod inclusions is kokchetavite. K-feldspar is present only in a few cases, probably the result of phase transformation/recrystallization from kokchetavite during rock exhumation. Electron diffractions further showed that kokchetavite rods are oriented parallel to clinopyroxene [0 0 1] direction and they exhibit the same epitaxial relation with the clinopyroxene host in both samples with the (Al, Si)O4 tetrahedra chains along the hexagonal a-axis of kokchetavite parallel to the single SiO4 chain along the c axis of clinopyroxene; i.e., [ 1 2 ¯ 1 0 ] Ko //[0 0 1]Cpx and (0 0 0 1)Ko//(1 0 0)Cpx. It is interesting to note that kokchetavite is always in association with phengite, tremolite, β-cristobalite, Si-rich (Al, K, Ca-bearing) low crystallinity phase, ±Si–Ca (Cl, As) phase, ±calcite, ±apatite, ±lollingite (FeAs2), forming compound rods. Furthermore, all these phases are also present within submicron-scale polyphase inclusion pockets in garnet within garnet-clinopyroxene rock sample. These kokchetavite compound rods are therefore most likely to have resulted from melt/fluid-clinopyroxene interactions leading to epitaxial deposition rather than exsolution sensu stricto from the clinopyroxene host. The suggested melt/fluid would have an “external” and/or an “internal” origin related to rock partial melting involving phengite breakdown. Discrete phlogopite and phengite needle-like inclusions with a needle width less than 1 μm, as well as phlogopite–phengite and kokchetavite-mica intergrowth needles, are also not uncommon in clinopyroxene cores. There are specific crystallographic orientation relationships among phases. These micas might have formed earlier than kokchetavite and have exsolved from clinopyroxene host, although the mass balance issue during exsolution and the temporal relation between phlogopite and phengite needles remain to be settled. Clinopyroxene rims generally contain mica needles only. Domains near fractures with coarse-grained kokchetavite compound rods or with coarse-grained phlogopite + quartz needles are not uncommon. These clinopyroxene rims/domains are low in K2O (

Zircons from two Myanmar jadeitite samples were separated for texture, mineral inclusion, U–Pb dating and trace element composition analyses. Three types of zircons, with respect to U–Pb isotope system, were recognized. Type I zircons are inherited ones, yielding an igneous protolith age of 160 ± 1 Ma; Type II zircons are metasomatic/hydrothermal ones, giving a (minimum) jadeitite formation age of 77 ± 3 Ma; and Type III zircons are incompletely recrystallized ones, with non-coherent and geologically meaningless ages from 153 to 105 Ma. These Myanmar jadeitites would therefore have formed through whole-sale metasomatic replacement processes. Compared with Type I zircons, Type II zircons show typical metasomatic/hydrothermal geochemical signatures, with low Th/U ratio ( The Myanmar jadeitites, based on the present study, might have formed during the Late Cretaceous subduction before the beginning of India–Asia continental collision at Paleocene. Previously proposed Late Jurassic ages for Myanmar jadeitites are suggested as results rooted on data retrieved from incompletely recrystallized inherited zircons.

Southeast Asia consists of several microcontinents that detached from the northeastern margin of Gondwanaland. The Song Ma belt in northern Vietnam consists of ophiolite, metabasite, metasedi- mentary rocks and eclogite, and it is thought to be a suture zone between the Indochina and South China blocks. However, the nature and boundaries of the Song Ma belt and the collision age of the two blocks have long been debated. In this article, petrological and geochemical studies on the Song Ma ophiolite and eclogite and first sensitive high-resolution ion microprobe (SHRIMP) age dating of eclogite provide new light to resolve such debate. Eclogite consisting of garnet, omphacite, phengite, quartz, barroisite and rutile is closely associated with garnet-phengite-quartz schist in the Nam Co antiform, a northern subunit of the Song Ma belt. The eclogite experienced a three-stage metamorphic evolution: (I) pre-eclogite stage (amphibolite facies) defined by inclusions of taramite, barroisite, quartz, zoisite ⁄ epidote, mica, rutile & rare chlorite in garnet, (II) eclogite stage and (III) retrograde stage of amphibolite to greenschist facies. The P-T conditions of the three stages are of 14-16 kbar and 520- 550 � C (I), 24-27 kbar and 650-750 � C (II), and 3-7 kbar and 430-510 � C (III), and show a clockwise P-T path based on their mineral assemblages and stability fields in the P-T pseudosection. Thermobarometric results yield similar peak pressure and temperature (26-28 kbar and 650-710 � C). These data suggest that the Song Ma eclogite underwent high-pressure metamorphism in subduction zone with a low thermal gradient 8 � Ck m )1 . The Song Ma ophiolite is composed of serpentinized peridotite, gabbro, basalt, mafic dyke and chert, and experienced ocean-floor metamorphism. Metabasalt and gabbro of ophiolite suite and eclogite all have MORB-type geochemical affinities. Zircon separates from eclogite have very low Th ⁄ U ratios of 0.01-0.05, indicating a metamorphic origin. SHRIMP U-Pb isotopic analyses of this zircon yield a 206 Pb ⁄ 238 U weighted mean age of 230.5 ± 8.2 Ma. This age is interpreted as the closure age of the Paleotethys Ocean that separated the South China and Indochina blocks, and the subsequent collision of the two blocks that took place at the Middle Triassic corresponding to the major episode of the Indosinian Orogeny.

The Dabie-Sulu Triassic collisional orogen in eastern Asia was created by northward subduction of the Yangtze continental-crust capped plate beneath the Sino-Korean craton. Eclogites, garnet peridotites, and surrounding country rock gneisses and marbles were all subjected to in situ UHP metamorphism, as indicated by the presence of rare but widespread coesite inclusions in eclogitic minerals and in zircon crystals in the country rocks, as well as by virtually identical metamorphic ages of various UHP rock types. Metamorphic P-T estimates, combined with investigations of mineral exsolution textures and high- P polymorphs, indicate that recovered depths of continental subduction may have exceeded 200 km. Parageneses of mineral inclusions in zoned zircon domains combined with U-Pb ages delineate a well-constrained P-T -time path, suggesting exhumation rates of 5–10 km/Myr. A similar P-T -time trajectory has been established for the microdiamond-bearing Kokchetav Massif. Thus far, however, diamond inclusions have not been confirmed from coesite-bearing zircon domains of Dabie-Sulu UHP rocks despite numerous detailed studies. Oxygen isotopes of minerals from many outcrop samples and the Chinese Continental Scientific Drilling (CCSD) project main hole cores indicate that Delta; 18 O depletion took place in a volume of Proterozoic protoliths exceeding 100 000 km 3 along the northern edge of the Yangtze craton. Evidently, passive-margin sediments and bimodal igneous rocks that had formed during rifting and breakup of the supercontinent Rodinia were subjected to extensive meteoric water-rock interactions attending terminal Neoproterozoic Snowball Earth conditions. Such hydrothermal alteration volatilized and depleted C from the relatively oxidized protoliths, accounting for the rare occurrences of graphite and apparent lack of microdiamond in Dabie-Sulu UHP rocks.

article i nfo A Late Cretaceous collision of the southernmost portion of the North American continental margin with an undetermined southern block was first established based on the sedimentation history of the plate's supracrustal cover, which is overthrust by harzburgite-dominated nappes of the Guatemala Suture Complex. The collision is also well registered in the metamorphic evolution of continental eclogites of the Chuacus complex, a geologic unit that represents Mesoproterozoic-Triassic sialic basement of North America at the boundary with the Caribbean plate. Garnet-clinopyroxene-phengite thermobarometry of eclogites hosted in Chuacus gneisses indicates near ultra-high-pressure conditions to ~700 °C and ~2.1-2.4 GPa. SHRIMP-RG U-Pb dating of eclogite metamorphic zircon yielded a 75.5±2 Ma age (95% confidence level). Chondrite- normalized rare-earth element patterns of zircon lack Eu anomalies and show depletions in heavy rare earths, consistent with zircon growing in a plagioclase-free, garnet-rich, eclogite-facies assemblage. Additionally, a Sm-Nd clinopyroxene-two garnet-whole rock isochron from an eclogite band yielded a less precise but consistent age of 77±13 Ma. The above features imply subduction to >60 km depth of at least a portion of the North American sialic basement during Late Cretaceous collision. The Chuacus complex was overprinted by an amphibolite-facies event. For instance, mafic high-pressure paragneiss contains symplectite, resorbed garnet, and amphibole+plagioclase poikiloblasts. Zircon rims from the paragneiss sample show rare-earth patterns consistent with plagioclase growth and garnet breakdown. Their74.5±3.5 MaSHRIMP-RGU-Pbage isthereforeinterpretedasthetimeofretrogression,whichisconsistent with previously published results. Within error, the ages of the eclogite-facies event and the amphibolite-facies retrogression are equivalent. Thus exhumation of the Chuacus slab from mantle to mid-crustal depth was quick, taking few million years. During exhumation, partial melting of Chuacus gneisses generated ubiquitous pegmatites. One of the pegmatites intruded the North Motagua melange, which is a serpentinite-rich subduction complex of the Guatemala Suture Complex containing Early Cretaceous oceanic eclogites. U-Pb, Rb-Sr, and K-Ar ages of the pegmatite range ~76-66 Ma. Thus initial juxtaposition of continental and oceanic high-pressure belts of the Guatemala Suture Complex predates Tertiary-present strike-slip faulting between the North-American and Caribbean plates.

The Kulet eclogite in the Kokchetav Massif, northern Kazakhstan, is identified as recording a prograde transformation from the amphibolite facies through transitional coronal eclogite to fully recrystallized eclogite (normal eclogite). In addition to minor bodies of normal eclogite with an assemblage of Grt + Omp + Qz + Rt ± Ph and fine-grained granoblastic texture (type A), most are pale greyish green bodies consisting of both coronal and normal eclogites (type B). The coronal eclogite is characterized by coarse-grained amphibole and zoisite of amphibolite facies, and the growth of garnet corona along phase boundaries between amphibole and other minerals as well as the presence of eclogitic domains. The Kulet eclogites experienced a four-stage metamorphic evolution: (I) pre-eclogite stage, (II) transition from amphibolite to eclogite, (III) a peak eclogite stage with prograde transformation from coronal eclogite to UHP eclogite and (IV) retrograde metamorphism. Previous studies made no mention of the presence of amphibole or zoisite in either the pre-eclogite stage or coronal eclogite, and so did not identify the four-stage evolution recognized here. P–T estimates using thermobarometry and Xprp and Xgrs isopleths of eclogitic garnet yield a clockwise P–T path and peak conditions of 27–33 kbar and 610–720 °C, and 27–35 kbar and 560–720 °C, respectively. P–T pseudosection calculations indicate that the coexistence of coronal and normal eclogites in a single body is chiefly due to different bulk compositions of eclogite. All eclogites have tholeiitic composition, and show flat or slightly LREE-enriched patterns [(La/Lu)N = 1.1–9.6] and negative Ba, Sr and Sc and positive Th, U and Ti anomalies. However, normal eclogite has higher TiO2 (1.35–2.65 wt%) and FeO (12.11–16.72 wt%) and REE contents than those of coronal eclogite (TiO2

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.

Kyanite (Ky)-quartzites occur in both the high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic belts in the southern Sulu orogen. The HP Ky-quartzites consist of quartz, kyanite and minor rutile with or without topaz and phengite, whereas those from the UHP unit consist of quartz, kyanite, phengite and rutile. The HP Ky-quartzites are characterized by high Al2O3 (up to 32.9 wt.%) and low SiO2 (down to 60.4 wt.%) with very low other oxides contents (

Comprehensive review on the characteristics of petrology, oxygen isotope, fluid inclusion and nominally anhydrous minerals (NAMs) for many Dabie–Sulu ultrahigh-pressure (UHP) metamorphic rocks including drill-hole core samples reveals that fluid has played important and multiple roles during complicated fluid–rock interactions attending the subduction and exhumation of supracrustal rocks. We have identified several distinct stages of fluid–rock interactions as follows: (1) The Neoproterozoic supercrustal protoliths of UHP rocks experienced variable degrees of hydration through interactions with cold meteoric water with extremely low oxygen isotope compositions during Neoproterozoic Snow-ball Earth time. (2) A series of dehydration reactions took place during Triassic subduction of the Yangtze plate beneath the Sino-Korean plate; the released fluid entered mainly into volatile-bearing high-pressure (HP) and UHP minerals, such as phengite, zoisite-epidote, talc, lawsonite and magnesite, as well as into UHP NAMs, such as garnet, omphacite and rutile. (3) Silicate-rich supercritical fluid (hydrous melt) existed during the UHP metamorphism at mantle depths >100 km which mobilized many normally fluid-immobile elements and caused unusual element fractionation. (4) The fluid exsolved from the NAMs during the early exhumation of the Dabie–Sulu terrane was the main source for HP hydrate retrogression and generation of HP veins. (5) Local amphibolite-facies retrogression at crustal depths took place by infiltration of aqueous fluid of various salinities possibly derived from an external source. (6) The greenschist-facies overprinting and low-pressure (LP) quartz veins were generated by fluid flow along ductile shear zones and brittle faults during late-stage uplift of the UHP terrane.

The Jiangzhuang ultrahigh-pressure (UHP) metamorphic peridotite from south Sulu, eastern China occurs as a layer within gneiss with eclogite blocks, and consists of coarse-grained garnet porphyroblasts and a fine-grained matrix assemblage of garnet + forsterite + enstatite + diopside ± phlogopite ± Ti-clinohumite ± magnesite. Both types of garnet are characterized by high MgO content and depletion of light rare earth element (LREE) and enrichment of heavy rare earth element, but the matrix garnet has lower MgO, TiO2 and higher Cr2O3 and REE contents. Diopside displays LREE enrichment, and has low but variable large-ion lithophile element (LILE) contents. Phlogopite is a major carrier of LILE. Ti-clinohumite contains high Nb, Ta, Cr, Ni, V and Co contents. The P–T conditions of 4.5–6.0 GPa and 850–950 °C were estimated for matrix mineral assemblages. Most peridotites are depleted in Al2O3, CaO and TiO2, and enriched in SiO2, K2O, REE and LILE. In contrast to phlogopite-free peridotites, the phlogopite-bearing peridotites have higher K2O, Zr, REE and LILE contents. Zircon occurs only in the phlogopite-bearing peridotites, shows no zoning, with low REE contents and Th/U ratios, and yields tight U–Pb ages of 225–220 Ma, indicating the peridotites experienced consistent Triassic UHP metamorphism with subducted supercrustal rocks. These data demonstrate that the Jiangzhuang peridotites were derived from the depleted mantle wedge of the North China Craton, and experienced various degrees of metasomatism. The phlogopite-free peridotites may have been subjected to an early cryptic metasomatism at UHP conditions of the mantle wedge, whereas the phlogopite-bearing peridotites were subjected to a subsequent strong metasomatism, characterized by distinctly enrichment in LILE, LREE, Zr and K as well as the growth of zircon and volatile-bearing minerals at UHP subduction conditions. The related metasomatism may have resulted from the filtration of fluids sourced mainly from deeply subducted supracrustal rocks.

Zircon is the best mineral to record the complex evolution history of ultrahigh-pressure (UHP) metamorphic rocks as mineralogical and geochemical tracers of UHP metamorphism are almost obliterated in matrix assemblages resulted from subsequent retrogression during exhumation. Zircons from Dabie–Sulu UHP rocks, including outcrop and core samples from drill holes ranging from 432 to 5158 m in depth contain abundant mineral inclusions of protolith, prograde, peak (UHP) and retrograde minerals in different domains; these minute inclusions were identified by laser Raman spectroscopy and/or electronic microprobe analysis. Systematic studies on inclusions in zircons from previous and present studies indicate that the Dabie–Sulu UHP terrane extends for >2000 km, is about 50 km wide, and has at least 10 km thick, probably the largest UHP terrane recognized in the world thus far. The internal structure of zircon revealed by cathodoluminescence (CL) imaging displays a distinct zonation, which comprises an inherited (magmatic or detrital) core, prograde, peak (UHP), and outmost retrograde domains, each with distinctive mineral inclusion assemblages. Low-pressure, igneous mineral inclusions are common in the inherited (magmatic or detrital) zircon cores. In contrast, quartz eclogite-facies inclusion assemblages occur in prograde domains, coesite eclogite-facies inclusion assemblages are preserved in UHP domains, and amphibolite-facies inclusion assemblages are enclosed in outmost retrograde rims. Parageneses and compositions of inclusion minerals preserved in distinct zircon domains were used to constrain the metamorphic P–T path of many Dabie–Sulu UHP rocks. The results indicate that Neoproterozoic supracrustal rocks together with minor mafic-ultramafic rocks were subjected to a prograde subduction-zone metamorphism at 570–690 °C and 1.7–2.1 GPa, and UHP metamorphism at 750–850 °C and 3.4–4.0 GPa, following by rapid decompression to amphibolite-facies retrograde metamorphism at 550–650 °C and 0.7–1.05 GPa. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb spot analyses of the zoned zircons show four discrete and meaningful ages of the Dabie–Sulu metamorphic evolution: (1) Neoproterozoic protolith ages (800–750 Ma); (2) 246–244 Ma for early-stage quartz eclogite-facies prograde metamorphism; (3) 235–225 Ma for UHP metamorphism; and (4) 215–208 Ma for late-stage amphibolite-facies retrogression. This indicates that Neoproterozoic voluminous igneous protoliths of orthogneiss in response to the breakup of Rodinia supercontinent, together with various sedimentary rocks, and minor mafic-ultramafic intrusive and extrusive rocks, were subjected to coeval Triassic subduction to mantle depths and exhumation during the collision between the South China Block and North China Block. The estimated subduction and exhumation rates for the Dabie–Sulu UHP terrane would be up to 4.7–9.3 km Myr−1 and 5.0–11.3 km Myr−1, respectively. The zonal distribution of mineral inclusions and the preservation of index UHP minerals such as coesite imply that zircon is the best mineral container for each metamorphic stage, particular for supracrustal rocks as their metamorphic evolution and UHP evidence have been almost or completely obliterated. Similar conclusions have been documented elsewhere for other UHP terranes.

Study of the Maobei garnet peridotite and pyroxenite are significant, because these rocks were subjected to subduction-zone UHP metamorphism that is indicative of deep continental subduction. In order to better understand the origin of the Maobei peridotite and pyroxenite and the processes of continental subduction, we performed geochemical analyses on whole-rock and mineral samples. The studied samples were recovered in the main-hole of the Chinese Continental Scientific Drilling in the Sulu UHP terrane, east-central China. The garnet peridotites occur as lenses or layers in pyroxenite; they comprise garnet wehrlite and garnet dunite with minor interlayers of garnet websterite and garnet. Mini-bands, nodules or veins of eclogite and garnet clinopyroxenite (“internal”) also irregularly occur in garnet wehrlite. The peridotites are composed of olivine (Fo 83–84 ), garnet (Prp 51–56 ), ± diopside ± enstatite (En 86–87 ) ± titanoclinochumite, and have lower Mg/(Mg + Fe total ) values of 0.79–0.83 and higher Al 2 O 3 and CaO contents in comparison with the gneiss-hosted Sulu mantle peridotites. Garnet wehrlite displays variable REE patterns, from LREE-enriched to slightly LREE depleted relative to MREE and HREE; whereas, other ultramafic rocks show roughly flat REE patterns. In general, Rb, Ba and LREE increase, and compatible element (e.g. Cr, Co and Ni) abundances decrease from peridotite, pyroxenite to “internal eclogite” and “internal Grt-clinopyroxenite”. The calculated REE partition coefficients (D REE ) between Cpx and Grt of the Maobei peridotite and pyroxenite decrease regularly from LREE (D Ce = 32–708) to HREE (D Yb = 0.01–0.10), which is comparable with natural mantle peridotite and eclogite crystallized at 800–1100 °C. The geochemical data and occurrence of these rocks suggest that the protoliths of the Maobei peridotites are cumulates derived by differentiation of basaltic magma at the lower continental crust. Thus, the “Internal eclogite and Grt-clinopyroxenite” result from post-cumulus crystallization of the trapped melt. Ti-clinohumite orthopyroxenite may have formed by reaction between peridotitic olivine and indigenous melt or country rock-derived (“exotic”) TiO 2 -bearing fluids at high-P condition during continental subduction to mantle depths. The peak P−T estimates of 795–840 °C and 5.3–6.8 GPa of the Maobei peridotites and the available petrological data of country rocks indicate that the continental crust was subducted to a depth of ∼ 200 km.

Optical microscopy, secondary electron microscopy and analytical electron microscopy were used to characterize crystallographic orientation relationships between oriented mineral inclusions and clinopyroxene (Cpx) host from the Hujialing garnet clinopyroxenite within the Sulu ultrahigh-pressure (UHP) terrane, eastern China. One garnet clinopyroxenite sample (2HJ-2C) and one megacrystic garnet-bearing garnet clinopyroxenite (RZ-11D) were studied. Porphyroblastic clinopyroxene from sample 2HJ-2C contains oriented inclusions of ilmenite (Ilm), spinel (Spl), magnetite and garnet, whereas clinopyroxene inclusions within megacrystic garnet from sample RZ-11D contain oriented inclusions of ilmenite and amphibole. Specific crystallographic relationships were observed between ilmenite/spinel plates and host clinopyroxene in sample 2HJ-2C and between ilmenite plates and host clinopyroxene in sample RZ-11D, i.e. [\( 1\bar{1}00 \)]Ilm//[\( 0\bar{1}0 \)]Cpx (0001)Ilm//(100)Cpx; and [110]Spl//[\( 0\bar{1}0 \)]Cpx (\( \bar{1}11 \))Spl//(100)Cpx. These inclusions are suggested to be primary precipitates via solid-state exsolutions. Most of the needle-like magnetite/spinel inclusions generally occur at the rims or along fractures of clinopyroxene within sample 2HJ-2C. Despite the epitaxial relation with host clinopyroxene, these magnetite/spinel needles would have resulted from fluid/melt infiltrations. Non-epitaxial garnet lamellae in clinopyroxene of sample 2HJ-2C were formed via fluid infiltration-deposition primarily along (010) and subordinately along (100) partings. Epitaxial amphibole plates (with a thickness

Major-element, trace-element and O-isotope analyses were carried out for garnetiferous Ti–clinohumite rocks and eclogites from the Kumdy Kol area, as well as talc–kyanite–phengite–almandine schists (previously named as whiteschists) and eclogites from the Kulet area of the Kokchetav ultrahigh-pressure (UHP) terrane, to explore their possible origins. Both garnetiferous Ti–clinohumite rocks and talc–kyanite–phengite–almandine schists are possibly genetically related to eclogites, as they occur in close association with eclogites enclosed within gneiss/metapelite in these two areas. Most eclogites from the Kumdy Kol and the Kulet areas exhibit chemical characteristics similar to that of ocean floor basalt, although a few show arc affinity with slight Ta and Nb depletions. Garnetiferous Ti–clinohumite rocks from the Kumdy Kol region are characterized by very high Mg, low Si, Ca, Na, and K contents compared with eclogites. They also exhibit LREE depleted patterns with negative Eu anomalies. Their O-isotope compositions are less than + 5.6‰. On the other hand, the rocks show comparable Al, Ni, Cr, Co and HREE contents as eclogites. The Mg enrichment of these garnetiferous Ti–clinohumite rocks is suggested to have originated from their mafic protoliths that had been subjected to mafic rock–ultramafic rock–fluid interactions. Talc–kyanite–phengite–almandine schists from the Kulet area are typically low in Ca and Na, but high in Mg, K, Rb, Ba, Cs, Th, U and O-isotope compositions (+ 6 to + 10‰) relative to eclogites. This peculiar chemical composition can be accounted for if the basaltic protoliths had been subjected to sequential high-temperature (enrichment of Mg) and low-temperature (enrichment of K, Rb, Ba, Cs, Th, U and O-isotope compositions) seawater–basalt interactions. One talc–kyanite–phengite–almandine schist sample KLW 7 is distinctly high in Si and K, but low in Mg and Fe, which can be attributed to more severe low-temperature hydrothermal alterations. However, except for Mg, the enrichment of Rb, Ba, Cs, Th, U and O-isotope compositions in these schist samples could also be enhanced by, or otherwise be interpreted as a result of, fluid influx sourced from enclosing metapelite during subduction.

Granitic leucosome and pegmatite are widely distributed within biotite-bearing orthogneiss in the northern part of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. A combined study of mineral inclusions, cathodoluminescence (CL) images, U–Pb SHRIMP dates, and in situ trace element and Lu–Hf isotope analyses of zircons provided insight into the nature and timing of partial melting in these rocks. Zircon grains separated from biotite-bearing orthogneiss typically have three distinct domains: (1) pre-metamorphic (magmatic) cores with Qtz + Kfs + Pl + Ap inclusions, which record a Neoproterozoic protolith age of ∼ 790 Ma, (2) mantles with Coe + Phe + Ap inclusions that record Triassic UHP age at 227 ± 3 Ma, and (3) narrow rims with quartz inclusions that record HP granulite-facies retrograde metamorphism at ∼ 210 ± 3 Ma. In contrast, zircons separated from granitic leucosome have only two distinct domains: (1) the central UHP areas with Coe + Phe + Ap inclusions record Triassic UHP age of 227 ± 3 Ma, and (2) outer magmatic areas with Qtz + Kfs + Ab + Ap inclusions that record partial melting time of 212 ± 2 Ma. Zircons separated from pegmatite contain mineral inclusions of Qtz + Kfs + Ap and show regular magmatic zoning from centre to edge. The centres record partial melting time of 212 ± 2 Ma in line with the outer domains of granitic leucosome, whereas the edges give a younger age of 201 ± 2 Ma related to Pb loss and partial recrystallization during late Triassic regional amphibolite-facies retrogression. These data indicate that partial melting in the north Sulu UHP gneissic rocks took place during post-UHP, retrograde HP granulite-facies metamorphism. Pre-metamorphic (magmatic) zircon cores from biotite-bearing orthogneiss give uniform 176Hf/177Hf of 0.28187 ± 0.00003 (2 SD; standard deviation) corresponding to eHf(790) and Hf model ages (TDM2) of about − 16.3 and 2.41 Ga, respectively. This is consistent with the generation of its protolith by reworking of Paleoproterozoic to late Archean crust. In contrast, UHP zircon domains from biotite-bearing orthogneiss and granitic leucosome are characterized by distinct trace element composition with low Lu/Hf (

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.

Kumdykolite, an orthorhombic polymorph of albite, has been identified for the first time by analytical electron microscopy. It occurs in association with diopside, quartz/cristobalite, phengite/phlogopite, an unidentified aluminosilicate, calcic amphibole, dolomite, calcite, or talc, as micrometer-scale mineral inclusions in omphacite of eclogite from the Kumdy Kol, Kokchetav ultrahigh-pressure massif in northern Kazakhstan. The unit-cell parameters of kumdykolite were determined to be a = 8.24(1) A, b = 8.68(1) A, and c = 4.84(1) A, V = 346.17 A3, and with Z = 2. The space group could be either P 2 nn or Pmnn, but is probably Pmnn . Kumdykolite is presumed to be a metastable phase formed at high temperatures followed by rapid cooling in the absence of water. It is further postulated that kumdykolite may have resulted from the interaction between infiltrated melt and omphacite when the Kokchetav massif was exhumed from mantle depths to the base of the crust.

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.

The Dabie–Sulu Triassic orogen in central-eastern China was created by northward subduction of the Yangtze cratonal plate beneath the Sino-Korean craton. Eclogites, garnet peridotites and the surrounding country rock gneisses in the collision zone were subjected to in situ ultrahigh-pressure (UHP) metamorphism based on the presence of widespread coesite inclusions in eclogitic minerals and in gneissic zircons, and on virtually identical metamorphic ages of the various UHP rocks. P–T estimates combined with investigations of mineral exsolution textures and high-P polymorphs indicate that recovered depths of continental subduction in east-central China may have reached 200 km or more. Parageneses of mineral inclusions reflecting pre-peak, peak, and retrograde metamorphism in zoned zircon prisms, combined with SHRIMP U–Pb age data imply exhumation rates of 5–10 km/Myr for the Dabie–Sulu UHP terrane. Petrochemical and zircon U–Pb + Hf isotopic studies of various rocks suggest that initial crustal growth of the Yangtze craton occurred in the Archean, but significant Neoproterozoic addition of continental crust along the northern margins of the Yangtze craton is indicated by the formation of abundant bimodal igneous rocks during supercontinental rifting in response to the breakup of Rodinia at about 750–780 Ma. Mineral oxygen isotope and zircon U–Pb data from many surface specimens, and CCSD-main hole core samples over a 3.3 km depth range indicate that three-dimensional 18 O depletion took place in more than 100,000 km 3 in the Dabie–Sulu collision zone; the northern edge of the Yangtze craton was attended by Snowball Earth conditions, and extensive meteoric water-rock interactions occurred during the Neoproterozoic. Most Sulu garnet peridotites were derived from a depleted and metasomatized mantle, representing the subcontinental lithosphere beneath the southern margin of the Sino-Korean craton. Although significant progress has been made in studies of the Dabie–Sulu orogenic belt, critical questions remain unanswered. With new approaches and advanced technologic applications, many petrochemical, mineralogical and isotopic surprises and new tectonic models are anticipated for the years to come.

Detailed field and petrological studies on the South Altyn Tagh in the last decade show that high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks are mainly exposed at Jianggalesayi and Qingshuiquan in the west, and at Yinggelisayi in the middle of the terrane. The South Altyn Tagh UHP rocks occur as lensoid blocks in granitic gneiss, paragneiss and marble and include eclogite, kyanite-garnet-bearing pelitic gneiss, magnesite-bearing garnet lherzolite, K-feldspar-bearing garnet clinopyroxenite and garnet-bearing granitic gneiss. Zircon U–Pb SHRIMP and LA-ICP-MS dating, combined with CL and trace element analysis yield geochronological data for different South Altyn Tagh HP/UHP rocks as follows: metamorphic ages of 493 ± 4 and 499 ± 27 Ma for the Jianggalesayi eclogite and its country rock respectively, 509 ± 12, 489 ± 8 and 487 ± 10 Ma for Yinggelisayi magnesite-bearing garnet lherzolite, K-feldspar-bearing garnet clinopyroxenite and garnet-bearing granitic gneiss, and 475 ± 4 and 486 ± 5 Ma for Qingshuiquan garnet clinopyroxenite and kyanite-bearing garnet-biotite gneiss. Magmatic protolith ages of eclogite, garnet lherzolite, garnet clinopyroxenite and garnet-bearing granitic gneiss are 754 ± 9, 844 ± 9, 821 ± 27 and 842 ± 18 Ma, respectively. Metamorphic ages of these rocks are consistent within errors, indicating that the South Altyn Tagh HP/UHP rocks were formed during 475–509 Ma; these are 20–80 Ma older than dated North Qaidam HP/UHP rocks (420–457 Ma). The occurrence of possible stishovite in pelitic gneiss and the older HP/UHP metamorphic ages suggest that the South Altyn Tagh belt may not necessarily be a western extension of the North Qaidam HP/UHP metamorphic belt, offset along the Altyn Tagh strike-slip fault.

The Yukahe eclogite-gneiss section in the western segment of the North Qaidam HP/UHP belt, NW China occurs two types of eclogites as lenses or layers in both granitic and pelitic gneisses. Zircon in situ LA-ICP-MS U–Th–Pb and trace element analyzes, combined with cathodoluminescence images yield metamorphic ages of 431 ± 4 to 436 ± 3 and 431 ± 3 to 432 ± 19 Ma for eclogites and gneisses, respectively. These nearly identical metamorphic ages suggest that the eclogites and host gneisses underwent coeval early Paleozoic HP/UHP metamorphism. A few inherited zircon cores of eclogites have magmatic characters of high REE and HREE abundance, HREE-enriched REE pattern and Th/U > 0.4, and yield protolith ages of >750 Ma. Geochemical data show that the Yukahe eclogite has alkali basaltic protoliths with high TiO2 and K2O + Na2O contents and Nb/Y ratios, REE patterns and trace element contents similar to those of within plate basalts (WPB). Nd isotope analyzes give epsilon Nd ranges from −5.34 to +4.47, suggesting an enriched mantle source. These characteristics together with the geochronological results conclude that the Yukahe eclogite protoliths likely formed in a Neoproterozoic continental rift setting, and have subjected to HP/UHP metamorphism during early Paleozoic continental subduction.

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 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.

The 5-km deep Chinese Continental Scientific Drilling Main Hole penetrated a sequence of ultrahigh pressure (UHP)-metamorphic rocks consisting mainly of eclogite, gneiss and garnet-peridotite with minor schist and quartzite. Zircon separates taken from thin layers of schist and gneiss within eclogite were investigated. Cathodoluminescence images of zircon grains show that they have oscillatory zoned magmatic cores and unzoned to patchy zoned metamorphic rims. Zircon rims contain rare coesite and calcite inclusions whereas cores contain inclusions of both low-P minerals (e.g. feldspar, biotite and quartz) and coesite and other eclogite-facies minerals such as phengite and jadeite. The zircon cores give highly variable 206Pb/238U ages ranging from 760 to 431 Ma for schist and from 698 to 285 Ma for gneiss, and relatively high but variable Th/U ratios (0.16–1.91). We suggest that the coesite and other eclogite facies mineral inclusions in zircon cores were not magmatic but formed through metasomatic processes caused by fluids during UHP metamorphism, and that the fluids contain components of SiO2, Al2O3, K2O, FeO, MgO, Na2O and H2O. Metasomatism of the Sulu UHP rocks during continental subduction to mantle depths has partly altered magmatic zircon cores and reset isotopic systems. This study provides key evidence that mineral inclusions within magmatic zircon domains are not unequivocal indicators of the formation conditions of the respective domain. This finding leads us to conclude that the routine procedure for dating of metamorphic events solely based on the occurrence of mineral inclusions in zoned zircon could be misleading and the data should be treated with caution.

Garnet clinopyroxenite and eclogite lenses are widespread in mantle-derived garnet peridotites from the Sulu UHP terrane, eastern China. In order to constrain their origin, major and trace element compositions of minerals from 6 eclogites and one garnet clinopyroxenite, and Hf isotopes of zircons from one eclogite, were analyzed by electron probe, LA-ICP-MS and LA- (MC) ICP-MS, respectively. Eclogitic garnets show a large variation in composition (Alm 20–48 Prp 24–50 Grs 15–38 Sps 1 ), weakly correlated with garnets in their host peridotites. Garnets show LREE depletion and flat HREE patterns, with La n of 0.02–0.52 and Lu n of 5–75. Some garnets show zoning in HREE and Y with a U-shaped, concave-downward profile, but locally exhibit positive sharp variations in HREE and Y contents; such feature reflects disturbance in chemical–physical conditions during garnet growth. Clinopyroxene exhibits humped (mostly) or sinusoidal REE patterns, with La n of 0.3–6.0, Nd n of 2–14 and (La/Lu) n of 1.8–113 with a few exceptions. Partition coefficients of REE ( D ) between coexisting clinopyroxene and garnet in eclogite show a large variation: D LREE and D HREE ranging from 10 to100 and from 0.1 to 0.01, respectively, similar to D REE values of the natural systems at 800–1100 °C. The D values (0.1–1) of HREE between Cpx and Grt from Grt clinopyroxenite and one eclogite differ distinctly from the other eclogites, which are consistent with high- T experimental data on garnet lherzolites. Zircons from the Xugou eclogite have no inherent core, which have low U/Th (most 176 Lu/ 177 Hf (0.000008–0.0000637), and relative high 176 Hf/ 177 Hf (0.28241–0.28262) ratios. These features indicate a metamorphic signature, and document that metasomatism affected the eclogite protolith. The model ages ( T DM1 ) of zircons mostly cluster around 1.0–1.1 Ga, which probably reflects the metasomatic source. Bulk compositions of eclogite and Grt clinopyroxenite differ from those of Sulu eclogites in gneiss and MORB. The protoliths of these rocks may reflect the intrusion of asthenosphere-derived melts in Proterozoic. Eclogites with convex REE pattern and garnet clinopyroxenite were formed by high-pressure crystal accumulation whereas the LREE-enriched eclogites may represent crystallized melt; both eclogite and clinopyroxenite together with their host peridotite were metamorphosed at 700–965 °C and ≥ 4 GPa during the Triassic northward subduction of the Yangtze craton.

Article history:Received 23 January 2008Accepted 29 June 2008Available online 10 July 2008Keywords:SHRIMP dataU/PbZirconKokchetavUHP metamorphismpartial melting The Kokchetav Massif of northern Kazakhstan is the best-known metamorphic diamond locality amongnumerous ultrahigh-pressure (UHP) terranes in the world. At the Kumdy-Kol deposit, diamondiferous rocksare interbedded with granitic gneisses, and biotite gneisses; some have been migmatized. Some granitegneisses and migmatites were formed by partial melting of diamondiferous rocks. To verify such suggestion,sensitive high resolution ion microprobe (SHRIMP) U–Pb dating of zoned zircons from migmatites at theKumdy-Kol regionwas performed to constrain the age of partial melting of the Kokchetav UHP metamorphicrocks. Most age data from core and rim domains of zircon separates are concordant. The apparent 206 Pb/ 238 Uages for core and rim domains of zirconsare nearly identical within analytical error. All SHRIMP analyses ofzircons from four samples fall in the range 508–538 Mawith the weighted mean age for all zircon domains at526±2.1 (MSWD=1.7). Our data show that migmatization of UHP pelites occurred later than the peakmetamorphism (537±9 Ma) and the decompression partial melting took place during exhumation ofdiamondiferous rocks from mantle depths to amphibolite-facies conditions at mid-crustal levels (507±8 Ma).© 2008 Elsevier B.V. All rights reserved.

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...

Inclusions of polycrystalline K-feldspar aggregates after K-cymrite (KAlSi 3 O 8 ·nH 2 O) were discovered in garnet from a Dulan eclogite in the Qaidam ultrahigh-pressure (UHP) terrane, western China. The eclogite consists of garnet (Alm 56 Grs 23 Prp 20 Sps 01 ), omphacite (Jd 35 Aeg 6 Aug 59 ), and minor rutile and apatite. The 20 to 200 μm inclusions vary in shape from prismatic, hexagonal to rounded, and exhibit palisade and mosaic textures. Host garnets show radial fractures, similar to those surrounding quartz pseudomorphs after coesite. Some inclusions consist of almost end-member K-feldspar (Or 99–100 Ab 0–1 ) polycrystalline aggregates, whereas others are composed of >90 vol% K-feldspar (Or 96–99 Ab 1–4 ) with minor secondary albite occurring along the margins of the inclusions. Raman spectra of K-feldspar crystalline aggregates vary slightly reflecting various degrees of Si-Al ordering, and show a Raman peak at ~390–395 cm −1 , typical for cymrite structure. These characteristics of the K-feldspar polycrystalline inclusions imply the presence of former K-cymrite in the Dulan eclogite formed at >3 GPa at ~720 °C. The occurrence of K-cymrite in UHP eclogite is significant because of its potential as an important carrier of crustal K and H 2 O to the upper mantle.

Fifty years ago, geologic conditions attending the formation of blueschists, eclogites, and garnet lherzolites were not known. But, with the advent of high-pressure phase-synthesis equipment and precise calorimetry, minerals like jadeite, aragonite, pyrope, and the dense polymorphs of SiO 2 and C were shown to be stable at elevated pressures and relatively low temperatures. Metamorphic conditions required by P - T stabilities of these minerals reflect the operation of plate tectonics, lithospheric subduction, and inferred mantle convection. Integration of phase equilibria with dynamic tectonic processes has illuminated the petrogenesis of the crust. Combined with geochemical, geophysical, and isotopic data, high-pressure phase equilibria are also providing new constraints on the constitution and evolution of the mantle. Circumpacific blueschists and eclogites occur in penetratively sheared nappes that are overturned seaward, indicating 30–50 km descent of an oceanic plate during metamorphism before partial exhumation of mainly low-density crustal material. Neoblastic coesite and microdiamond inclusions in tough, rigid host minerals show that continental collision involves fragmentary recovery of subducted rocks from depths of 100–130 km, far deeper than traditionally thought. Even more surprising, garnet peridotites from the central Alps, western Norway, Bohemia, and China display intergrowths and exsolution lamellae reflecting the former existence of majoritic garnet, stishovite, and other phases requiring depths of origin >300 km. Exsolved nanominerals attest to the decompression of precursor phases that had formed at profound depths preceding mantle upwelling. Times of deep-seated storage and rates of exhumation remain as major problems. Fluid-rock and lithosphere-asthenosphere interactions have recycled volatiles to the deep Earth through subduction of both hydrous and nominally anhydrous minerals. Mantle petrochemistry and plume-plate dynamics control the evolving architecture of the Earth’s crust and the interdependent biosphere. Applications of advanced technologies to condensed materials are leading to a fuller understanding of the planetary interior in time and space.

The complex vein associations hosted in southern Sulu ultrahigh-pressure (UHP) eclogites contain quartz ± omphacite (or jadeite) ± kyanite ± allanite ± zoisite ± rutile ± garnet. These minerals have chemical compositions similar to those of host eclogites. Inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet, and coesite inclusions were found in vein zircon. These facts suggest that the veins together with host eclogites have been subjected to synchronous UHP metamorphism. The vein minerals contain relatively high concentrations of rare earth elements (REE), high-field-strength elements (HFSE) and transition metal elements (TME). A kyanite–quartz vein has a whole-rock composition similar to adjacent UHP metamorphic granitic gneisses. Abundant primary multi-solid fluid inclusions trapped within UHP vein minerals contain complex daughter minerals of muscovite, calcite, anhydrite, magnetite, pyrite, apatite, celestite and liquid and gas phase of H 2 O with solids up to 30–70% of the inclusion volume. The presence of daughter minerals anhydrite and magnetite indicates the subduction fluids were oxidizing, and provides a possible interpretation for the high oxygen fugacity of subduction zone magmas. These characteristics imply that the UHP vein minerals were crystallized from supercritical silicate-rich aqueous fluids that were in equilibrium with peak-UHP minerals, and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as HREE, HFSE and TME. Such fluids might have resulted in major fractionation between Nb and Ta, i.e. the UHP fluids have subchondritic Nb/Ta values, whereas the host eclogites after extraction of the fluids have suprachondritic Nb/Ta values. Therefore, voluminous residual eclogites with high Nb/Ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs.

In situ, spot U-Pb ages of zircons from rhyolite, tuffaceous chert, spherule-bearing chert, and granite in the North Pole Dome obtained by laser ablation ICP-MS (LA-ICP-MS) combined with cathode luminescence imaging (CLI) reveal zircon growth patterns that reflect their igneous origins. Felsic lava forming the structural top of the succession, previously dated as 3458 + 9.1/-4.2 Ma by TIMS, has been dated as 3660 ± 52 Ma. These 3.6 Ga zircons from felsic lava, initially considered to be xenocrysts, are interpreted as euhedral igneous crystals as their CL images show no overgrowth rims and/or evidence of melting. Progressive downward in the section, zircons from tuffaceous cherts yield ages of 3463 ± 34 Ma and 3454 ± 46 Ma. This Archean volcaniclastic sequence was intruded by the North Pole Monzogranite at 3391 ± 33 Ma. Our reconstructed stratigraphy shows that the chert-greenstone secession, previously thought to be a single intact stratigraphic unit, actually consists of five units; most are typified by ...

A ~5 cm thick garnet-rich layer, with relict spinel grains in its center, is intercalated within garnet pyroxenite at Rongcheng. Both the garnetite layer and the host garnet pyroxenite underwent the same P-T evolution, but exhibit quite different mineral assemblages, microtextures, and associated mineral reactions. Microtextures and mineral compositions confirm our previous suggestion that the Rongcheng garnet pyroxenite recrystallized from a spinel pyroxenite cumulate derived from a gabbroic magma. The present study concludes that: (1) The garnet-rich layer resulted from a net transfer reaction between primary spinel and clinopyroxene, Ca1.5(FeMg)0.5 Si2O6 + (FeMg)Al2O4 + TiO2 = Ca1.5(FeMg)1.5Al2(Si2Ti)O12, and mass-balance reactions between clinopyroxene end-members, 2[Ca0.75(FeMg)0.25][(FeMg)0.5Al0.5] (Si1.5Al0.5)O6 = Ca1.5(FeMg)1.5Al2Si3O12, under UHP conditions that produced garnet-I. (2) Garnet-I decomposed during exhumation of the UHP terrane, leading to the assemblage of Grt-II + Ilm + Cpx-II + Am...

We here summarize recent studies of the occurrence, petrology, geochemistry, and geochronology of a new UHP metamorphic belt in the southwestern Tianshan. Eclogites here are grouped into three types: (1) lenticular eclogite bodies surrounded by blueschists; (2) eclogites with relict pillow structures; and (3) banded eclogite layers within marbles. They contain inclusions of coesite pseudomorphs in garnet, coesite exsolution rods in omphacite, and the possible stable association of aragonite + magnesite after dolomite. Rocks record three discrete stages of recrystallization: a peak UHP eclogite stage at 560-600°C and 4.9-5.0 GPa, a main UHP eclogite stage at 598-496°C and 2.6-2.7 GPa and retrograde overprinting under epidote blueschist-facies conditions. Protoliths possess geochemical characteristics of ocean-island basalts that originated from an enriched mantle with ∊Nd = -1.4 to -0.4, NMORB from depleted mantle with ∊Nd = +6.7 to +7.4, and EMORB from enriched mantle with ∊Nd = -2.5 to +3.2. SHRIMP datin...

Garnet clinopyroxenite at Rizhao occurs as a lens 100 x 225 m2 within a serpentinized peridotite body, in fault contact with felsic gneiss. Minor dunite also occurs within the same peridotite. The clinopyroxenite consists of porphyroblastic Grt-bearing (type 1), megacrystic Grt-bearing (type 2), porphyroblastic Cpx-bearing (type 3), and equigranular (type 4) garnet clinopyroxenite. Lamellaerich, coarse-grained Cpx occurs as inclusions in megacrystic Grt from type 2 clinopyroxenite, and as porphyroblasts in a matrix of Cpx + Grt + Ilm from type 3 clinopyroxenite; these clinopyroxenes contain abundant exsolution lamellae of Grt (Prp16-19Grs62Alm19-21Sps1) + Ilm ± Mag ± Amp (Mg#: 0.88-0.98; Na2O: 0.5-3.3 wt%; K2O: 0.7-1.0 wt%; TiO2: 0.1-0.2 wt%). Megacrystic and porphyroblastic garnet (Prp35-42Grs34-45Alm19-24Sps1) also occurs in a matrix of fine-grained Cpx + Grt + Ilm; spinel (Mg#: 0.60-0.62) is present as inclusions in both porphyroblastic Grt and lamellae-rich Cpx. Petrochemical data support a mantle ori...

The Triassic Dabie-Sulu collision zone along with recently identified ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic terranes of the western Tianshan, Altyn Tagh, northern Qaidam, and northern Qinling belts forms a huge chain of orogens from northwestern to east-central China. The western Tianshan HP/UHP terrane formed by Pacific-type subduction of a Paleozoic South Tianshan ocean, and was exhumed to higher crustal levels during collision of the Tarim and Yili-central Tianshan plates. HP/UHP recrystallization took place either at ~345 Ma or 232-215 Ma. The Altyn Tagh, northern Qaidam, and northern Qinling UHP belts are Early Paleozoic collisional orogens. Coesite-bearing eclogites, garnet peridotites, and the country rock gneisses in the Dabie-Sulu UHP terrane were subjected to in situ UHP metamorphism based on the presence of abundant coesite inclusions in zircons and identical metamorphic ages in both eclogite and gneiss. Some garnet peridotites are crust-hosted whereas others were mantle d...

Low-temperature eclogite and eclogite facies metapelite together with serpentinite and marble occur as blocks within foliated blueschist that was originated from greywacke matrix; they formed a high-pressure low-temperature (HPLT) subduction complex (melange) in the North Qilian oceanic-type suture zone, NW China. Phengite–eclogite (type I) and epidote–eclogite (type II) were recognized on the basis of mineral assemblage. Relic lawsonite and lawsonite pseudomorphs occur as inclusions in garnet from both types of eclogite. Garnet–omphacite–phengite geothermobarometry yields metamorphic conditions of 460–510 °C and 2.20–2.60 GPa for weakly deformed eclogite, and 475–500 °C and 1.75–1.95 GPa for strongly foliated eclogite. Eclogite facies metasediments include garnet–omphacite–phengite–glaucophane schist and various chloritoid-bearing schists. Mg-carpholite was identified in some high-Mg chloritoid schists. P–T estimates yield 2.60–2.15 GPa and 495–540 °C for Grt–Omp–Phn–Gln schist, and 2.45–2.50 GPa and 525–530 °C for the Mg-carpholite schist. Mineral assemblages and P–T estimates, together with isotopic ages, suggest that the oceanic lithosphere as well as pelagic to semi-pelagic sediments have been subducted to the mantle depths (≥75 km) before 460 Ma. Blueschist facies retrogression occurred at c. 454–446 Ma and led to eclogite deformation and dehydration of lawsonite during exhumation. The peak P–Tconditions for eclogite and metapelite in the North Qilian suture zone demonstrate the existence of cold subduction-zone gradients (6–7 °C km−1), and this cold subduction brought a large amount of H2O to the deep mantle in the Early Palaeozoic times.